使用[ALAssetsLibrary enumerateGroupsWithTypes:]时的可可线程同步

框架不会在单独的线程上运行这些块。它只是在同一个运行循环中将它们作为附加事件运行。为了证明这一点,试试这个

    [library enumerateGroupsWithTypes:ALAssetsGroupAll 
                           usingBlock:[^(ALAssetsGroup * group, BOOL * stop)
                             {
                               if([NSThread isMainThread])
                               {
                                  NSLog(@"main");
                               }
                               else
                               {
                                 NSLog(@"non-main");
                               }
                             } copy] 
           failureBlock:^(NSError * err)
                          {NSLog(@"Erorr: %@", [err localizedDescription] );}];
    [library release];
    if([NSThread isMainThread])
    {
        NSLog(@"main");
    }
    else
    {
        NSLog(@"non-main");
    }

main
main
main

编辑: 例如,使用此块

^(ALAssetsGroup * group, BOOL * stop)
{
    if(group == nil)
    {
        // we've enumerated all the groups 
        // do something to return a value somehow (maybe send a selector to a delegate)
    }
}

答案是这样使用NSConditionLock类。。。

typedef enum {
    completed = 0,
    running = 1
} threadState;

...

NSConditionLock *lock = [[NSConditionLock alloc] initWithCondition:running];

然后剥离线程,或者在我的例子中调用[ALAssetsLibrary enumerateGroupsWithTypes:]。然后用这个阻止父线程。。。

// Await completion of the worker threads 
[lock lockWhenCondition:completed];
[lock unlockWithCondition:completed];

儿童/工人

// Signal the waiting thread
[lock lockWhenCondition:running];
[lock unlockWithCondition:completed];

简单地使用这个:

[library enumerateGroupsWithTypes:ALAssetsGroupAll 
                           usingBlock:[^(ALAssetsGroup * group, BOOL * stop)
{
    if(group == nil)
    {
        // this is end of enumeration
    }
}
.
.
.