Background image load api

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## master
 * Add your own contributions to the next release on the line below this with your name.
+- [ASNetworkImageNode] Allow delegate methods to be called on background thread. [Max Wang](https://github.com/wsdwsd0829). [#1007](https://github.com/TextureGroup/Texture/pull/1007)
 - [ASLayoutTransition] Add support for preserving order after node moves during transitions. (This order defines the z-order as well.) [Kevin Smith](https://github.com/wiseoldduck) [#1006]
 - [ASDisplayNode] Adds support for multiple interface state delegates. [Garrett Moon](https://github.com/garrettmoon) [#979](https://github.com/TextureGroup/Texture/pull/979)
 - [ASDataController] Add capability to renew supplementary views (update map) when size change from zero to non-zero.[Max Wang](https://github.com/wsdwsd0829) [#842](https://github.com/TextureGroup/Texture/pull/842)

Source/ASNetworkImageNode.h

@@ -56,6 +56,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nullable, weak) id<ASNetworkImageNodeDelegate> delegate;
 
+/**
+ * The delegate will receive callbacks on main thread. Default to YES.
+ */
+@property (class) BOOL useMainThreadDelegateCallbacks;
+
 /**
  * The image to display.
  *
@@ -156,7 +161,7 @@ NS_ASSUME_NONNULL_BEGIN
  * @param image The newly-loaded image.
  * @param info Additional information about the image load.
  *
- * @discussion Called on a background queue.
+ * @discussion Called on the main thread if useMainThreadDelegateCallbacks=YES (the default), otherwise on a background thread.
  */
 - (void)imageNode:(ASNetworkImageNode *)imageNode didLoadImage:(UIImage *)image info:(ASNetworkImageLoadInfo *)info;
 
@@ -166,7 +171,7 @@ NS_ASSUME_NONNULL_BEGIN
  * @param imageNode The sender.
  * @param image The newly-loaded image.
  *
- * @discussion Called on a background queue.
+ * @discussion Called on the main thread if useMainThreadDelegateCallbacks=YES (the default), otherwise on a background thread.
  */
 - (void)imageNode:(ASNetworkImageNode *)imageNode didLoadImage:(UIImage *)image;
 
@@ -185,7 +190,7 @@ NS_ASSUME_NONNULL_BEGIN
  * @param imageNode The sender.
  * @param error The error with details.
  *
- * @discussion Called on a background queue.
+ * @discussion Called on the main thread if useMainThreadDelegateCallbacks=YES (the default), otherwise on a background thread.
  */
 - (void)imageNode:(ASNetworkImageNode *)imageNode didFailWithError:(NSError *)error;
 

Source/ASNetworkImageNode.mm

@@ -86,6 +86,8 @@ @interface ASNetworkImageNode ()
 
 @implementation ASNetworkImageNode
 
+static std::atomic_bool _useMainThreadDelegateCallbacks(true);
+
 @dynamic image;
 
 - (instancetype)initWithCache:(id<ASImageCacheProtocol>)cache downloader:(id<ASImageDownloaderProtocol>)downloader
@@ -430,6 +432,16 @@ - (void)didEnterPreloadState
   [self _lazilyLoadImageIfNecessary];
 }
 
++ (void)setUseMainThreadDelegateCallbacks:(BOOL)useMainThreadDelegateCallbacks
+{
+  _useMainThreadDelegateCallbacks = useMainThreadDelegateCallbacks;
+}
+
++ (BOOL)useMainThreadDelegateCallbacks
+{
+  return _useMainThreadDelegateCallbacks;
+}
+
 #pragma mark - Progress
 
 - (void)handleProgressImage:(UIImage *)progressImage progress:(CGFloat)progress downloadIdentifier:(nullable id)downloadIdentifier
@@ -743,11 +755,15 @@ - (void)_lazilyLoadImageIfNecessary
           }
 
           if (calloutBlock) {
-            ASPerformBlockOnMainThread(^{
-              if (let strongSelf = weakSelf) {
-                calloutBlock(strongSelf);
-              }
-            });
+            if (ASNetworkImageNode.useMainThreadDelegateCallbacks) {
+              ASPerformBlockOnMainThread(^{
+                if (auto strongSelf = weakSelf) {
+                  calloutBlock(strongSelf);
+                }
+              });
+            } else {
+              calloutBlock(self);
+            }
           }
         });
       };