Add a "Null Object" for a layout inspector if no no flow layout is given

The purpose of the ASCollectionViewCustomLayoutInspector is to provide a default implementation if given a custom layout, but no layout inspector.

Author: maicki

AsyncDisplayKit/ASCollectionView.mm

@@ -98,7 +98,7 @@ @interface ASCollectionView () <ASRangeControllerDataSource, ASRangeControllerDe
   ASCollectionDataController *_dataController;
   ASRangeController *_rangeController;
   ASCollectionViewLayoutController *_layoutController;
-  ASCollectionViewFlowLayoutInspector *_flowLayoutInspector;
+  id<ASCollectionViewLayoutInspecting> _defaultLayoutInspector;
   NSMutableSet *_cellsForVisibilityUpdates;
   id<ASCollectionViewLayoutFacilitatorProtocol> _layoutFacilitator;
 
@@ -244,11 +244,19 @@ - (instancetype)_initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionV
   // and should not trigger a relayout.
   _ignoreMaxSizeChange = CGSizeEqualToSize(_maxSizeForNodesConstrainedSize, CGSizeZero);
 
-  // Register the default layout inspector delegate for flow layouts only, custom layouts
-  // will need to roll their own ASCollectionViewLayoutInspecting implementation and set a layout delegate
   if ([layout asdk_isFlowLayout]) {
-    _layoutInspector = [self flowLayoutInspector];
+    // Register the default layout inspector delegate for flow layouts only
+    UICollectionViewFlowLayout *layout = (UICollectionViewFlowLayout *)self.collectionViewLayout;
+    ASDisplayNodeAssertNotNil(layout, @"Collection view layout must be a flow layout to use the built-in inspector");
+    _defaultLayoutInspector = [[ASCollectionViewFlowLayoutInspector alloc] initWithCollectionView:self flowLayout:layout];
+  } else {
+    // Custom layouts will need to roll their own ASCollectionViewLayoutInspecting implementation and set a layout
+    // delegate. In the meantime ASDK provides a custom layout inspector that does not provide any implementation
+    // and asserts for methods that should be implemented in the <ASCollectionViewLayoutInspecting>
+    _defaultLayoutInspector = [[ASCollectionViewCustomLayoutInspector alloc] init];
   }
+  _layoutInspector = _defaultLayoutInspector;
+  
   _layoutFacilitator = layoutFacilitator;
 
   _proxyDelegate = [[ASCollectionViewProxy alloc] initWithTarget:nil interceptor:self];
@@ -275,19 +283,6 @@ - (void)dealloc
   [self setAsyncDataSource:nil];
 }
 
-/**
- * A layout inspector implementation specific for the sizing behavior of UICollectionViewFlowLayouts
- */
-- (ASCollectionViewFlowLayoutInspector *)flowLayoutInspector
-{
-  if (_flowLayoutInspector == nil) {
-    UICollectionViewFlowLayout *layout = (UICollectionViewFlowLayout *)self.collectionViewLayout;
-    ASDisplayNodeAssertNotNil(layout, @"Collection view layout must be a flow layout to use the built-in inspector");
-    _flowLayoutInspector = [[ASCollectionViewFlowLayoutInspector alloc] initWithCollectionView:self flowLayout:layout];
-  }
-  return _flowLayoutInspector;
-}
-
 #pragma mark -
 #pragma mark Overrides.
 
@@ -405,7 +400,9 @@ - (void)setAsyncDelegate:(id<ASCollectionViewDelegate>)asyncDelegate
 
   super.delegate = (id<UICollectionViewDelegate>)_proxyDelegate;
 
-  [_layoutInspector didChangeCollectionViewDelegate:asyncDelegate];
+  if ([_layoutInspector respondsToSelector:@selector(didChangeCollectionViewDelegate:)]) {
+    [_layoutInspector didChangeCollectionViewDelegate:asyncDelegate];
+  }
 }
 
 - (void)setTuningParameters:(ASRangeTuningParameters)tuningParameters forRangeType:(ASLayoutRangeType)rangeType
@@ -864,28 +861,21 @@ - (ASCellNodeBlock)dataController:(ASDataController *)dataController nodeBlockAt
 
 - (ASSizeRange)dataController:(ASDataController *)dataController constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
 {
-  ASSizeRange constrainedSize = kInvalidSizeRange;
-  if (_layoutInspector) {
-    constrainedSize = [_layoutInspector collectionView:self constrainedSizeForNodeAtIndexPath:indexPath];
-  }
-  
+  ASSizeRange constrainedSize = [_layoutInspector collectionView:self constrainedSizeForNodeAtIndexPath:indexPath];
   if (!ASSizeRangeEqualToSizeRange(constrainedSize, kInvalidSizeRange)) {
     return constrainedSize;
   }
 
-  // TODO: Move this logic into the flow layout inspector. Create a simple inspector for non-flow layouts that don't
-  // implement a custom inspector.
-  if (_asyncDataSourceFlags.asyncDataSourceConstrainedSizeForNode) {
-    constrainedSize = [_asyncDataSource collectionView:self constrainedSizeForNodeAtIndexPath:indexPath];
+  CGSize maxSize = _maxSizeForNodesConstrainedSize;
+  if (CGSizeEqualToSize(_maxSizeForNodesConstrainedSize, CGSizeZero)) {
+    maxSize = self.bounds.size;
+  }
+  if (ASScrollDirectionContainsHorizontalDirection([self scrollableDirections])) {
+    maxSize.width = FLT_MAX;
   } else {
-    CGSize maxSize = CGSizeEqualToSize(_maxSizeForNodesConstrainedSize, CGSizeZero) ? self.bounds.size : _maxSizeForNodesConstrainedSize;
-    if (ASScrollDirectionContainsHorizontalDirection([self scrollableDirections])) {
-      maxSize.width = FLT_MAX;
-    } else {
-      maxSize.height = FLT_MAX;
-    }
-    constrainedSize = ASSizeRangeMake(CGSizeZero, maxSize);
+    maxSize.height = FLT_MAX;
   }
+  constrainedSize = ASSizeRangeMake(CGSizeZero, maxSize);
 
   return constrainedSize;
 }
@@ -947,19 +937,16 @@ - (NSArray *)supplementaryNodeKindsInDataController:(ASCollectionDataController
 
