Add default ASCollectionViewLayoutInspecting for custom ASCollectionViewLayout

AsyncDisplayKit/ASCollectionView.mm

@@ -254,7 +254,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
     // Custom layouts will need to roll their own ASCollectionViewLayoutInspecting implementation and set a layout
     // delegate. In the meantime ASDK provides a null layout inspector that does not provide any implementation
     // and throws an exception for methods that should be implemented in the <ASCollectionViewLayoutInspecting>
-    _defaultLayoutInspector = [[ASCollectionViewNullLayoutInspector alloc] init];
+    _defaultLayoutInspector = [[ASCollectionViewDefaultCustomLayoutInspector alloc] initWithCollectionView:self];
   }
   _layoutInspector = _defaultLayoutInspector;
   

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.h

@@ -60,10 +60,14 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 /**
- * Simple "Null Object" inspector for non-flow layouts that does throws exceptions if methods are called
- * from <ASCollectionViewLayoutInspecting>
+ * A layout inspector for non-flow layouts that returns a constrained size to let the cells layout itself as
+ * far as possible based on the scrollable direction of the collection view. It throws exceptions for delegate
+ * methods that are related to supplementary node's management.
  */
-@interface ASCollectionViewNullLayoutInspector : NSObject <ASCollectionViewLayoutInspecting>
+@interface ASCollectionViewDefaultCustomLayoutInspector : NSObject <ASCollectionViewLayoutInspecting>
+
+- (instancetype)init NS_UNAVAILABLE;
+- (instancetype)initWithCollectionView:(ASCollectionView *)collectionView NS_DESIGNATED_INITIALIZER;
 
 @end
 
@@ -74,7 +78,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property (nonatomic, weak, readonly) UICollectionViewFlowLayout *layout;
 
-- (instancetype)initWithCollectionView:(ASCollectionView *)collectionView flowLayout:(UICollectionViewFlowLayout *)flowLayout;
+- (instancetype)init NS_UNAVAILABLE;
+- (instancetype)initWithCollectionView:(ASCollectionView *)collectionView flowLayout:(UICollectionViewFlowLayout *)flowLayout NS_DESIGNATED_INITIALIZER;
 
 @end
 

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.m

@@ -15,14 +15,57 @@
 
 #define kDefaultItemSize CGSizeMake(50, 50)
 
-#pragma mark - ASCollectionViewNullLayoutInspector
+#pragma mark - Helper Functions
 
-@implementation ASCollectionViewNullLayoutInspector
+// Returns a constrained size to let the cells layout itself as far as possible based on the scrollable direction
+// of the collection view
+static ASSizeRange ASDefaultConstrainedSizeForNodeForCollectionView(ASCollectionView *collectionView) {
+  CGSize maxSize = collectionView.bounds.size;
+  if (ASScrollDirectionContainsHorizontalDirection(collectionView.scrollableDirections)) {
+    maxSize.width = FLT_MAX;
+  } else {
+    maxSize.height = FLT_MAX;
+  }
+  return ASSizeRangeMake(CGSizeZero, maxSize);
+}
+
+#pragma mark - ASCollectionViewDefaultCustomLayoutInspector
+
+@implementation ASCollectionViewDefaultCustomLayoutInspector {
+  struct {
+    unsigned int implementsConstrainedSizeForNodeAtIndexPath:1;
+  } _dataSourceFlags;
+}
+
+#pragma mark Lifecycle
+
+- (instancetype)initWithCollectionView:(ASCollectionView *)collectionView
+{
+  self = [super init];
+  if (self != nil) {
+    [self didChangeCollectionViewDataSource:collectionView.asyncDataSource];
+  }
+  return self;
+}
+
+#pragma mark ASCollectionViewLayoutInspecting
+
+- (void)didChangeCollectionViewDataSource:(id<ASCollectionDataSource>)dataSource
+{
+  if (dataSource == nil) {
+    memset(&_dataSourceFlags, 0, sizeof(_dataSourceFlags));
+  } else {
+    _dataSourceFlags.implementsConstrainedSizeForNodeAtIndexPath = [dataSource respondsToSelector:@selector(collectionView:constrainedSizeForNodeAtIndexPath:)];
+  }
+}
 
