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，可以下载一系列图像，稍后我将使用UIImageView的UIImageView属性来使用这些图像来动画。我想知道我的循环中的所有块什么时候已经完成执行，这样我就可以开始动画了，我想知道如何才能知道它们什么时候完成呢？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. 

Error -999 编辑：的问题

在执行提供的答案中的代码时，我遇到了以下问题：Domain=NSURLErrorDomain Code=-999 "The operation couldn’t be completed. (NSURLErrorDomain error -999.)"

快速搜索reveals，Error -999的意思是“在前一个请求完成之前发出了另一个请求”.这当然是这样的，因为我正在快速地提出几个要求。推荐的修补程序建议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)，而前面的那个失败了。我想知道在这里还是在AFNetworking有我应该考虑的解决办法？谢谢!

编辑2:基于@David的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

创建调度组，在for循环中输入组，在完成块中离开组。然后可以使用dispatch_group_notify查找所有块何时完成：

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
});

Answer 2

数一数你完成了多少。最具挑战性的部分是让它的线程安全。我建议为此创建一个原子计数器类。

通用解决方案！

+ (void)runBlocksInParallel:(NSArray *)blocks completion:(CompletionBlock)completion {
    AtomicCounter *completionCounter = [[AtomicCounter alloc] initWithValue:blocks.count];
    for (AsyncBlock block in blocks) {
        block(^{
            if ([completionCounter decrementAndGet] == 0) {
                if (completion) completion();
            }
        });
    }
    if (blocks.count == 0) {
        if (completion) completion();
    }
}

NSMutableArray *asyncBlocks = [NSMutableArray array];
for (PFObject *pictureObject in objects){
    [asyncBlocks addObject:^(CompletionBlock completion) {
        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);
        } completion:completion];
    }];
}
[BlockRunner runBlocksInParallel:[asyncBlocks copy] completion:^{
    //Do your final completion here!
}];

Answer 3

设置一个属性并将其初始化为循环数- objects.count。在完成块时，将数字降下来。当你达到零，你就完蛋了。