ALAssetsLibrary enumerateGroupsWithTypes：-线程同步

Question

我使用ALAssetsLibrary enumerateGroupsWithTypes:将ALAssets存储在数组中。因为这是一个异步操作，所以我需要等待它完成，然后才能继续我的工作。

我阅读了Cocoa thread synchronisation when using [ALAssetsLibrary enumerateGroupsWithTypes:]并尝试了推荐的NSConditionLock。然而，块总是在主线程中执行的，因此，如果我使用条件锁等待，主线程被阻塞，这些块不会被执行，我被卡住了。我甚至尝试在一个新线程上运行loadAssets方法，但仍然在主线程上执行这些块。

我找不到一种方法来真正等待枚举完成。有没有办法将这些块强制到不同于主线程的线程，或者我可以做的任何其他事情？

代码如下：

- (void)loadAssets
{
    assets = [NSMutableArray array];
    NSConditionLock *threadLock = [[NSConditionLock alloc] initWithCondition:THREADRUNNING];

    void (^assetEnumerator)(ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop)
    {
        if(result != nil)
        {
            [assets addObject:result];
        }
    };

    void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) = ^(ALAssetsGroup *group, BOOL *stop)
    {
        if(group != nil)
        {
            [group enumerateAssetsUsingBlock:assetEnumerator];
        }

        [threadLock lock];
        [threadLock unlockWithCondition:THREADFINISHED];
    };

    void (^assetFailureBlock)(NSError *) = ^(NSError *error)
    {
        [threadLock lock];
        [threadLock unlockWithCondition:THREADFINISHED];
    };

    ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
    [assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:assetGroupEnumerator failureBlock:assetFailureBlock];

    [threadLock lockWhenCondition:THREADFINISHED];
    [threadLock unlock];

    [assetsLibrary release];
    [threadLock release];
}

Answer 1

您将在assetsLibrary对象完成枚举对象之前终止该对象。将其移动到您的.h文件中，并在dealloc中释放它。

Answer 2

我知道这个帖子可能已经死了，但我之所以回答它，是因为这是我通过谷歌搜索登陆的地方

诀窍是锁定后台线程，直到枚举产生nil-group。此类别是如何使用它的一个示例，您可以使用category方法替代ALAssetsLibrary枚举方法。

@implementation ALAssetsLibrary (EEEConcurrency)

- (NSUInteger)eee_enumerateGroupsLockedWithTypes:(ALAssetsGroupType)types
                                      usingBlock:(ALAssetsLibraryGroupsEnumerationResultsBlock)enumerationBlock
                                    failureBlock:(ALAssetsLibraryAccessFailureBlock)failureBlock
{
    NSAssert(![NSThread isMainThread], @"This would create a deadlock (main thread waiting for main thread to complete)");

    enum
    {
        EEEAssetsLibraryDone,
        EEEAssetsLibraryBusy
    };

    NSConditionLock *assetsLibraryConditionLock = [[NSConditionLock alloc] initWithCondition:EEEAssetsLibraryBusy];

    __block NSUInteger numberOfGroups = 0;
    [self enumerateGroupsWithTypes:types
                        usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
                            enumerationBlock(group, stop);
                            if (group) numberOfGroups++;
                            if (!group || *stop)
                            {
                                [assetsLibraryConditionLock lock];
                                [assetsLibraryConditionLock unlockWithCondition:EEEAssetsLibraryDone];
                            }
                        }
                      failureBlock:^(NSError *error) {
                          failureBlock(error);
                          [assetsLibraryConditionLock lock];
                          [assetsLibraryConditionLock unlockWithCondition:EEEAssetsLibraryDone];
                      }];

    [assetsLibraryConditionLock lockWhenCondition:EEEAssetsLibraryDone];
    [assetsLibraryConditionLock unlock];

    return numberOfGroups;
}

@end

请在https://gist.github.com/epologee/8890692下载该类别

Answer 3

这个代码块应该对您有效-->

    ALAssetsLibrary *assetsLib = [[ALAssetsLibrary alloc] init];
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    __block int numberOfGroups = 0;

    dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [assetsLib enumerateGroupsWithTypes:ALAssetsGroupAlbum usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
            if (group) {
                numberOfGroups++;
            }
            else {
                dispatch_semaphore_signal(sema);
            }
        } failureBlock:^(NSError *error) {
            NSLog(@"enumerateGroupsWithTypes failure %@", [error localizedDescription]);
            dispatch_semaphore_signal(sema);
        }];
    });

    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    NSLog( @"number of groups is: %d", numGroups);

尽管您可能希望将时间设置为从现在起一秒后，而不是DISPATCH_TIME_FOREVER