HUBViewControllerExperimentalImplementation.m

/*
 *  Copyright (c) 2016 Spotify AB.
 *
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you under the Apache License, Version 2.0 (the
 *  "License"); you may not use this file except in compliance
 *  with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing,
 *  software distributed under the License is distributed on an
 *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
 *  under the License.
 */

#import "HUBViewControllerExperimentalImplementation.h"

#import "HUBIdentifier.h"
#import "HUBViewModelLoaderImplementation.h"
#import "HUBViewModel.h"
#import "HUBComponentModel.h"
#import "HUBComponentImageData.h"
#import "HUBComponentTarget.h"
#import "HUBComponentWithImageHandling.h"
#import "HUBComponentWithChildren.h"
#import "HUBComponentContentOffsetObserver.h"
#import "HUBComponentViewObserver.h"
#import "HUBComponentWrapper.h"
#import "HUBComponentWrapperImageLoader.h"
#import "HUBComponentCollectionViewCell.h"
#import "HUBUtilities.h"
#import "HUBCollectionViewFactory.h"
#import "HUBCollectionView.h"
#import "HUBCollectionViewLayout.h"
#import "HUBCollectionContainerView.h"
#import "HUBContentReloadPolicy.h"
#import "HUBViewControllerScrollHandler.h"
#import "HUBComponentReusePool.h"
#import "HUBActionContextImplementation.h"
#import "HUBActionHandlerWrapper.h"
#import "HUBViewModelDiff.h"
#import "HUBFeatureInfo.h"
#import "HUBOperation.h"
#import "HUBOperationQueue.h"

NS_ASSUME_NONNULL_BEGIN

@interface HUBViewControllerExperimentalImplementation () <
    HUBViewModelLoaderDelegate,
    HUBComponentWrapperDelegate,
    UICollectionViewDataSource,
    HUBCollectionViewDelegate
>

@property (nonatomic, strong, readonly) id<HUBFeatureInfo> featureInfo;
@property (nonatomic, strong, readonly) id<HUBViewModelLoader> viewModelLoader;
@property (nonatomic, strong, readonly) HUBCollectionViewFactory *collectionViewFactory;
@property (nonatomic, strong, readonly) HUBComponentReusePool *componentReusePool;
@property (nonatomic, strong, readonly) id<HUBActionHandler> actionHandler;
@property (nonatomic, strong, readonly) id<HUBViewControllerScrollHandler> scrollHandler;
@property (nonatomic, strong, nullable, readonly) id<HUBContentReloadPolicy> contentReloadPolicy;
@property (nonatomic, strong, readonly) HUBComponentWrapperImageLoader *componentWrapperImageLoader;
@property (nonatomic, strong, nullable) HUBCollectionView *collectionView;
@property (nonatomic, strong, nullable) id<HUBViewModel> lastRenderedViewModel;
@property (nonatomic, assign) BOOL collectionViewIsScrolling;
@property (nonatomic, strong, readonly) NSHashTable<id<HUBComponentContentOffsetObserver>> *contentOffsetObservingComponentWrappers;
@property (nonatomic, strong, readonly) NSHashTable<id<HUBComponentActionObserver>> *actionObservingComponentWrappers;
@property (nonatomic, strong, nullable) HUBComponentWrapper *headerComponentWrapper;
@property (nonatomic, strong, readonly) NSMutableArray<HUBComponentWrapper *> *overlayComponentWrappers;
@property (nonatomic, strong, readonly) NSMutableDictionary<NSUUID *, HUBComponentWrapper *> *componentWrappersByIdentifier;
@property (nonatomic, strong, readonly) NSMutableDictionary<NSUUID *, HUBComponentWrapper *> *componentWrappersByCellIdentifier;
@property (nonatomic, strong, readonly) NSMutableDictionary<NSString *, HUBComponentWrapper *> *componentWrappersByModelIdentifier;
@property (nonatomic, strong, nullable) HUBComponentWrapper *highlightedComponentWrapper;
@property (nonatomic, strong, readonly) HUBOperationQueue *renderingOperationQueue;
@property (nonatomic, strong, nullable) id<HUBViewModel> viewModel;
@property (nonatomic, assign) BOOL viewHasBeenLaidOut;
@property (nonatomic) BOOL viewModelHasChangedSinceLastLayoutUpdate;
@property (nonatomic) CGFloat visibleKeyboardHeight;
@property (nonatomic, assign) CGPoint lastContentOffset;
@property (nonatomic, copy, nullable) void(^pendingScrollAnimationCallback)(void);

@end

@implementation HUBViewControllerExperimentalImplementation

@synthesize viewModel = _viewModel;
@synthesize viewURI = _viewURI;

#pragma mark - Lifecycle

- (instancetype)initWithViewURI:(NSURL *)viewURI
                    featureInfo:(id<HUBFeatureInfo>)featureInfo
                viewModelLoader:(id<HUBViewModelLoader>)viewModelLoader
          collectionViewFactory:(HUBCollectionViewFactory *)collectionViewFactory
             componentReusePool:(HUBComponentReusePool *)componentReusePool
                  actionHandler:(id<HUBActionHandler>)actionHandler
                  scrollHandler:(id<HUBViewControllerScrollHandler>)scrollHandler
                    imageLoader:(id<HUBImageLoader>)imageLoader
{
    NSParameterAssert(viewURI != nil);
    NSParameterAssert(featureInfo != nil);
    NSParameterAssert(viewModelLoader != nil);
    NSParameterAssert(collectionViewFactory != nil);
    NSParameterAssert(componentReusePool != nil);
    NSParameterAssert(actionHandler != nil);
    NSParameterAssert(scrollHandler != nil);
    NSParameterAssert(imageLoader != nil);

    if (!(self = [super initWithNibName:nil bundle:nil])) {
        return nil;
    }

    _viewURI = [viewURI copy];
    _featureInfo = featureInfo;
    _viewModelLoader = viewModelLoader;
    _collectionViewFactory = collectionViewFactory;
    _componentReusePool = componentReusePool;
    _actionHandler = actionHandler;
    _scrollHandler = scrollHandler;
    _componentWrapperImageLoader = [[HUBComponentWrapperImageLoader alloc] initWithImageLoader:imageLoader];
    _contentOffsetObservingComponentWrappers = [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory];
    _actionObservingComponentWrappers = [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory];
    _overlayComponentWrappers = [NSMutableArray new];
    _componentWrappersByCellIdentifier = [NSMutableDictionary new];
    _componentWrappersByModelIdentifier = [NSMutableDictionary new];
    _renderingOperationQueue = [HUBOperationQueue new];

    viewModelLoader.delegate = self;
    if (HUBConformsToProtocol(viewModelLoader, @protocol(HUBViewModelLoaderWithActions))) {
        ((id<HUBViewModelLoaderWithActions>)viewModelLoader).actionPerformer = self;
    }

    self.automaticallyAdjustsScrollViewInsets = [_scrollHandler shouldAutomaticallyAdjustContentInsetsInViewController:self];

    return self;
}

- (void)dealloc
{
    _collectionView.delegate = nil;
    _collectionView.dataSource = nil;
}

