Customized post insertion animation

Summary:
The current animation is the collection default animation - fade in/out: demo: https://pxl.cl/bj8R
Based on our design, we want the Shelf to be push down: demo https://pxl.cl/bj9R

1. Created **IGListCollectionViewDelegate** which inherits **UICollectionViewDelegateFlowLayout**
2. **IGListAdapter ** conforms **UICollectionViewDelegateFlowLayout**
3. add **transitionDelegate** to IGListSectionController
4. **IGFeedSectionController** sets transitionDelegate as itself and handles IGListCollectionViewDelegate methods

Reviewed By: rnystrom

Differential Revision: D6785726

fbshipit-source-id: bdf19f84fef05264ca0e082c6a326a31494a20da

Source/IGListCollectionViewDelegateLayout.h

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+/**
+ Conform to `IGListCollectionViewDelegateLayout` to provide customized layout information for a collection view.
+ */
+@protocol IGListCollectionViewDelegateLayout <UICollectionViewDelegateFlowLayout>
+
+/**
+ Asks the delegate to customize and return the starting layout information for an item being inserted into the collection view.
+
+ @param collectionView The collection view to perform the transition on.
+ @param collectionViewLayout The layout to use with the collection view.
+ @param attributes The starting layout information for an item being inserted into the collection view.
+ @param indexPath The index path of the item being inserted.
+ */
+- (UICollectionViewLayoutAttributes *)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout customizedInitialLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes atIndexPath:(NSIndexPath *)indexPath;
+
+/**
+ Asks the delegate to customize and return the final layout information for an item that is about to be removed from the collection view.
+
+ @param collectionView The collection view to perform the transition on.
+ @param collectionViewLayout The layout to use with the collection view.
+ @param attributes The final layout information for an item that is about to be removed from the collection view.
+ @param indexPath The index path of the item being deleted.
+ */
+- (UICollectionViewLayoutAttributes *)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout customizedFinalLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes atIndexPath:(NSIndexPath *)indexPath;
+
+@end
+

Source/IGListCollectionViewLayout.mm

@@ -14,6 +14,7 @@
 #import <vector>
 
 #import <IGListKit/IGListAssert.h>
+#import <IGListKit/IGListCollectionViewDelegateLayout.h>
 
 static CGFloat UIEdgeInsetsLeadingInsetInDirection(UIEdgeInsets insets, UICollectionViewScrollDirection direction) {
     switch (direction) {
@@ -206,6 +207,30 @@ static void adjustZIndexForAttributes(UICollectionViewLayoutAttributes *attribut
 
 #pragma mark - UICollectionViewLayout
 
+- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath {
+    UICollectionViewLayoutAttributes *attributes = [super initialLayoutAttributesForAppearingItemAtIndexPath:itemIndexPath];
+    id<IGListCollectionViewDelegateLayout> delegate = (id<IGListCollectionViewDelegateLayout>)self.collectionView.delegate;
+    if ([delegate respondsToSelector:@selector(collectionView:layout:customizedInitialLayoutAttributes:atIndexPath:)]) {
+        return [delegate collectionView:self.collectionView
+                                 layout:self
+      customizedInitialLayoutAttributes:attributes
+                            atIndexPath:itemIndexPath];
+    }
+    return attributes;
+}
+
+- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath{
+    UICollectionViewLayoutAttributes *attributes = [super finalLayoutAttributesForDisappearingItemAtIndexPath:itemIndexPath];
+    id<IGListCollectionViewDelegateLayout> delegate = (id<IGListCollectionViewDelegateLayout>)self.collectionView.delegate;
+    if ([delegate respondsToSelector:@selector(collectionView:layout:customizedFinalLayoutAttributes:atIndexPath:)]) {
+        return [delegate collectionView:self.collectionView
+                                 layout:self
+        customizedFinalLayoutAttributes:attributes
+                            atIndexPath:itemIndexPath];
+    }
+    return attributes;
+}
+
 - (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
     IGAssertMainThread();
 

Source/IGListSectionController.h

@@ -14,6 +14,7 @@
 #import <IGListKit/IGListScrollDelegate.h>
 #import <IGListKit/IGListSupplementaryViewSource.h>
 #import <IGListKit/IGListWorkingRangeDelegate.h>
+#import <IGListKit/IGListTransitionDelegate.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -200,6 +201,15 @@ NS_SWIFT_NAME(ListSectionController)
  */
 @property (nonatomic, weak, nullable) id <IGListScrollDelegate> scrollDelegate;
 
