1.同步请求
ASIHTTPRequest*request=  [ASIHTTPRequest requestWithURL:url];
    [request setRequestMethod:@"GET"];
    [request setTimeOutSeconds:50];
    [request startSynchronous];
    NSError *error=request.error;
    if (error==nil) {
       NSData *data= request.responseData;
       UIImage*image= [UIImage imageWithData:data];
        self.image=image;
        
    }else{
        NSLog(@"请求出错");
    }
    

2.异步请求
-(void)setImageURL:(NSURL *)url{
  ASIHTTPRequest*request=  [ASIHTTPRequest requestWithURL:url];

    [request setRequestMethod:@"GET"];
    [request setTimeOutSeconds:50];
    request.delegate=self;//这个是与同步请求不一样的地方
    [request startAsynchronous];
}
- (void)requestFinished:(ASIHTTPRequest *)request{
    NSData*imagedata= request.responseData;
   UIImage *image= [UIImage imageWithData:imagedata];
       self.image=image;  //监听还是主线程，所以不用跳
}
- (void)requestFailed:(ASIHTTPRequest *)request{
    NSLog(@"%@失败",request.error);
}