24

我想等待这段代码执行后再继续，但是由于这些块是异步调用的，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)
    }];
}
4

5 回答 5

44

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);
于 2012-05-18T06:33:44.340 回答
6

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

于 2011-09-05T16:31:21.990 回答
4

请注意assetForURL:resultBlock:failureBlock: 如果主线程正在等待而没有运行RunLoop，则会卡住。这是替代（清洁器:-)）解决方案：

#import <libkern/OSAtomic.h>

...

ALAssetsLibrary *library;
NSMutableArray *assets;
...
__block int32_t counter = 0;
for (NSURL *url in urls) {
    OSAtomicIncrement32(&counter);
    [library assetForURL:url resultBlock:^(ALAsset *asset) {
        if (asset)
            [assets addObject:asset];
        OSAtomicDecrement32(&counter);
    } failureBlock:^(NSError *error) {
        OSAtomicDecrement32(&counter);
    }];
}
while (counter > 0) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.01]];
}
于 2012-06-14T03:49:15.967 回答
1

最简单的做法是将代码移动到内部（末尾）resultBlock或failureBlock. 这样，您的代码将以正确的顺序运行，并且您还将保留异步行为。

于 2011-08-29T19:37:34.430 回答
1

这是一种简单的方法。也许不如使用 GCD 优雅，但它应该可以完成工作......这将使您的方法阻塞而不是非阻塞。

__block BOOL isFinished = NO;
NSURL *asseturl;
NSMutableArray *tmpListAsset = [[NSMutableArray alloc] init];

ALAssetsLibrary *library = [[[ALAssetsLibrary alloc] init];
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);
        }
        if (objectsToRemove.count + tmpListAsset.count == assetsList.count) {
            isFinished = YES;
        }
    } failureBlock:^(NSError *error) {
        // Type your code here for failure (when user doesn't allow location in your app)
        isFinished = YES;
    }];
}

while (!isFinished) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.01f]];
}
于 2014-06-19T21:48:50.430 回答