objective c - GCD queue with completion block that runs sequentially

Question

I have code that needed to be run in sequentially, but most answer that I've searched doesn't include the case where it has a completion block. For example:

//global variable
dispatch_queue_t serialQueue = dispatch_queue_create("com.my.queue", DISPATCH_QUEUE_SERIAL); 
    
-(void)executeTask:(int)taskNum{
    dispatch_async(serialQueue, ^{
            //TIME CONSUMING TASK
            [self doTask:taskNum withCompletion:^{
                  NSLog(@"completed task number: %d", taskNum);
            }];
        }); 
}

-(void)startTask:(int)taskNum{
    [self executeTask:taskNum];
}

startTask: may be called multiple times, and it shall run sequentially once the completion handler of the previous task is completed.

Which means, if startTask: gets called 5 times, in the order of 1, 2, 3, 4, 5, the expected output would be:

completed task number: 1
completed task number: 2
completed task number: 3
completed task number: 4
completed task number: 5

Any idea how to achieve this? Thanks in advance.

question from:https://stackoverflow.com/questions/65844195/gcd-queue-with-completion-block-that-runs-sequentially

Answer 1

Waitting for answers