Fixes some issues that appeared while moving the rendering logic, and

clarifies the reasoning behind the forcing of layoutSubviews when
reloading.

Author: goos

sources/HUBViewControllerImplementation.m

@@ -226,8 +226,8 @@ - (void)viewDidLayoutSubviews
 
     self.viewHasBeenLaidOut = YES;
 
-    if (self.viewModel != nil && self.viewModelHasChangedSinceLastLayoutUpdate) {
-        if (!CGRectEqualToRect(self.collectionView.frame, self.view.bounds)) {
+    if (self.viewModel != nil) {
+        if (self.viewModelHasChangedSinceLastLayoutUpdate || !CGRectEqualToRect(self.collectionView.frame, self.view.bounds)) {
             self.collectionView.frame = self.view.bounds;
             id<HUBViewModel> const viewModel = self.viewModel;
             [self reloadCollectionViewWithViewModel:viewModel animated:NO];

sources/HUBViewModelRenderer.h

@@ -41,8 +41,8 @@ NS_ASSUME_NONNULL_BEGIN
  * Renders the provided view model in the collection view.
  * 
  * @param viewModel The view model to render.
- * @param usingBatchUpdates @c YES if the renderer should render using batch updates, @c NO otherwise.
- * @param animated @c YES if the renderer should render with animations, @c NO otherwise.
+ * @param usingBatchUpdates Whether the renderer should render using batch updates or not.
+ * @param animated Whether the renderer should render with animations or not.
  * @param completionBlock The block to be called once the rendering is completed.
  */
 - (void)renderViewModel:(id<HUBViewModel>)viewModel

sources/HUBViewModelRenderer.m

@@ -28,7 +28,7 @@
 @interface HUBViewModelRenderer ()
 
 @property (nonatomic, strong, readonly) UICollectionView *collectionView;
-@property (nonatomic, strong) id<HUBViewModel> lastRenderedViewModel;
+@property (nonatomic, strong, nullable) id<HUBViewModel> lastRenderedViewModel;
 
 @end
 
@@ -50,7 +50,8 @@ - (void)renderViewModel:(id<HUBViewModel>)viewModel
 {
     HUBViewModelDiff *diff;
     if (self.lastRenderedViewModel != nil) {
-        diff = [HUBViewModelDiff diffFromViewModel:self.lastRenderedViewModel toViewModel:viewModel];
+        id<HUBViewModel> nonnullViewModel = self.lastRenderedViewModel;
+        diff = [HUBViewModelDiff diffFromViewModel:nonnullViewModel toViewModel:viewModel];
     }
 
     HUBCollectionViewLayout * const layout = (HUBCollectionViewLayout *)self.collectionView.collectionViewLayout;
@@ -60,10 +61,18 @@ - (void)renderViewModel:(id<HUBViewModel>)viewModel
 
         [layout computeForCollectionViewSize:self.collectionView.frame.size viewModel:viewModel diff:diff];
 
-        /* Forcing a re-layout as the reloadData-call doesn't trigger the numberOfItemsInSection:-calls
-         by itself, and batch update calls don't play well without having an initial item count. */
-        [self.collectionView setNeedsLayout];
-        [self.collectionView layoutIfNeeded];
+        /* Below is a workaround for an issue caused by UICollectionView not asking for numberOfItemsInSection
+           before viewDidAppear is called or instantly after a call to reloadData. If reloadData is called
+           after viewDidAppear has been called, followed by a call to performBatchUpdates, UICollectionView will
+           ask for the initial number of items right before the batch updates, and for the new count while inside
+           the update block. This will often trigger an assertion if there are any insertions / deletions, as
+           the data model has already changed before the update. Forcing a layoutSubviews however, manually
+           triggers the numberOfItems call.
+         */
+        if (usingBatchUpdates && diff == nil) {
+            [self.collectionView setNeedsLayout];
+            [self.collectionView layoutIfNeeded];
+        }
         completionBlock();
     } else {
         void (^updateBlock)() = ^{