 - (ASSizeRange)dataController:(ASCollectionDataController *)dataController constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
 {
-  ASDisplayNodeAssert(_layoutInspector != nil, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
   return [_layoutInspector collectionView:self constrainedSizeForSupplementaryNodeOfKind:kind atIndexPath:indexPath];
 }
 
 - (NSUInteger)dataController:(ASCollectionDataController *)dataController supplementaryNodesOfKind:(NSString *)kind inSection:(NSUInteger)section
 {
-  ASDisplayNodeAssert(_layoutInspector != nil, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
   return [_layoutInspector collectionView:self supplementaryNodesOfKind:kind inSection:section];
 }
 
 - (NSUInteger)dataController:(ASCollectionDataController *)dataController numberOfSectionsForSupplementaryNodeOfKind:(NSString *)kind;
 {
-  ASDisplayNodeAssert(_layoutInspector != nil, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
   return [_layoutInspector collectionView:self numberOfSectionsForSupplementaryNodeOfKind:kind];
 }
 

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.h

@@ -47,6 +47,17 @@
 
 @end
 
+/**
+ * Simple "Null Object" inspector for non-flow layouts that does not implement a custom inspector, provides a zero
+ * constrained size and throws an exception if methods are called from <ASCollectionViewLayoutInspecting>
+ */
+@interface ASCollectionViewCustomLayoutInspector : NSObject <ASCollectionViewLayoutInspecting>
+
+@end
+
+/**
+ * A layout inspector implementation specific for the sizing behavior of UICollectionViewFlowLayouts
+ */
 @interface ASCollectionViewFlowLayoutInspector : NSObject <ASCollectionViewLayoutInspecting>
 
 @property (nonatomic, weak) UICollectionViewFlowLayout *layout;

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.m

@@ -13,6 +13,35 @@
 #import "ASAssert.h"
 #import "ASEqualityHelpers.h"
 
+
+
+@implementation ASCollectionViewCustomLayoutInspector
+
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
+{
+  return ASSizeRangeMake(CGSizeZero, CGSizeZero);
+}
+
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
+{
+  ASDisplayNodeAssert(NO, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
+  return ASSizeRangeMake(CGSizeZero, CGSizeZero);
+}
+
+- (NSUInteger)collectionView:(ASCollectionView *)collectionView numberOfSectionsForSupplementaryNodeOfKind:(NSString *)kind
+{
+  ASDisplayNodeAssert(NO, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
+  return 0;
+}
+
+- (NSUInteger)collectionView:(ASCollectionView *)collectionView supplementaryNodesOfKind:(NSString *)kind inSection:(NSUInteger)section
+{
+  ASDisplayNodeAssert(NO, @"To support supplementary nodes in ASCollectionView, it must have a layoutDelegate for layout inspection. (See ASCollectionViewFlowLayoutInspector for an example.)");
+  return 0;
+}
+
+@end
+ 
 @implementation ASCollectionViewFlowLayoutInspector {
   BOOL _delegateImplementsReferenceSizeForHeader;
   BOOL _delegateImplementsReferenceSizeForFooter;
@@ -50,8 +79,11 @@ - (void)didChangeCollectionViewDelegate:(id<ASCollectionDelegate>)delegate;
 
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
 {
-  // TODO: Provide constrained size for flow layout item nodes
-  return ASSizeRangeMake(CGSizeZero, CGSizeZero);
+  if ([collectionView.asyncDataSource respondsToSelector:@selector(collectionView:constrainedSizeForNodeAtIndexPath:)]) {
+    return [collectionView.asyncDataSource collectionView:collectionView constrainedSizeForNodeAtIndexPath:indexPath];
+  } else {
+    return ASSizeRangeMake(_layout.itemSize, _layout.itemSize);
+  }
 }
 
 - (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath

AsyncDisplayKitTests/ASCollectionViewTests.m

@@ -108,7 +108,10 @@ - (void)testThatItDoesNotSetALayoutInspectorForCustomLayouts
 {
   UICollectionViewLayout *layout = [[UICollectionViewLayout alloc] init];
   ASCollectionView *collectionView = [[ASCollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
-  XCTAssert(collectionView.layoutInspector == nil, @"should not set a layout delegate for custom layouts");
+  XCTAssert(collectionView.layoutInspector != nil, @"should automatically set a layout delegate for custom layouts");
+  XCTAssert([collectionView.layoutInspector isKindOfClass:[ASCollectionViewCustomLayoutInspector class]], @"should have a internal custom layout inspector by default");
+  XCTAssert(ASSizeRangeEqualToSizeRange([collectionView.layoutInspector collectionView:collectionView constrainedSizeForNodeAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:0]], ASSizeRangeMake(CGSizeZero, CGSizeZero)), @"should return a zero constrainted size range in internal custom layout inspector");
+  XCTAssertThrows([collectionView.layoutInspector collectionView:collectionView supplementaryNodesOfKind:UICollectionElementKindSectionHeader inSection:0], @"should throw an exception for <ASCollectionViewLayoutInspecting> methods");
 }
 
 - (void)testThatRegisteringASupplementaryNodeStoresItForIntrospection