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
 {
-  ASDisplayNodeAssert(NO, @"To support a custom collection view layout in ASCollectionView, it must have a layoutInspector for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
-  return ASSizeRangeMake(CGSizeZero, CGSizeZero);
+  if (_dataSourceFlags.implementsConstrainedSizeForNodeAtIndexPath) {
+    return [collectionView.asyncDataSource collectionView:collectionView constrainedSizeForNodeAtIndexPath:indexPath];
+  }
+  
+  return ASDefaultConstrainedSizeForNodeForCollectionView(collectionView);
 }
 
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
@@ -64,7 +107,7 @@
   } _dataSourceFlags;
 }
 
-#pragma mark - Accessors
+#pragma mark Lifecycle
 
 - (instancetype)initWithCollectionView:(ASCollectionView *)collectionView flowLayout:(UICollectionViewFlowLayout *)flowLayout;
 {
@@ -80,6 +123,8 @@
   return self;
 }
 
+#pragma mark ASCollectionViewLayoutInspecting
+
 - (void)didChangeCollectionViewDelegate:(id<ASCollectionDelegate>)delegate;
 {
   if (delegate == nil) {
@@ -100,29 +145,18 @@
   }
 }
 
-#pragma mark - ASCollectionViewLayoutInspecting
-
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
 {
-  // First check if delegate provides a constrained size
   if (_dataSourceFlags.implementsConstrainedSizeForNodeAtIndexPath) {
     return [collectionView.asyncDataSource collectionView:collectionView constrainedSizeForNodeAtIndexPath:indexPath];
   }
   
-  // Check if item size as constrained size is given
   CGSize itemSize = _layout.itemSize;
   if (!CGSizeEqualToSize(itemSize, kDefaultItemSize)) {
     return ASSizeRangeMake(itemSize, itemSize);
   }
   
-  // No constrained size is given try to let the cells layout itself as far as possible based on the scrollable direction
-  CGSize maxSize = collectionView.bounds.size;
-  if (ASScrollDirectionContainsHorizontalDirection([collectionView scrollableDirections])) {
-    maxSize.width = FLT_MAX;
-  } else {
-    maxSize.height = FLT_MAX;
-  }
-  return ASSizeRangeMake(CGSizeZero, maxSize);
+  return ASDefaultConstrainedSizeForNodeForCollectionView(collectionView);
 }
 
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath

AsyncDisplayKitTests/ASCollectionViewTests.m

@@ -124,14 +124,12 @@
   XCTAssert([collectionView.layoutInspector isKindOfClass:[ASCollectionViewFlowLayoutInspector class]], @"should have a flow layout inspector by default");
 }
 
-- (void)testThatItDoesNotSetALayoutInspectorForCustomLayouts
+- (void)testThatISetALayoutInspectorForCustomLayouts
 {
   UICollectionViewLayout *layout = [[UICollectionViewLayout alloc] init];
   ASCollectionView *collectionView = [[ASCollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
-  XCTAssert(collectionView.layoutInspector != nil, @"should automatically set a layout delegate for custom layouts");
-  XCTAssert([collectionView.layoutInspector isKindOfClass:[ASCollectionViewNullLayoutInspector class]], @"should have a null layout inspector by default if no layout inspector is given for a custom layout");
-  XCTAssertThrows([collectionView.layoutInspector collectionView:collectionView constrainedSizeForNodeAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:0]], @"should throw an exception for <ASCollectionViewLayoutInspecting> methods");
-  XCTAssertThrows([collectionView.layoutInspector collectionView:collectionView supplementaryNodesOfKind:UICollectionElementKindSectionHeader inSection:0], @"should throw an exception for <ASCollectionViewLayoutInspecting> methods");
+  XCTAssert(collectionView.layoutInspector != nil, @"should automatically set a layout delegate for flow layouts");
+  XCTAssert([collectionView.layoutInspector isKindOfClass:[ASCollectionViewDefaultCustomLayoutInspector class]], @"should have a flow layout inspector by default");
 }
 
 - (void)testThatRegisteringASupplementaryNodeStoresItForIntrospection