实现异步，无非三种：用线程(比如GCD)、Runloop、CADisplayLink

- (void)display {
     dispatch_async(backgroundQueue, ^{
         CGContextRef ctx = CGBitmapContextCreate(...);
         // draw in context...
         CGImageRef img = CGBitmapContextCreateImage(ctx);
         CFRelease(ctx);
         dispatch_async(mainQueue, ^{
             layer.contents = img;
         });
     });
  }

Q:

• AsyncDisplayKit异步加载的原理
• YYAsyncLayer异步加载的原理
• SDWebImage如何做到异步加载的
  用NSCache做内存缓存
• AFNetworking的UIImageView如何做到异步加载的
• YTKNetwork