#pragma mark - UIViewController

- (void)loadView
{
    HUBCollectionContainerView * const containerView = [[HUBCollectionContainerView alloc] initWithFrame:CGRectZero];

    HUBCollectionView * const collectionView = [self.collectionViewFactory createCollectionView];
    self.collectionView = collectionView;
    collectionView.showsVerticalScrollIndicator = [self.scrollHandler shouldShowScrollIndicatorsInViewController:self];
    collectionView.showsHorizontalScrollIndicator = collectionView.showsVerticalScrollIndicator;
    collectionView.keyboardDismissMode = [self.scrollHandler keyboardDismissModeForViewController:self];
    collectionView.decelerationRate = [self.scrollHandler scrollDecelerationRateForViewController:self];
    collectionView.bounces = YES;
    collectionView.dataSource = self;
    collectionView.delegate = self;

    self.lastContentOffset = self.collectionView.contentOffset;

    containerView.contentView = self.collectionView;
    self.view = containerView;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    NSNotificationCenter * const notificationCenter = [NSNotificationCenter defaultCenter];

    [notificationCenter addObserver:self
                           selector:@selector(handleKeyboardWillShowNotification:)
                               name:UIKeyboardWillShowNotification
                             object:nil];

    [notificationCenter addObserver:self
                           selector:@selector(handleKeyboardWillHideNotification:)
                               name:UIKeyboardWillHideNotification
                             object:nil];

    if (self.viewModel == nil) {
        self.viewModel = self.viewModelLoader.initialViewModel;
    }

    [self.viewModelLoader loadViewModel];

    for (NSIndexPath * const indexPath in self.collectionView.indexPathsForVisibleItems) {
        HUBComponentCollectionViewCell * const cell = (HUBComponentCollectionViewCell *)[self.collectionView cellForItemAtIndexPath:indexPath];
        [self collectionViewCellWillAppear:cell ignorePreviousAppearance:YES];
    }

    [self headerAndOverlayComponentViewsWillAppear];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];

    NSNotificationCenter * const notificationCenter = [NSNotificationCenter defaultCenter];
    [notificationCenter removeObserver:self name:UIKeyboardWillShowNotification object:nil];
    [notificationCenter removeObserver:self name:UIKeyboardWillHideNotification object:nil];

    self.viewHasBeenLaidOut = NO;
}

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];

    self.viewHasBeenLaidOut = YES;

    if (self.viewModel == nil) {
        return;
    }

    if (self.viewModelHasChangedSinceLastLayoutUpdate || !CGRectEqualToRect(self.collectionView.frame, self.view.bounds)) {
        self.collectionView.frame = self.view.bounds;
        HUBOperation * const reloadOperation = [self createReloadCollectionViewOperation];
        [self.renderingOperationQueue addOperation:reloadOperation];
    }
}

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
{
    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext>  _Nonnull context) {
        [self reconfigureVisibleCellsForSize:size];
        [self.collectionView.collectionViewLayout invalidateLayout];
    } completion:nil];
}

#pragma mark - HUBViewController

- (NSString *)featureIdentifier
{
    return self.featureInfo.identifier;
}

- (NSDictionary<NSIndexPath *, UIView *> *)visibleComponentViewsForComponentType:(HUBComponentType)componentType
{
    NSMutableDictionary<NSIndexPath *, UIView *> * const visibleViewIndexPaths = [NSMutableDictionary new];
    NSMutableArray<HUBComponentWrapper *> * const visibleComponents = [NSMutableArray array];

    for (HUBComponentWrapper * const rootComponentWrapper in [self rootComponentWrappersForComponentType:componentType]) {
        [self addComponentWrapper:rootComponentWrapper toArray:visibleComponents];
    }

    for (HUBComponentWrapper * const visibleComponent in visibleComponents) {
        NSIndexPath * const indexPath = visibleComponent.model.indexPath;
        visibleViewIndexPaths[indexPath] = HUBComponentLoadViewIfNeeded(visibleComponent);
    }

    return [visibleViewIndexPaths copy];
}

- (nullable UIView *)visibleViewForComponentOfType:(HUBComponentType)componentType indexPath:(NSIndexPath *)indexPath
{
    NSUInteger const rootIndex = [indexPath indexAtPosition:0];

    if (rootIndex == NSNotFound) {
        return nil;
    }

    HUBComponentWrapper *componentWrapper;

    switch (componentType) {
        case HUBComponentTypeHeader:
            componentWrapper = self.headerComponentWrapper;
            break;
        case HUBComponentTypeBody: {
            NSIndexPath * const rootIndexPath = [NSIndexPath indexPathForItem:(NSInteger)rootIndex inSection:0];
            HUBComponentCollectionViewCell * const cell = (HUBComponentCollectionViewCell *)[self.collectionView cellForItemAtIndexPath:rootIndexPath];

            if (cell == nil) {
                return nil;
            }

            componentWrapper = [self componentWrapperFromCell:cell];
            break;
        }
        case HUBComponentTypeOverlay: {
            if (rootIndex >= self.overlayComponentWrappers.count) {
                return nil;
            }

            componentWrapper = self.overlayComponentWrappers[rootIndex];
            break;
        }
    }

    if (indexPath.length == 1) {
        return componentWrapper.view;
    }

    for (NSUInteger indexPosition = 1; indexPosition < indexPath.length; indexPosition++) {
        NSArray<HUBComponentWrapper *> * const visibleChildren = componentWrapper.visibleChildren;
        NSUInteger const childIndex = [indexPath indexAtPosition:indexPosition];

        if (childIndex >= visibleChildren.count) {
            return nil;
        }

        componentWrapper = visibleChildren[childIndex];
    }

    return componentWrapper.view;
}

- (NSArray<HUBComponentWrapper *> *)rootComponentWrappersForComponentType:(HUBComponentType)componentType
{
    NSMutableArray<HUBComponentWrapper *> * const rootComponentWrappers = [NSMutableArray array];

    switch (componentType) {
        case HUBComponentTypeHeader: {
            if (self.headerComponentWrapper != nil) {
                HUBComponentWrapper * const headerComponentWrapper = self.headerComponentWrapper;
                [rootComponentWrappers addObject:headerComponentWrapper];
            }
            break;
        }
        case HUBComponentTypeBody: {
            for (HUBComponentCollectionViewCell * const cell in self.collectionView.visibleCells) {
                HUBComponentWrapper * const wrapper = [self componentWrapperFromCell:cell];
                [rootComponentWrappers addObject:wrapper];
            }
            break;
        }
        case HUBComponentTypeOverlay: {
            // All root overlay components are implicitly visible.
            [rootComponentWrappers addObjectsFromArray:self.overlayComponentWrappers];
            break;
        }
    }

    return rootComponentWrappers;
}

- (void)addComponentWrapper:(HUBComponentWrapper *)componentWrapper toArray:(NSMutableArray<HUBComponentWrapper *> *)array
{
    [array addObject:componentWrapper];
    for (HUBComponentWrapper *childComponentWrapper in componentWrapper.visibleChildren) {
        [self addComponentWrapper:childComponentWrapper toArray:array];
    }
}

