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up vote 1 down vote favorite
1

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!

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3 Answers 3

up vote 0 down vote accepted

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;
};
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Thanks for your tips, I just found an answer after searching keywords dispatch_semaphore_create. Link: stackoverflow.com/a/4326754/1677041 –  Itachi Jan 19 at 6:03
up vote 0 down vote

I would considering the following re-structure:

    __block NSObject * result;
    void (^executableBlock)(NSObject *) =  ^void (NSObject * obj)
    {
        [self performAnActionWithBlock:^(BOOL success)
        {
            if (success) {
                result = obj;
                NSLog(@"results: %@", result);
            }else{
               result = nil;
            }

        }];
    };
    executableBlock(@"Hello");


//where:
-(void)performAnActionWithBlock:(void(^)(BOOL))block{
    block(YES);
}

You should then be able to call result from elsewhere

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up vote 0 down vote 
void (^executableBlock)(NSObject*) = ^(NSObject *arg) {

[Utility performAction:^(NSObject *arg) {
     if (conditionA)
          arg = fooA;
     else
          arg = fooB;
}];

};

__block NSObject *result = nil;

executableBlock(result); // result will be changed in block

share|improve this answer
    
Please add more detail about your answer and why this answer works. –  SAM Jan 19 at 5:27

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