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
71bf8a3e48