- (CGRect)frameForBodyComponentAtIndex:(NSUInteger)index
{
    if (index >= (NSUInteger)[self.collectionView numberOfItemsInSection:0]) {
        return CGRectZero;
    }

    NSIndexPath * const indexPath = [NSIndexPath indexPathForItem:(NSInteger)index inSection:0];
    return [self.collectionView.collectionViewLayout layoutAttributesForItemAtIndexPath:indexPath].frame;
}

- (NSUInteger)indexOfBodyComponentAtPoint:(CGPoint)point
{
    point.y += self.collectionView.contentOffset.y;

    NSIndexPath * const indexPath = [self.collectionView indexPathForItemAtPoint:point];

    if (indexPath == nil) {
        return NSNotFound;
    }

    return (NSUInteger)indexPath.item;
}

- (void)scrollToContentOffset:(CGPoint)contentOffset animated:(BOOL)animated
{
    const CGFloat x = contentOffset.x;
    const CGFloat y = contentOffset.y - self.collectionView.contentInset.top;

    [self setContentOffset:CGPointMake(x, y) animated:animated];
}

- (void)scrollToComponentOfType:(HUBComponentType)componentType
                      indexPath:(NSIndexPath *)indexPath
                 scrollPosition:(HUBScrollPosition)scrollPosition
                       animated:(BOOL)animated
                     completion:(void (^ _Nullable)(NSIndexPath *))completion
{
    if (componentType == HUBComponentTypeBody) {
        NSAssert([indexPath indexAtPosition:0] < (NSUInteger)[self.collectionView numberOfItemsInSection:0],
                 @"Root index %@ specified but there are only %@ components in the list.",
                 @([indexPath indexAtPosition:0]), @([self.collectionView numberOfItemsInSection:0]));
    } else if (componentType == HUBComponentTypeHeader) {
        NSAssert(self.headerComponentWrapper != nil, @"Attempted to scroll to component within header, but no header was found.");
    } else if (componentType == HUBComponentTypeOverlay) {
        NSAssert([indexPath indexAtPosition:0] < self.overlayComponentWrappers.count,
                 @"Root index %@ specified but there are only %@ overlays in the list.",
                 @([indexPath indexAtPosition:0]), @(self.overlayComponentWrappers.count));
    }

    [self scrollToRemainingComponentsOfType:componentType
                              startPosition:0
                                  indexPath:indexPath
                                  component:nil
                             scrollPosition:scrollPosition
                                   animated:animated
                                 completion:completion];
}

-(void)reload
{
    [self.viewModelLoader reloadViewModel];
}

#pragma mark - HUBViewModelLoaderDelegate

- (void)viewModelLoader:(id<HUBViewModelLoader>)viewModelLoader didLoadViewModel:(id<HUBViewModel>)viewModel
{
    HUBOperation * const willUpdateDelegateOperation = [HUBOperation synchronousOperationWithBlock:^{
        [self.delegate viewController:self willUpdateWithViewModel:viewModel];
    }];

    HUBOperation * const updateViewModelOperation = [HUBOperation synchronousOperationWithBlock:^{
        HUBCopyNavigationItemProperties(self.navigationItem, viewModel.navigationItem);
        self.viewModel = viewModel;
        self.viewModelHasChangedSinceLastLayoutUpdate = YES;
        [self.view setNeedsLayout];
    }];

    HUBOperation * const reloadCollectionViewOperation = [self createReloadCollectionViewOperation];

    HUBOperation * const didUpdateDelegateOperation = [HUBOperation synchronousOperationWithBlock:^{
        [self.delegate viewControllerDidUpdate:self];
    }];

    [self.renderingOperationQueue addOperations:@[
                                                  willUpdateDelegateOperation,
                                                  updateViewModelOperation,
                                                  reloadCollectionViewOperation,
                                                  didUpdateDelegateOperation
                                                  ]];
}

- (void)viewModelLoader:(id<HUBViewModelLoader>)viewModelLoader didFailLoadingWithError:(NSError *)error
{
    [self.delegate viewController:self didFailToUpdateWithError:error];
}

- (BOOL)selectComponentWithModel:(id<HUBComponentModel>)componentModel customData:(nullable NSDictionary<NSString *, id> *)customData
{
    HUBComponentWrapper * const componentWrapper = self.componentWrappersByModelIdentifier[componentModel.identifier];

    if (componentWrapper != nil) {
        [componentWrapper updateViewForSelectionState:HUBComponentSelectionStateSelected];

        // Deselect after a short time, to enable the user to see the selection for a brief time
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            [componentWrapper updateViewForSelectionState:HUBComponentSelectionStateNone];
        });

        if (componentWrapper == self.highlightedComponentWrapper) {
            self.highlightedComponentWrapper = nil;
        }
    }

    BOOL selectionHandled = NO;

    for (HUBIdentifier * const identifier in componentModel.target.actionIdentifiers) {
        selectionHandled = [self performActionForTrigger:HUBActionTriggerSelection
                                        customIdentifier:identifier
                                              customData:customData
                                          componentModel:componentModel];

        if (selectionHandled) {
            break;
        }
    }

    if (!selectionHandled) {
        selectionHandled = [self performActionForTrigger:HUBActionTriggerSelection
                                        customIdentifier:nil
                                              customData:customData
                                          componentModel:componentModel];
    }

    if (selectionHandled) {
        [self.delegate viewController:self componentSelectedWithModel:componentModel];
    }

    return selectionHandled;
}

- (void)cancelComponentSelection
{
    [self.highlightedComponentWrapper updateViewForSelectionState:HUBComponentSelectionStateNone];
}

