ios - 如何判断循环中的 block 是否全部执行完毕？

Question

我设置了一个循环，用于下载一系列图像，稍后我将使用这些图像使用 UIImageView 的 animationImages 属性制作动画。我想知道我的循环中的所有 block 何时完成执行以便我可以开始动画，并且想知道我如何才能知道它们何时完成？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:错误 -999 有问题

我在执行提供的答案中的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 reveals Error -999 的意思是“在上一个请求完成之前发出了另一个请求”......这里肯定是这种情况，因为我正在快速连续发出多个请求。建议修复建议 here对我不起作用，因为它只会成功下载一个 UIImage(最后一个请求)，而之前的失败。我想知道这里或 AFNetworking 中是否有我应该考虑的解决方法？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循环中进入组，在完成 block 中离开组。然后你可以使用 dispatch_group_notify 找出所有 block 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
});