等待assetForURL块完成

Question

我想等待这个代码继续执行之前继续但是因为这些块被异步调用我不知道该怎么做???

NSURL *asseturl;
NSMutableArray *tmpListAsset = [[NSMutableArray alloc] init];

ALAssetsLibrary *library = [[[ALAssetsLibrary alloc] init] autorelease];
NSMutableArray *objectsToRemove = [[NSMutableArray alloc] init];
for (NSDictionary *dico in assetsList) {
    asseturl = [NSURL URLWithString:[dico objectForKey:@"assetUrl"]];
    NSLog(@"asset url %@", asseturl);
    // Try to load asset at mediaURL
    [library assetForURL:asseturl resultBlock:^(ALAsset *asset) {
        // If asset doesn't exists
        if (!asset){
            [objectsToRemove addObject:dico];
        }else{
            [tmpListAsset addObject:[asseturl absoluteString]];
            NSLog(@"tmpListAsset : %@", tmpListAsset);
        }
    } failureBlock:^(NSError *error) {
        // Type your code here for failure (when user doesn't allow location in your app)
    }];
}

Answer 1

GCD信号量方法:

dispatch_semaphore_t sema = dispatch_semaphore_create(0);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

for (NSURL *url in self.assetUrls) {
    dispatch_async(queue, ^{
        [library assetForURL:url resultBlock:^(ALAsset *asset) {
            [self.assets addObject:asset];
            dispatch_semaphore_signal(sema);
        } failureBlock:^(NSError *error) {
            dispatch_semaphore_signal(sema);
        }];
    });
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}
dispatch_release(sema);

/* Check out ALAssets */
NSLog(@"%@", self.assets);

Answer 2

解决方案在这里 http://omegadelta.net/2011/05/10/how-to-wait-for-ios-methods-with-completion-blocks-to-finish/