Fix retain cycle in basic, cache-less ASNetworkImageNodes

This should not directly affect more complex apps that don't use the ASBasicImageDownloader.

Also disables the default-on text placeholders, as they churn memory during the measurement pass.
These were intended to be written with pure layers (without using backing stores), so I don't
think it is a reasonable default-on behavior until that is fixed.

AsyncDisplayKit/ASNetworkImageNode.h

@@ -23,10 +23,10 @@
 @interface ASNetworkImageNode : ASImageNode
 
 /**
- * The designated initializer.
+ * The designated initializer.  Cache and Downloader are WEAK references.
  *
- * @param cache The object that implements a cache of images for the image node.
+ * @param cache The object that implements a cache of images for the image node.  Weak reference.
- * @param downloader The object that implements image downloading for the image node.  Must not be nil.
+ * @param downloader The object that implements image downloading for the image node.  Must not be nil.  Weak reference.
  *
  * @discussion If `cache` is nil, the receiver will not attempt to retrieve images from a cache before downloading them.
  *

AsyncDisplayKit/ASNetworkImageNode.mm

@@ -16,8 +16,8 @@
 @interface ASNetworkImageNode ()
 {
   ASDN::RecursiveMutex _lock;
-  id<ASImageCacheProtocol> _cache;
+  __weak id<ASImageCacheProtocol> _cache;
-  id<ASImageDownloaderProtocol> _downloader;
+  __weak id<ASImageDownloaderProtocol> _downloader;
 
   // Only access any of these with _lock.
   __weak id<ASNetworkImageNodeDelegate> _delegate;
@@ -51,7 +51,7 @@
 
 - (instancetype)init
 {
-  return [self initWithCache:nil downloader:[[ASBasicImageDownloader alloc] init]];
+  return [self initWithCache:nil downloader:[ASBasicImageDownloader sharedImageDownloader]];
 }
 
 - (void)dealloc

AsyncDisplayKit/ASTextNode.h

@@ -79,6 +79,15 @@ typedef NS_ENUM(NSUInteger, ASTextNodeHighlightStyle) {
 
 #pragma mark - Placeholders
 
+/**
+ * @abstract ASTextNode has a special placeholder behavior when placeholderEnabled is YES.
+ *
+ * @discussion Defaults to NO.  When YES, it draws rectangles for each line of text,
+ * following the true shape of the text's wrapping.  This visually mirrors the overall
+ * shape and weight of paragraphs, making the appearance of the finished text less jarring.
+ */
+@property (nonatomic, assign) BOOL placeholderEnabled;
+
 /**
  @abstract The placeholder color.
  */

AsyncDisplayKit/ASTextNode.mm

@@ -131,7 +131,8 @@ static NSString *ASTextNodeTruncationTokenAttributeName = @"ASTextNodeTruncation
     _constrainedSize = CGSizeMake(-INFINITY, -INFINITY);
 
     // Placeholders
-    self.placeholderEnabled = YES;
+    // Disabled by default in ASDisplayNode, but add a few options for those who toggle
+    // on the special placeholder behavior of ASTextNode.
     _placeholderColor = ASDisplayNodeDefaultPlaceholderColor();
     _placeholderInsets = UIEdgeInsetsMake(1.0, 0.0, 1.0, 0.0);
   }
@@ -747,6 +748,8 @@ static NSString *ASTextNodeTruncationTokenAttributeName = @"ASTextNodeTruncation
 
 - (UIImage *)placeholderImage
 {
+  // FIXME: Replace this implementation with reusable CALayers that have .backgroundColor set.
+  // This would completely eliminate the memory and performance cost of the backing store.
   CGSize size = self.calculatedSize;
   UIGraphicsBeginImageContext(size);
   [self.placeholderColor setFill];

AsyncDisplayKit/Details/ASBasicImageDownloader.h

@@ -14,4 +14,6 @@
  */
 @interface ASBasicImageDownloader : NSObject <ASImageDownloaderProtocol>
 
++ (instancetype)sharedImageDownloader;
+
 @end

AsyncDisplayKit/Details/ASBasicImageDownloader.mm

@@ -200,6 +200,16 @@ static const char *kContextKey = NSStringFromClass(ASBasicImageDownloaderContext
 
 @implementation ASBasicImageDownloader
 
++ (instancetype)sharedImageDownloader
+{
+  static ASBasicImageDownloader *sharedImageDownloader = nil;
+  static dispatch_once_t once = 0;
+  dispatch_once(&once, ^{
+    sharedImageDownloader = [[ASBasicImageDownloader alloc] init];
+  });
+  return sharedImageDownloader;
+}
+
 #pragma mark Lifecycle.
 
 - (instancetype)init

examples/Kittens/Sample/ViewController.m

@@ -166,8 +166,6 @@ static const NSInteger kMaxLitterSize = 100;        // max number of kitten cell
 
 - (void)tableView:(UITableView *)tableView willBeginBatchFetchWithContext:(ASBatchContext *)context
 {
-  NSLog(@"adding kitties");
-    
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     sleep(1);
     dispatch_async(dispatch_get_main_queue(), ^{
@@ -189,8 +187,6 @@ static const NSInteger kMaxLitterSize = 100;        // max number of kitten cell
       [tableView insertRowsAtIndexPaths:indexPaths withRowAnimation:UITableViewRowAnimationFade];
 
       [context completeBatchFetching:YES];
-
-      NSLog(@"kittens added");
     });
   });
 }