Only clear image contents on memory warning

Summary:
Some apps are complaining about flashing images when performing navigation transitions. An example issue would be:

1. Load a master list view with many images
2. Click on an image to go to a detail view
3. Go back to the master list view

At step (3), users see a number of images flash from a placeholder image back to the final image because `-[RCTImageView didMoveToWindow]` calls `clearImage` when the image view exits the view hierarchy between (1) and (2) and calls `reloadImage` (which sets the image property asynchronously) when the image view re-enters the view hiearchy between (2) and (3).

This diff fixes the issue by being less aggressive about clearing image contents. It only clears image contents when the app receives a memory warning or the app goes into the background.

For comparison, CKNetworkImageComponent in ComponentKit doesn't have this purging behavior at all.

Reviewed By: javache

Differential Revision: D3325009

fbshipit-source-id: efca10099cdfdb49afbb3f550854d4b8a40511d0

Libraries/Image/RCTImageView.m

@@ -62,10 +62,25 @@ static BOOL RCTShouldReloadImageForSizeChange(CGSize currentSize, CGSize idealSi
 {
   if ((self = [super init])) {
     _bridge = bridge;
+
+    NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
+    [center addObserver:self
+               selector:@selector(clearImageIfDetached)
+                   name:UIApplicationDidReceiveMemoryWarningNotification
+                 object:nil];
+    [center addObserver:self
+               selector:@selector(clearImageIfDetached)
+                   name:UIApplicationDidEnterBackgroundNotification
+                 object:nil];
   }
   return self;
 }
 
+- (void)dealloc
+{
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
 RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
 - (void)updateImage
@@ -172,6 +187,13 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   self.image = nil;
 }
 
+- (void)clearImageIfDetached
+{
+  if (!self.window) {
+    [self clearImage];
+  }
+}
+
 - (void)reloadImage
 {
   [self cancelImageLoad];
@@ -294,24 +316,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 {
   [super didMoveToWindow];
 
-  if (!self.window) {
-    // Don't keep self alive through the asynchronous dispatch, if the intention
-    // was to remove the view so it would deallocate.
-    __weak typeof(self) weakSelf = self;
-
-    dispatch_async(dispatch_get_main_queue(), ^{
-      __strong typeof(self) strongSelf = weakSelf;
-      if (!strongSelf) {
-        return;
-      }
-
-      // If we haven't been re-added to a window by this run loop iteration,
-      // clear out the image to save memory.
-      if (!strongSelf.window) {
-        [strongSelf clearImage];
-      }
-    });
-  } else if (!self.image || self.image == _defaultImage) {
+  if (self.window && (!self.image || self.image == _defaultImage)) {
     [self reloadImage];
   }
 }