#pragma mark - HUBComponentWrapperDelegate

- (void)componentWrapper:(HUBComponentWrapper *)componentWrapper
willUpdateSelectionState:(HUBComponentSelectionState)selectionState
{
    if (selectionState == HUBComponentSelectionStateHighlighted) {
        self.highlightedComponentWrapper = componentWrapper;
    }
}

- (void)componentWrapper:(HUBComponentWrapper *)componentWrapper
 didUpdateSelectionState:(HUBComponentSelectionState)selectionState
{
    switch (selectionState) {
        case HUBComponentSelectionStateNone:
            if (componentWrapper == (HUBComponentWrapper *)self.highlightedComponentWrapper) {
                self.highlightedComponentWrapper = nil;
            }

            break;
        case HUBComponentSelectionStateHighlighted:
            break;
        case HUBComponentSelectionStateSelected:
            [self selectComponentWithModel:componentWrapper.model customData:nil];
            break;
    }
}

- (HUBComponentWrapper *)componentWrapper:(HUBComponentWrapper *)componentWrapper
                   childComponentForModel:(id<HUBComponentModel>)model
{
    CGSize const containerViewSize = [self childComponentContainerViewSizeForParentWrapper:componentWrapper];

    HUBComponentWrapper * const childComponentWrapper = [self.componentReusePool componentWrapperForModel:model
                                                                                                 delegate:self
                                                                                                   parent:componentWrapper];

    UIView * const childComponentView = HUBComponentLoadViewIfNeeded(childComponentWrapper);
    [self configureComponentWrapper:childComponentWrapper withModel:model containerViewSize:containerViewSize];
    [self didAddComponentWrapper:childComponentWrapper];

    CGSize const preferredViewSize = [childComponentWrapper preferredViewSizeForDisplayingModel:model
                                                                              containerViewSize:containerViewSize];

    childComponentView.frame = CGRectMake(0, 0, preferredViewSize.width, preferredViewSize.height);

    [self.componentWrapperImageLoader loadImagesForComponentWrapper:childComponentWrapper containerViewSize:self.view.frame.size];
    [childComponentWrapper viewDidMoveToSuperview:HUBComponentLoadViewIfNeeded(componentWrapper)];

    return childComponentWrapper;
}

- (void)componentWrapper:(HUBComponentWrapper *)componentWrapper
          childComponent:(nullable HUBComponentWrapper *)childComponent
               childView:(UIView *)childView
       willAppearAtIndex:(NSUInteger)childIndex
{
    id<HUBComponentModel> const componentModel = componentWrapper.model;

    if (childIndex >= componentModel.children.count) {
        return;
    }

    if (childComponent != nil) {
        HUBComponentWrapper *nonNilChildComponent = childComponent;
        [self.componentWrapperImageLoader loadImagesForComponentWrapper:nonNilChildComponent containerViewSize:self.view.frame.size];
    }

    id<HUBComponentModel> const childComponentModel = componentModel.children[childIndex];
    NSSet<HUBComponentLayoutTrait> * const layoutTraits = childComponent.layoutTraits ?: [NSSet new];

    [self.delegate viewController:self
               componentWithModel:childComponentModel
                     layoutTraits:layoutTraits
                 willAppearInView:childView];

    [self addComponentWrapperToLookupTables:childComponent];
}

- (void)componentWrapper:(HUBComponentWrapper *)componentWrapper
          childComponent:(nullable HUBComponentWrapper *)childComponent
               childView:(UIView *)childView
     didDisappearAtIndex:(NSUInteger)childIndex
{
    id<HUBComponentModel> const componentModel = componentWrapper.model;

    if (childIndex >= componentModel.children.count) {
        return;
    }

    id<HUBComponentModel> const childComponentModel = componentModel.children[childIndex];
    NSSet<HUBComponentLayoutTrait> * const layoutTraits = childComponent.layoutTraits ?: [NSSet new];

    [self.delegate viewController:self
               componentWithModel:childComponentModel
                     layoutTraits:layoutTraits
             didDisappearFromView:childView];

    [self removeComponentWrapperFromLookupTables:childComponent];
}

- (void)componentWrapper:(HUBComponentWrapper *)componentWrapper
    childSelectedAtIndex:(NSUInteger)childIndex
              customData:(nullable NSDictionary<NSString *, id> *)customData
{
    id<HUBComponentModel> const componentModel = componentWrapper.model;

    if (childIndex >= componentModel.children.count) {
        return;
    }

    id<HUBComponentModel> const childComponentModel = componentModel.children[childIndex];
    [self selectComponentWithModel:childComponentModel customData:customData];
}

- (BOOL)componentWrapper:(HUBComponentWrapper *)componentWrapper performActionWithIdentifier:(HUBIdentifier *)identifier customData:(nullable NSDictionary<NSString *, id> *)customData
{
    return [self performActionForTrigger:HUBActionTriggerComponent
                        customIdentifier:identifier
                              customData:customData
                          componentModel:componentWrapper.model];
}

- (void)sendComponentWrapperToReusePool:(HUBComponentWrapper *)componentWrapper
{
    [self.componentReusePool addComponentWrappper:componentWrapper];

    if (componentWrapper.view) {
        UIView *componentView = componentWrapper.view;
        [self.delegate viewController:self willReuseComponentWithView:componentView];
    }
}

#pragma mark - HUBActionPerformer

- (BOOL)performActionWithIdentifier:(HUBIdentifier *)identifier customData:(nullable NSDictionary<NSString *, id> *)customData
{
    return [self performActionForTrigger:HUBActionTriggerContentOperation
                        customIdentifier:identifier
                              customData:customData
                          componentModel:nil];
}

#pragma mark - UICollectionViewDataSource

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return (NSInteger)self.viewModel.bodyComponentModels.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    id<HUBComponentModel> const componentModel = self.viewModel.bodyComponentModels[(NSUInteger)indexPath.item];
    NSString * const cellReuseIdentifier = componentModel.componentIdentifier.identifierString;

    HUBComponentCollectionViewCell * const cell = [self.collectionView dequeueReusableCellWithReuseIdentifier:cellReuseIdentifier
                                                                                                 forIndexPath:indexPath
                                                                                    cellClassWhenUnregistered:[HUBComponentCollectionViewCell class]];

    HUBComponentWrapper * const componentWrapper = [self.componentReusePool componentWrapperForModel:componentModel
                                                                                            delegate:self
                                                                                              parent:nil];

    self.componentWrappersByCellIdentifier[cell.identifier] = componentWrapper;
    cell.component = componentWrapper;
    [componentWrapper viewDidMoveToSuperview:cell];
    [self didAddComponentWrapper:componentWrapper];

    [self configureComponentWrapper:componentWrapper withModel:componentModel containerViewSize:collectionView.frame.size];

