是否存在UIImage完成加载的事件

我正在使用以下代码显示UIImage视图:

NSURL *imageUrl = [[NSURL alloc]initWithString:@"http://..."];
NSData *imageData = [[NSData alloc] initWithContentsOfURL:imageUrl];
UIImage *audioArt =[[UIImage alloc] initWithData:imageData];
UIImageView *artView =[[UIImageView alloc] initWithImage:audioArt];

artView.contentMode = UIViewContentModeScaleAspectFit;
artView.autoresizesSubviews = NO;
artView.frame = viewRef.bounds;
[artView setAutoresizingMask:UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight];
[artView setBackgroundColor:[UIColor blackColor]];

[viewRef setBackgroundColor:[UIColor blackColor]];
[viewRef addSubview:artView];

在Objective C中是否有一个事件告诉UIImage何时完成加载或UIImageView何时完成显示图像？

非常感谢.