+/**
+ An object that handles transition events for the section controller. Can be `nil`.
+
+ @return An object that conforms to `IGListTransitionDelegat` or `nil`.
+
+ @note You may wish to return `self` if your section controller implements this protocol.
+ */
+@property (nonatomic, weak, nullable) id<IGListTransitionDelegate> transitionDelegate;
+
 @end
 
 NS_ASSUME_NONNULL_END

Source/IGListTransitionDelegate.h

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+/**
+ Conform to `IGListTransitionDelegate` to provide customized layout information for a collection view.
+ */
+@protocol IGListTransitionDelegate
+
+/**
+ Asks the delegate to customize and return the starting layout information for an item being inserted into the collection view.
+
+ @param listAdapter The adapter controlling the list.
+ @param attributes The starting layout information for an item being inserted into the collection view.
+ @param sectionController The section controller to perform the transition on.
+ @param index The index of the item being inserted.
+ */
+- (UICollectionViewLayoutAttributes *)listAdapter:(IGListAdapter *)listAdapter customizedInitialLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes sectionController:(IGListSectionController *)sectionController atIndex:(NSInteger)index;
+
+/**
+ Asks the delegate to customize and return the final layout information for an item that is about to be removed from the collection view.
+
+ @param listAdapter The adapter controlling the list.
+ @param attributes The final layout information for an item that is about to be removed from the collection view.
+ @param sectionController The section controller to perform the transition on.
+ @param index The index of the item being deleted.
+ */
+- (UICollectionViewLayoutAttributes *)listAdapter:(IGListAdapter *)listAdapter customizedFinalLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes sectionController:(IGListSectionController *)sectionController atIndex:(NSInteger)index;
+
+@end
+

Source/Internal/IGListAdapter+UICollectionView.h

@@ -10,10 +10,11 @@
 #import <UIKit/UIKit.h>
 
 #import <IGListKit/IGListAdapter.h>
+#import <IGListKit/IGListCollectionViewDelegateLayout.h>
 
 @interface IGListAdapter (UICollectionView)
 <
 UICollectionViewDataSource,
-UICollectionViewDelegateFlowLayout
+IGListCollectionViewDelegateLayout
 >
 @end

Source/Internal/IGListAdapter+UICollectionView.m

@@ -211,4 +211,34 @@
     return [self sizeForSupplementaryViewOfKind:UICollectionElementKindSectionFooter atIndexPath:indexPath];
 }
 
+#pragma mark - IGListCollectionViewDelegateLayout
+
+- (UICollectionViewLayoutAttributes *)collectionView:(UICollectionView *)collectionView
+                                              layout:(UICollectionViewLayout*)collectionViewLayout
+                   customizedInitialLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes
+                                         atIndexPath:(NSIndexPath *)indexPath {
+    IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
+    if (sectionController.transitionDelegate) {
+        return [sectionController.transitionDelegate listAdapter:self
+                               customizedInitialLayoutAttributes:attributes
+                                               sectionController:sectionController
+                                                         atIndex:indexPath.item];
+    }
+    return attributes;
+}
+
+- (UICollectionViewLayoutAttributes *)collectionView:(UICollectionView *)collectionView
+                                              layout:(UICollectionViewLayout*)collectionViewLayout
+                     customizedFinalLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes
+                                         atIndexPath:(NSIndexPath *)indexPath {
+    IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
+    if (sectionController.transitionDelegate) {
+        return [sectionController.transitionDelegate listAdapter:self
+                                 customizedFinalLayoutAttributes:attributes
+                                               sectionController:sectionController
+                                                         atIndex:indexPath.item];
+    }
+    return attributes;
+}
+
 @end

Source/Internal/IGListAdapterProxy.m

@@ -10,6 +10,7 @@
 #import "IGListAdapterProxy.h"
 
 #import <IGListKit/IGListAssert.h>
+#import "IGListCollectionViewDelegateLayout.h"
 
 /**
  Define messages that you want the IGListAdapter object to intercept. Pattern copied from
@@ -34,7 +35,10 @@ static BOOL isInterceptedSelector(SEL sel) {
             sel == @selector(scrollViewDidScroll:) ||
             sel == @selector(scrollViewWillBeginDragging:) ||
             sel == @selector(scrollViewDidEndDragging:willDecelerate:) ||
-            sel == @selector(scrollViewDidEndDecelerating:)
+            sel == @selector(scrollViewDidEndDecelerating:) ||
+            // IGListCollectionViewDelegateLayout
+            sel == @selector(collectionView:layout:customizedInitialLayoutAttributes:atIndexPath:) ||
+            sel == @selector(collectionView:layout:customizedFinalLayoutAttributes:atIndexPath:)
             );
 }