    [self.componentWrapperImageLoader loadImagesForComponentWrapper:componentWrapper
                                                  containerViewSize:self.view.frame.size];

    return cell;
}

- (void)reconfigureVisibleCellsForSize:(CGSize)size
{
    for (HUBComponentCollectionViewCell *visibleCell in [self.collectionView visibleCells]) {
        HUBComponentWrapper * const componentWrapper = self.componentWrappersByCellIdentifier[visibleCell.identifier];
        [componentWrapper reconfigureViewWithContainerViewSize:size];
        [self.componentWrapperImageLoader loadImagesForComponentWrapper:componentWrapper
                                                      containerViewSize:size];
    }
}

#pragma mark - HUBCollectionViewDelegate

- (void)collectionView:(UICollectionView *)collectionView
       willDisplayCell:(UICollectionViewCell *)cell
    forItemAtIndexPath:(NSIndexPath *)indexPath
{
    [self collectionViewCellWillAppear:(HUBComponentCollectionViewCell *)cell
              ignorePreviousAppearance:self.collectionViewIsScrolling];

    HUBComponentWrapper * const componentWrapper = [self componentWrapperFromCell:(HUBComponentCollectionViewCell *)cell];

    [self addComponentWrapperToLookupTables:componentWrapper];
}

- (void)collectionView:(UICollectionView *)collectionView
  didEndDisplayingCell:(UICollectionViewCell *)cell
    forItemAtIndexPath:(NSIndexPath *)indexPath
{
    HUBComponentWrapper * const componentWrapper = [self componentWrapperFromCell:(HUBComponentCollectionViewCell *)cell];

    [self.delegate viewController:self
               componentWithModel:componentWrapper.model
                     layoutTraits:componentWrapper.layoutTraits
             didDisappearFromView:cell];

    [self removeComponentWrapperFromLookupTables:componentWrapper];
}

- (BOOL)collectionViewShouldBeginScrolling:(HUBCollectionView *)collectionView
{
    id<HUBViewControllerDelegate> const delegate = self.delegate;

    if (delegate == nil) {
        return YES;
    }

    return [delegate viewControllerShouldStartScrolling:self];
}

#pragma mark - UIScrollViewDelegate

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    for (HUBComponentWrapper * const componentWrapper in self.contentOffsetObservingComponentWrappers) {
        [componentWrapper updateViewForChangedContentOffset:scrollView.contentOffset];
    }

    [self.highlightedComponentWrapper updateViewForSelectionState:HUBComponentSelectionStateNone];
}

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    CGRect const contentRect = [self contentRectForScrollView:scrollView];
    [self.scrollHandler scrollingWillStartInViewController:self currentContentRect:contentRect];
    self.collectionViewIsScrolling = YES;

    [self.highlightedComponentWrapper updateViewForSelectionState:HUBComponentSelectionStateNone];
}

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                     withVelocity:(CGPoint)velocity
              targetContentOffset:(inout CGPoint *)targetContentOffset
{
    CGVector const velocityVector = CGVectorMake(velocity.x, velocity.y);

    *targetContentOffset = [self.scrollHandler targetContentOffsetForEndedScrollInViewController:self
                                                                                        velocity:velocityVector
                                                                                    contentInset:scrollView.contentInset
                                                                            currentContentOffset:scrollView.contentOffset
                                                                           proposedContentOffset:*targetContentOffset];

    if (targetContentOffset->y >= (scrollView.contentSize.height - CGRectGetHeight(scrollView.frame))) {
        if (!self.viewModelLoader.isLoading) {
            [self.viewModelLoader loadNextPageForCurrentViewModel];
        }
    }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    self.collectionViewIsScrolling = NO;
    [self notifyScrollingDidEndInScrollView:scrollView];
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate) {
        [self notifyScrollingDidEndInScrollView:scrollView];
    }
}

- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
{
    if (self.pendingScrollAnimationCallback) {
        self.pendingScrollAnimationCallback();
        self.pendingScrollAnimationCallback = nil;
    }
}

- (void)notifyScrollingDidEndInScrollView:(UIScrollView *)scrollView
{
    CGRect const contentRect = [self contentRectForScrollView:scrollView];
    [self.scrollHandler scrollingDidEndInViewController:self currentContentRect:contentRect];
}

- (CGRect)contentRectForScrollView:(UIScrollView *)scrollView
{
    CGRect contentRect = CGRectZero;
    contentRect.origin = scrollView.contentOffset;
    contentRect.size = scrollView.frame.size;
    contentRect.size.height = MIN(CGRectGetHeight(contentRect),
                                  scrollView.contentSize.height - CGRectGetMinY(contentRect));
    return contentRect;
}

#pragma mark - UIGestureRecognizerDelegate

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer
{
    return YES;
}

#pragma mark - Notification handling

- (void)handleKeyboardWillShowNotification:(NSNotification *)notification
{
    CGRect const keyboardEndFrame = [notification.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue];
    self.visibleKeyboardHeight = CGRectGetHeight(keyboardEndFrame);
    [self updateOverlayComponentCenterPointsWithKeyboardNotification:notification];
}

- (void)handleKeyboardWillHideNotification:(NSNotification *)notification
{
    self.visibleKeyboardHeight = 0;
    [self updateOverlayComponentCenterPointsWithKeyboardNotification:notification];
}

#pragma mark - Rendering

- (void)renderViewModel:(id<HUBViewModel>)viewModel
               animated:(BOOL)animated
        addHeaderMargin:(BOOL)addHeaderMargin
             completion:(void (^)(void))completionBlock
{
    __weak __typeof(self) weakSelf = self;
    void (^renderBlock)() = ^{
        __strong __typeof(self) strongSelf = weakSelf;
        [strongSelf renderViewModel:viewModel
                    addHeaderMargin:addHeaderMargin
                         completion:completionBlock];
    };

    if (animated) {
        renderBlock();
    } else {
        [UIView performWithoutAnimation:renderBlock];
    }
}

- (void)renderViewModel:(id<HUBViewModel>)viewModel
        addHeaderMargin:(BOOL)addHeaderMargin
             completion:(void (^)(void))completionBlock
{
    HUBViewModelDiff *diff;
    if (self.lastRenderedViewModel != nil) {
        id<HUBViewModel> nonnullViewModel = self.lastRenderedViewModel;
        diff = [HUBViewModelDiff diffFromViewModel:nonnullViewModel toViewModel:viewModel];
    }

