Clarify default value behavior of the layoutDelegate for flow layouts

AsyncDisplayKit/ASCollectionView.mm

@@ -228,7 +228,9 @@ static BOOL _isInterceptedSelector(SEL sel)
   
   // Register the default layout inspector delegate for flow layouts, custom layouts
   // will need to roll their own ASCollectionViewLayoutInspecting implementation.
-  _layoutDelegate = [self flowLayoutInspector];
+  if ([layout asdk_isFlowLayout]) {
+    _layoutDelegate = [self flowLayoutInspector];
+  }
   
   _registeredSupplementaryKinds = [NSMutableArray array];
   
@@ -249,8 +251,6 @@ static BOOL _isInterceptedSelector(SEL sel)
 
 /**
  * A layout inspector implementation specific for the sizing behavior of UICollectionViewFlowLayouts
- *
- * @discussion Will return nil if the current layout is not a flow layout
  */
 - (ASCollectionViewFlowLayoutInspector *)flowLayoutInspector
 {

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.m

@@ -21,11 +21,12 @@
 
 - (instancetype)initWithCollectionView:(ASCollectionView *)collectionView flowLayout:(UICollectionViewFlowLayout *)flowLayout
 {
+    self = [super init];
+
     if (flowLayout == nil) {
         return nil;
     }
-
-    self = [super init];
+    
     if (self != nil) {
         self.collectionView = collectionView;
         _layout = flowLayout;

AsyncDisplayKitTests/ASCollectionViewTests.m

@@ -6,7 +6,8 @@
 //
 
 #import <XCTest/XCTest.h>
-#import <AsyncDisplayKit/ASCollectionView.h>
+#import "ASCollectionView.h"
+#import "ASCollectionViewFlowLayoutInspector.h"
 
 @interface ASCollectionViewTestDelegate : NSObject <ASCollectionViewDataSource, ASCollectionViewDelegate>
 
@@ -79,7 +80,23 @@
 
 @implementation ASCollectionViewTests
 
-- (void)DISABLED_testCollectionViewController {
+- (void)testThatItSetsALayoutInspectorForFlowLayouts
+{
+  UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
+  ASCollectionView *collectionView = [[ASCollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
+  XCTAssert(collectionView.layoutDelegate != nil, @"should automatically set a layout delegate for flow layouts");
+  XCTAssert([collectionView.layoutDelegate isKindOfClass:[ASCollectionViewFlowLayoutInspector class]], @"should have a flow layout inspector by default");
+}
+
+- (void)testThatItDoesNotSetALayoutInspectorForCustomLayouts
+{
+  UICollectionViewLayout *layout = [[UICollectionViewLayout alloc] init];
+  ASCollectionView *collectionView = [[ASCollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
+  XCTAssert(collectionView.layoutDelegate == nil, @"should not set a layout delegate for custom layouts");
+}
+
+- (void)DISABLED_testCollectionViewController
+{
   ASCollectionViewTestController *testController = [[ASCollectionViewTestController alloc] initWithNibName:nil bundle:nil];
 
   UIView *containerView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];