Objective-C Block: return __block variable from block inside

Question

OK, here is the code.

NSObject* (^executableBlock)(void) = ^NSObject*() {
    __block NSObject *refObj = nil;

    [Utility performAction:^() {
         if (conditionA)
              refObj = fooA;
         else
              refObj = fooB;
    };

    return refObj;
};

NSObject *result = executableBlock();   // result is nil

After executing the executableBlock, the result is nil and performAction block didn't be executed immediately and returned my expected value.

I know performAction block is executed within another thread and using the shared nil pointer refObj. Refer to Working with Blocks.

Here is my through, if I use GCD to call the performAction block and wait for its finish, how to rewrite it? Thanks!

Answer 1

By using semaphore, you can wait until a block is done.
But completion-block may be suitable in your case, i think.

NSObject* (^executableBlock)(void) = ^NSObject*() {
    __block NSObject *refObj = nil;

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    [self performAction:^() {
        if (conditionA)
            refObj = fooA;
        else
            refObj = fooB;

        dispatch_semaphore_signal(semaphore);
    }];

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    return refObj;
};