    BOOL const hasDiffChanges = (diff == nil || diff.hasChanges);
    UICollectionView *collectionView = self.collectionView;
    HUBCollectionViewLayout * const layout = (HUBCollectionViewLayout *)collectionView.collectionViewLayout;

    if (hasDiffChanges) {
        [collectionView reloadData];
    }

    [layout computeForCollectionViewSize:collectionView.frame.size
                               viewModel:viewModel
                                    diff:diff
                         addHeaderMargin:addHeaderMargin];

    self.lastRenderedViewModel = viewModel;
    completionBlock();
}

#pragma mark - Private utilities

- (HUBOperation *)createReloadCollectionViewOperation
{
    return [HUBOperation asynchronousOperationWithBlock:^(HUBOperationCompletionBlock completionHandler){
        if (!self.viewHasBeenLaidOut) {
            completionHandler();
            return;
        }

        [self saveStatesForVisibleComponents];
        [self configureHeaderComponent];
        [self configureOverlayComponents];
        [self adjustCollectionViewContentInsetWithProposedTopValue:[self calculateTopContentInset]];

        self.viewModelHasChangedSinceLastLayoutUpdate = NO;

        BOOL const shouldAddHeaderMargin = [self shouldAutomaticallyManageTopContentInset];
        id<HUBViewModel> const viewModel = self.viewModel;

        [self renderViewModel:viewModel
                     animated:NO
              addHeaderMargin:shouldAddHeaderMargin
                   completion:^{
                       [self headerAndOverlayComponentViewsWillAppear];
                       [self adjustCollectionViewContentInsetWithProposedTopValue:[self calculateTopContentInset]];
                       [self.delegate viewControllerDidFinishRendering:self];
                       completionHandler();
                   }];
    }];
}

- (void)saveStatesForVisibleComponents
{
    for (HUBComponentCollectionViewCell *cell in self.collectionView.visibleCells) {
        HUBComponentWrapper *wrapper = [self componentWrapperFromCell:cell];
        [wrapper saveComponentUIState];
    }
}

- (void)didAddComponentWrapper:(HUBComponentWrapper *)wrapper
{
    wrapper.delegate = self;
    self.componentWrappersByIdentifier[wrapper.identifier] = wrapper;
}

- (void)configureComponentWrapper:(HUBComponentWrapper *)wrapper withModel:(id<HUBComponentModel>)model containerViewSize:(CGSize)containerViewSize
{
    NSString * const currentModelIdentifier = wrapper.model.identifier;

    if (self.componentWrappersByModelIdentifier[currentModelIdentifier] == wrapper) {
        self.componentWrappersByModelIdentifier[currentModelIdentifier] = nil;
    }

    [wrapper configureViewWithModel:model containerViewSize:containerViewSize];
    self.componentWrappersByModelIdentifier[model.identifier] = wrapper;
}

- (CGSize)childComponentContainerViewSizeForParentWrapper:(HUBComponentWrapper *)parentWrapper
{
    if (parentWrapper.isRootComponent && parentWrapper.model.type == HUBComponentTypeBody) {
        NSIndexPath * const indexPath = [NSIndexPath indexPathForItem:(NSInteger)parentWrapper.model.index inSection:0];
        return [self.collectionView.collectionViewLayout layoutAttributesForItemAtIndexPath:indexPath].frame.size;
    }

    return HUBComponentLoadViewIfNeeded(parentWrapper).frame.size;
}

- (nullable HUBComponentWrapper *)componentWrapperFromCell:(HUBComponentCollectionViewCell *)cell
{
    return self.componentWrappersByCellIdentifier[cell.identifier];
}

- (CGFloat)calculateTopContentInset
{
    if (![self shouldAutomaticallyManageTopContentInset]) {
        return 0;
    }

    if (self.headerComponentWrapper != nil) {
        return 0;
    }

    CGFloat const statusBarWidth = CGRectGetWidth([UIApplication sharedApplication].statusBarFrame);
    CGFloat const statusBarHeight = CGRectGetHeight([UIApplication sharedApplication].statusBarFrame);
    CGFloat const navigationBarWidth = CGRectGetWidth(self.navigationController.navigationBar.frame);
    CGFloat const navigationBarHeight = CGRectGetHeight(self.navigationController.navigationBar.frame);
    CGFloat const topBarHeight = MIN(statusBarWidth, statusBarHeight) + MIN(navigationBarWidth, navigationBarHeight);
    return topBarHeight;
}

- (BOOL)shouldAutomaticallyManageTopContentInset
{
    id<HUBViewControllerDelegate> delegate = self.delegate;

    if (delegate == nil) {
        return YES;
    }

    return [delegate viewControllerShouldAutomaticallyManageTopContentInset:self];
}

- (void)configureHeaderComponent
{
    id<HUBComponentModel> const componentModel = self.viewModel.headerComponentModel;

    if (componentModel == nil) {
        if (self.headerComponentWrapper != nil) {
            HUBComponentWrapper * const headerComponentWrapper = self.headerComponentWrapper;
            [self removeComponentWrapper:headerComponentWrapper];
            self.headerComponentWrapper = nil;
        }

        return;
    }

    self.headerComponentWrapper = [self configureHeaderOrOverlayComponentWrapperWithModel:componentModel
                                                                 previousComponentWrapper:self.headerComponentWrapper];
}

- (void)configureOverlayComponents
{
    NSMutableArray * const currentOverlayComponentWrappers = [self.overlayComponentWrappers mutableCopy];
    [self.overlayComponentWrappers removeAllObjects];

    for (id<HUBComponentModel> const componentModel in self.viewModel.overlayComponentModels) {
        HUBComponentWrapper *componentWrapper = nil;

        if (self.overlayComponentWrappers.count < currentOverlayComponentWrappers.count) {
            NSUInteger const componentIndex = self.overlayComponentWrappers.count;
            componentWrapper = currentOverlayComponentWrappers[componentIndex];
            [currentOverlayComponentWrappers removeObjectAtIndex:componentIndex];
        }

        componentWrapper = [self configureHeaderOrOverlayComponentWrapperWithModel:componentModel
                                                          previousComponentWrapper:componentWrapper];

        [self.overlayComponentWrappers addObject:componentWrapper];

        componentWrapper.view.center = [self overlayComponentCenterPoint];
    }

    for (HUBComponentWrapper * const unusedOverlayComponentWrapper in currentOverlayComponentWrappers) {
        [self removeComponentWrapper:unusedOverlayComponentWrapper];
    }
}

- (CGPoint)overlayComponentCenterPoint
{
    CGRect frame = self.view.bounds;
    frame.origin.y = self.collectionView.contentInset.top;
    frame.size.height -= self.visibleKeyboardHeight + CGRectGetMinY(frame);

