Cocoa thread synchronisation when using [ALAssetsLibrary enumerateGroupsWithTypes:]

Question

I have recently, like a few people, discovered that [ALAssetsLibrary enumerateGroupsWithTypes] likes to run its blocks on another thread. What a shame that Apple didn't document that :-)

In my current circumstance I need to wait for the enumeration to complete, before the main thread returns any results. I clearly need some sort of thread synchronisation.

I've read about NSLock & NSConditionLock, but nothing yet seems to fit the requirement of 'signal a blocked thread that this worker thread has completed'. It seems like a simple enough need - can anyone point me in the right direction?

Your clue & boos, are most welcome as always,

M.

Answer 1

The framework doesn't run these blocks on a separate thread. It just runs them as additional events in the same run-loop. To prove it, try this

    [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");
    }

My output from this was

main
main
main

Meaning that the block was being called in the main thread. It's just a separate event. To solve your problem, you just need to return your value somehow from within the block when you reach the last step. You can tell it's the last step because your block will be called with nil for the group object.

EDIT: for instance use this block

^(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)
    }
}

Answer 2

The answer is to use the NSConditionLock class thusly ...

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

...

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

Then spin off your thread, or in my case a call to [ALAssetsLibrary enumerateGroupsWithTypes:]. Then block the parent thread with this ...

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

When all work is done in the child/worker thread, unblock the parent with this ...

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

Answer 3

Simply use this:

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