Issue ASNetworkImageNode callbacks off main thread (#908)

* The main thread is under a bunch of contention on startup, let's avoid using it.

* Update CHANGELOG

CHANGELOG.md

@@ -47,6 +47,7 @@
 - Introduces `ASRecursiveUnfairLock` as an experiment to improve locking performance. [Adlai Holler](https://github.com/Adlai-Holler)
 - Adds an experiment to shorten init time. [Adlai Holler](https://github.com/Adlai-Holler)
 - Adds a check that Texture is compiled with stdc++11 as specified by the podfile. gnu++11 can cause subtle issues that are currently being investigated. [Adlai Holler](https://github.com/Adlai-Holler)
+- Adds an experiment to call ASNetworkImageNode callbacks off main. [Garrett Moon](https://github.com/garrettmoon)
 
 ## 2.6
 - [Xcode 9] Updated to require Xcode 9 (to fix warnings) [Garrett Moon](https://github.com/garrettmoon)

Source/ASExperimentalFeatures.h

@@ -25,6 +25,7 @@ typedef NS_OPTIONS(NSUInteger, ASExperimentalFeatures) {
   ASExperimentalInterfaceStateCoalescing = 1 << 2,          // exp_interface_state_coalesce
   ASExperimentalUnfairLock = 1 << 3,                        // exp_unfair_lock
   ASExperimentalLayerDefaults = 1 << 4,                     // exp_infer_layer_defaults
+  ASExperimentalNetworkImageQueue = 1 << 5,                 // exp_network_image_queue
   ASExperimentalFeatureAll = 0xFFFFFFFF
 };
 

Source/ASExperimentalFeatures.m

@@ -18,7 +18,8 @@ NSArray<NSString *> *ASExperimentalFeaturesGetNames(ASExperimentalFeatures flags
                                       @"exp_text_node",
                                       @"exp_interface_state_coalesce",
                                       @"exp_unfair_lock",
-                                      @"exp_infer_layer_defaults"]));
+                                      @"exp_infer_layer_defaults",
+                                      @"exp_network_image_queue"]));
   
   if (flags == ASExperimentalFeatureAll) {
     return allNames;

Source/ASNetworkImageNode.mm

@@ -124,6 +124,20 @@
   [self _cancelImageDownloadWithResumePossibility:NO];
 }
 
+- (dispatch_queue_t)callbackQueue
+{
+    static dispatch_once_t onceToken;
+    static dispatch_queue_t callbackQueue;
+    dispatch_once(&onceToken, ^{
+        if (ASActivateExperimentalFeature(ASExperimentalNetworkImageQueue)) {
+            callbackQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
+        } else {
+            callbackQueue = dispatch_get_main_queue();
+        }
+    });
+    return callbackQueue;
+}
+
 #pragma mark - Public methods -- must lock
 
 /// Setter for public image property. It has the side effect of setting an internal _imageWasSetExternally that prevents setting an image internally. Setting an image internally should happen with the _setImage: method
@@ -455,7 +469,7 @@
   // Unbind from the previous download.
   if (oldDownloadIDForProgressBlock != nil) {
     as_log_verbose(ASImageLoadingLog(), "Disabled progress images for %@ id: %@", self, oldDownloadIDForProgressBlock);
-    [_downloader setProgressImageBlock:nil callbackQueue:dispatch_get_main_queue() withDownloadIdentifier:oldDownloadIDForProgressBlock];
+    [_downloader setProgressImageBlock:nil callbackQueue:[self callbackQueue] withDownloadIdentifier:oldDownloadIDForProgressBlock];
   }
 
   // Bind to the current download.
@@ -464,7 +478,7 @@
     as_log_verbose(ASImageLoadingLog(), "Enabled progress images for %@ id: %@", self, newDownloadIDForProgressBlock);
     [_downloader setProgressImageBlock:^(UIImage * _Nonnull progressImage, CGFloat progress, id  _Nullable downloadIdentifier) {
       [weakSelf handleProgressImage:progressImage progress:progress downloadIdentifier:downloadIdentifier];
-    } callbackQueue:dispatch_get_main_queue() withDownloadIdentifier:newDownloadIDForProgressBlock];
+    } callbackQueue:[self callbackQueue] withDownloadIdentifier:newDownloadIDForProgressBlock];
   }
 
   // Update state local state with lock held.
@@ -484,7 +498,7 @@
     // In this case another thread changed the _downloadIdentifierForProgressBlock before we finished registering
     // the new progress block for newDownloadIDForProgressBlock ID. Let's clear it now and reattempt to register
     if (newDownloadIDForProgressBlock) {
-      [_downloader setProgressImageBlock:nil callbackQueue:dispatch_get_main_queue() withDownloadIdentifier:newDownloadIDForProgressBlock];
+      [_downloader setProgressImageBlock:nil callbackQueue:[self callbackQueue] withDownloadIdentifier:newDownloadIDForProgressBlock];
     }
     [self _updateProgressImageBlockOnDownloaderIfNeeded];
   }
@@ -556,7 +570,7 @@
 
 
     downloadIdentifier = [_downloader downloadImageWithURL:url
-                                             callbackQueue:dispatch_get_main_queue()
+                                             callbackQueue:[self callbackQueue]
                                           downloadProgress:NULL
                                                 completion:^(id <ASImageContainerProtocol> _Nullable imageContainer, NSError * _Nullable error, id  _Nullable downloadIdentifier, id _Nullable userInfo) {
                                                   if (finished != NULL) {
@@ -757,7 +771,7 @@
           }
         };
         [_cache cachedImageWithURL:URL
-                     callbackQueue:dispatch_get_main_queue()
+                     callbackQueue:[self callbackQueue]
                         completion:completion];
       } else {
         [self _downloadImageWithCompletion:^(id<ASImageContainerProtocol> imageContainer, NSError *error, id downloadIdentifier, id userInfo) {