    CGPoint proposedCenterPoint = CGPointMake(CGRectGetMidX(frame), CGRectGetMidY(frame));

    id<HUBViewControllerDelegate> delegate = self.delegate;
    if (delegate == nil) {
        return proposedCenterPoint;
    }

    return [delegate centerPointForOverlayComponentInViewController:self
                                                proposedCenterPoint:proposedCenterPoint];
}

- (void)updateOverlayComponentCenterPointsWithKeyboardNotification:(NSNotification *)notification
{
    NSTimeInterval const animationDuration = [notification.userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue];
    UIViewAnimationCurve const animationCurve = [notification.userInfo[UIKeyboardAnimationCurveUserInfoKey] integerValue];

    [UIView beginAnimations:@"com.spotify.hub.keyboard" context:nil];
    [UIView setAnimationDuration:animationDuration];
    [UIView setAnimationCurve:animationCurve];

    for (HUBComponentWrapper * const overlayComponentWrapper in self.overlayComponentWrappers) {
        overlayComponentWrapper.view.center = [self overlayComponentCenterPoint];
    }

    [UIView commitAnimations];
}

- (HUBComponentWrapper *)configureHeaderOrOverlayComponentWrapperWithModel:(id<HUBComponentModel>)componentModel
                                                  previousComponentWrapper:(nullable HUBComponentWrapper *)previousComponentWrapper
{
    if ([previousComponentWrapper.model isEqual:componentModel]) {
        return (HUBComponentWrapper *)previousComponentWrapper;
    }

    BOOL const shouldReuseCurrentComponent = [previousComponentWrapper.model.componentIdentifier isEqual:componentModel.componentIdentifier];
    HUBComponentWrapper *componentWrapper;

    if (shouldReuseCurrentComponent) {
        [previousComponentWrapper prepareViewForReuse];
        componentWrapper = previousComponentWrapper;
    } else {
        if (previousComponentWrapper != nil) {
            HUBComponentWrapper * const nonNilPreviousComponentWrapper = previousComponentWrapper;
            [self removeComponentWrapper:nonNilPreviousComponentWrapper];
        }

        componentWrapper = [self.componentReusePool componentWrapperForModel:componentModel delegate:self parent:nil];
        [self didAddComponentWrapper:componentWrapper];
    }

    CGSize const containerViewSize = self.view.frame.size;
    CGSize const componentViewSize = [componentWrapper preferredViewSizeForDisplayingModel:componentModel
                                                                         containerViewSize:containerViewSize];

    UIView * const componentView = HUBComponentLoadViewIfNeeded(componentWrapper);
    [self configureComponentWrapper:componentWrapper withModel:componentModel containerViewSize:containerViewSize];
    componentView.frame = CGRectMake(0, 0, componentViewSize.width, componentViewSize.height);

    [self.componentWrapperImageLoader loadImagesForComponentWrapper:componentWrapper
                                                  containerViewSize:self.view.frame.size];

    if (!shouldReuseCurrentComponent) {
        [self.view addSubview:componentView];
        [componentWrapper viewDidMoveToSuperview:self.view];
    }

    [self addComponentWrapperToLookupTables:componentWrapper];

    return componentWrapper;
}

- (void)removeComponentWrapper:(HUBComponentWrapper *)wrapper
{
    self.componentWrappersByIdentifier[wrapper.identifier] = nil;
    self.componentWrappersByModelIdentifier[wrapper.model.identifier] = nil;
    [wrapper.view removeFromSuperview];
}

- (void)adjustCollectionViewContentInsetWithProposedTopValue:(CGFloat)topContentInset
{
    UIEdgeInsets contentInsets = self.collectionView.contentInset;
    contentInsets.top = topContentInset;

    contentInsets = [self.scrollHandler contentInsetsForViewController:self
                                                 proposedContentInsets:contentInsets];

    self.collectionView.contentInset = contentInsets;
    self.collectionView.scrollIndicatorInsets = contentInsets;
}

- (void)collectionViewCellWillAppear:(HUBComponentCollectionViewCell *)cell
            ignorePreviousAppearance:(BOOL)ignorePreviousAppearance
{
    HUBComponentWrapper * const wrapper = [self componentWrapperFromCell:cell];

    if (wrapper == nil) {
        return;
    }

    if (wrapper.viewHasAppearedSinceLastModelChange) {
        if (!ignorePreviousAppearance) {
            return;
        }
    }

    [self componentWrapperWillAppear:wrapper];

    id<HUBComponent> const component = cell.component;

    if (component == nil) {
        return;
    }

    UIView * const componentView = HUBComponentLoadViewIfNeeded(component);

    [self.delegate viewController:self
               componentWithModel:wrapper.model
                     layoutTraits:wrapper.layoutTraits
                 willAppearInView:componentView];
}

- (void)headerAndOverlayComponentViewsWillAppear
{
    id<HUBViewControllerDelegate> const delegate = self.delegate;

    if (self.headerComponentWrapper != nil) {
        HUBComponentWrapper * const headerComponentWrapper = self.headerComponentWrapper;
        [self componentWrapperWillAppear:headerComponentWrapper];

        UIView * const componentView = HUBComponentLoadViewIfNeeded(headerComponentWrapper);

        [delegate viewController:self
              componentWithModel:headerComponentWrapper.model
                    layoutTraits:headerComponentWrapper.layoutTraits
                willAppearInView:componentView];
    }

    for (HUBComponentWrapper * const overlayComponentWrapper in self.overlayComponentWrappers) {
        [self componentWrapperWillAppear:overlayComponentWrapper];

        UIView * const componentView = HUBComponentLoadViewIfNeeded(overlayComponentWrapper);

        [delegate viewController:self
              componentWithModel:overlayComponentWrapper.model
                    layoutTraits:overlayComponentWrapper.layoutTraits
                willAppearInView:componentView];
    }
}

- (void)componentWrapperWillAppear:(HUBComponentWrapper *)componentWrapper
{
    [componentWrapper viewWillAppear];

    BOOL wasContentOffsetUpdated = componentWrapper.appearanceCount == 1 ||
    !CGPointEqualToPoint(self.lastContentOffset, self.collectionView.contentOffset);

    if (componentWrapper.isContentOffsetObserver && wasContentOffsetUpdated) {
        [componentWrapper updateViewForChangedContentOffset:self.collectionView.contentOffset];
    }
}

- (BOOL)performActionForTrigger:(HUBActionTrigger)trigger
               customIdentifier:(nullable HUBIdentifier *)customIdentifier
                     customData:(nullable NSDictionary<NSString *, id> *)customData
                 componentModel:(nullable id<HUBComponentModel>)componentModel
{
    if (self.viewModel == nil) {
        return NO;
    }

    id<HUBViewModel> const viewModel = self.viewModel;

    id<HUBActionContext> const context = [[HUBActionContextImplementation alloc] initWithTrigger:trigger
                                                                          customActionIdentifier:customIdentifier
                                                                                      customData:customData
                                                                                     featureInfo:self.featureInfo
                                                                                         viewURI:self.viewURI
                                                                                       viewModel:viewModel
                                                                                  componentModel:componentModel
                                                                                  viewController:self];

