如何判断循环中的块是否都已完成执行？

Question

我有一个循环设置,下载一系列图像,我将在稍后用于使用animationImages属性的动画UIImageView.我想知道我的循环中的所有块何时完成执行所以我可以开始动画,并且想知道我怎么能够告诉他们什么时候完成完成？谢谢!

for (PFObject *pictureObject in objects){

    PFFile *imageFile = [pictureObject objectForKey:@"image"];
    NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
    NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];

    [tokenImageView setImageWithURLRequest:imageRequest placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
       [self.downloadedUIImages addObject:image]; //This is a mutableArray that will later be set to an UIImageView's animnationImages

    } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
        NSLog(@"Error %@", error);
    }];

}
//When I know all the blocks have finished downloading, I will then to animate the downloaded images. 

编辑:有问题 Error -999

我在提供的答案中执行代码时遇到以下问题: Domain=NSURLErrorDomain Code=-999 "The operation couldn’t be completed. (NSURLErrorDomain error -999.)"

快速搜索显示,这Error -999意味着" 在上一个请求完成之前发出了另一个请求 "......由于我快速连续发出了几个请求,因此这肯定是这种情况.建议的修复建议在这里,我没有工作,因为它只会成功下载一个UIImage的(最后一个请求),与前面的失败.我想知道这里是否有解决方法或AFNetworking我应该考虑？谢谢!

编辑2:基于@David解决方案的工作代码

for (PFObject *pictureObject in objects){

    PFFile *imageFile = [pictureObject objectForKey:@"image"];
    NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
    NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];
    AFHTTPRequestOperation *requestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:imageRequest];
    requestOperation.responseSerializer = [AFImageResponseSerializer serializer];

    dispatch_group_enter(group);
    [requestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        NSLog(@"Response: %@", responseObject);
        UIImage *retrivedImage = (UIImage *)responseObject;
        [self.downloadedUIImages addObject:retrivedImage];

        dispatch_group_leave(group);

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"Image error: %@", error);

        dispatch_group_leave(group);
    }];

    [requestOperation start];
    counter ++;
}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"Horray everything has completed");
    NSLog(@"What is here %@", self.downloadedUIImages);
    NSLog(@"Done");

});

Answer 1

创建一个调度组,在for循环中输入组,在完成块中离开该组.然后你可以使用dispatch_group_notify来找出所有块何时完成:

dispatch_group_t    group = dispatch_group_create();

for (PFObject *pictureObject in objects){

    PFFile *imageFile = [pictureObject objectForKey:@"image"];
    NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
    NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];

    dispatch_group_enter(group);
    [tokenImageView setImageWithURLRequest:imageRequest placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
        [self.downloadedUIImages addObject:image]; //This is a mutableArray that will later be set to an UIImageView's animnationImages
        dispatch_group_leave(group);
    } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
        NSLog(@"Error %@", error);
        dispatch_group_leave(group);
    }];

}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // do your completion stuff here
});