iOS下载网络图片

-(void)setWebImage:(NSURL *)aUrl placeHolder:(UIImage *)placeHolder downloadFlag:(int)flag{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        //配置下载路径
        NSString *path=[NSHomeDirectory() stringByAppendingFormat:@"/Library/Caches/%lu",(unsigned long)aUrl.description.hash];
        NSData *data=[NSData dataWithContentsOfFile:path];
        if (!data) {
            NSLog(@"准备下载到沙盒路径:%@",path);
            data=[NSData dataWithContentsOfURL:aUrl];
            [data writeToFile:path atomically:YES];
        }
        UIImage *image=[UIImage imageWithData:data];
        dispatch_async(dispatch_get_main_queue(), ^{
            if (self.tag==flag) {
                [self setImage:image];
            }
        });
    });
}