    BOOL actionWasHandled = [self.actionHandler handleActionWithContext:context];

    for (HUBComponentWrapper *componentWrapper in self.actionObservingComponentWrappers) {
        id<HUBComponentActionObserver> observer = componentWrapper;
        [observer actionPerformedWithContext:context];
    }

    return actionWasHandled;
}

- (void)addComponentWrapperToLookupTables:(nullable HUBComponentWrapper *)componentWrapper
{
    if (componentWrapper.isContentOffsetObserver) {
        [self.contentOffsetObservingComponentWrappers addObject:componentWrapper];
    }

    if (componentWrapper.isActionObserver) {
        [self.actionObservingComponentWrappers addObject:componentWrapper];
    }
}

- (void)removeComponentWrapperFromLookupTables:(nullable HUBComponentWrapper *)componentWrapper
{
    if (componentWrapper.isContentOffsetObserver) {
        [self.contentOffsetObservingComponentWrappers removeObject:componentWrapper];
    }

    if (componentWrapper.isActionObserver) {
        [self.actionObservingComponentWrappers removeObject:componentWrapper];
    }
}

- (void)scrollToRootBodyComponentAtIndex:(NSUInteger)componentIndex
                          scrollPosition:(HUBScrollPosition)scrollPosition
                                animated:(BOOL)animated
                              completion:(void (^)())completion
{
    NSParameterAssert(componentIndex <= (NSUInteger)[self.collectionView numberOfItemsInSection:0]);

    CGPoint const contentOffset = [self.scrollHandler contentOffsetForDisplayingComponentAtIndex:componentIndex
                                                                                  scrollPosition:scrollPosition
                                                                                    contentInset:self.collectionView.contentInset
                                                                                     contentSize:self.collectionView.contentSize
                                                                                  viewController:self];

    // If the target offset is the same, the completion handler can be called instantly.
    if (CGPointEqualToPoint(contentOffset, self.collectionView.contentOffset)) {
        completion();
        // If the scrolling is animated, the animation has to end before the new component can be retrieved.
    } else if (animated) {
        self.pendingScrollAnimationCallback = completion;
        [self setContentOffset:contentOffset animated:animated];
        // If there's no animations, the UICollectionView will still not update its visible cells until having layouted.
    } else {
        [self setContentOffset:contentOffset animated:animated];
        [self.collectionView setNeedsLayout];
        [self.collectionView layoutIfNeeded];
        completion();
    }
}

- (void)scrollToRemainingComponentsOfType:(HUBComponentType)componentType
                            startPosition:(NSUInteger)startPosition
                                indexPath:(NSIndexPath *)indexPath
                                component:(nullable HUBComponentWrapper *)componentWrapper
                           scrollPosition:(HUBScrollPosition)scrollPosition
                                 animated:(BOOL)animated
                               completion:(void (^ _Nullable)(NSIndexPath *))completionHandler
{
    NSUInteger const childIndex = [indexPath indexAtPosition:startPosition];

    if (startPosition > 0) {
        NSAssert(childIndex < componentWrapper.model.children.count,
                 @"Attempted to scroll to child %@ in component %@, but it only has %@ children",
                 @(childIndex), componentWrapper.model.identifier, @(componentWrapper.model.children.count));
    }

    __weak HUBViewControllerExperimentalImplementation *weakSelf = self;
    void (^stepCompletionHandler)() = ^{
        HUBViewControllerExperimentalImplementation *strongSelf = weakSelf;

        HUBComponentWrapper *childComponentWrapper = nil;
        if (startPosition == 0) {
            if (componentType == HUBComponentTypeBody) {
                NSIndexPath * const rootIndexPath = [NSIndexPath indexPathForItem:(NSInteger)childIndex inSection:0];
                HUBComponentCollectionViewCell * const cell = (HUBComponentCollectionViewCell *)[strongSelf.collectionView cellForItemAtIndexPath:rootIndexPath];
                childComponentWrapper = [strongSelf componentWrapperFromCell:cell];
            } else if (componentType == HUBComponentTypeHeader) {
                childComponentWrapper = strongSelf.headerComponentWrapper;
            } else if (componentType == HUBComponentTypeOverlay) {
                childComponentWrapper = strongSelf.overlayComponentWrappers[startPosition];
            }
        } else {
            childComponentWrapper = [componentWrapper visibleChildComponentAtIndex:childIndex];
        }

        if (completionHandler != nil) {
            NSUInteger const currentIndexPathLength = startPosition + 1;
            NSUInteger currentIndexes[currentIndexPathLength];
            [indexPath getIndexes:currentIndexes range:NSMakeRange(0, currentIndexPathLength)];
            NSIndexPath * const currentIndexPath = [NSIndexPath indexPathWithIndexes:currentIndexes length:currentIndexPathLength];
            completionHandler(currentIndexPath);
        }

        NSUInteger const nextPosition = startPosition + 1;
        if (childComponentWrapper != nil && nextPosition < indexPath.length) {
            [strongSelf scrollToRemainingComponentsOfType:componentType
                                            startPosition:nextPosition
                                                indexPath:indexPath
                                                component:childComponentWrapper
                                           scrollPosition:scrollPosition
                                                 animated:animated
                                               completion:completionHandler];
        }
    };

    // Any other root components than body components don't need to be scrolled to, as they are always visible.
    if (startPosition == 0) {
        if (componentType == HUBComponentTypeBody) {
            [self scrollToRootBodyComponentAtIndex:childIndex
                                    scrollPosition:scrollPosition
                                          animated:animated
                                        completion:stepCompletionHandler];
        } else {
            stepCompletionHandler();
        }
    } else {
        [componentWrapper scrollToComponentAtIndex:childIndex
                                    scrollPosition:scrollPosition
                                          animated:animated
                                        completion:^{
                                            /* This solves a case where the UICollectionView hasn't updated its visible cells until the next cycle
                                             when changing the content offset without animations. */
                                            if (!animated) {
                                                dispatch_async(dispatch_get_main_queue(), stepCompletionHandler);
                                            } else {
                                                stepCompletionHandler();
                                            }
                                        }];
    }
}

- (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated
{
    self.lastContentOffset = contentOffset;
    [self.collectionView setContentOffset:contentOffset animated:animated];
}

@end

NS_ASSUME_NONNULL_END