Objective-C: Waiting for the last call in method

Question

I have a method that is being called several times. But I need to act only in the very last method call. I tried dispatch_async but didn't work because still is been queue the calls:

-(void)doingSomething:(NSString*)someValue
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [NSThread sleepForTimeInterval:1.0f];
        dispatch_async(dispatch_get_main_queue(), ^{
            // do something with the last call
        });
    });
}

Any of you knows a way to queue the calls and only use the very last call?

I'll really appreciate your help

you want to wait for the last doingSomething call or what do you mean. im afraid I dont get it at all — Daij-Djan, Aug 24 '17 at 19:25
[Implement a debounced coalesced pattern in Cocoa Touch like layoutsubviews](https://stackoverflow.com/q/28153632) — jscs, Aug 26 '17 at 17:08

Charles Srstka · Answer 1 · 2017-08-24T19:36:42.560

2

My suggestion would be to use a dispatch_group. Call dispatch_group_enter before you call dispatch_async, and call dispatch_group_leave and the end of the block that's executed by dispatch_async. Then, after you've enqueued all the blocks, use dispatch_group_notify to schedule the completion block, which will run after all the other dispatch_async blocks have finished.

dispatch_group_t group = dispatch_group_create();

for (...) {
    dispatch_group_enter(group);

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        ...
        dispatch_group_leave(group);
    });
}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // This gets called when all the other blocks have finished
});

Alternatively, you could use an NSOperationQueue instead of libdispatch, and make a completion operation which lists every other operation as a dependency. This does have the disadvantage that the completion operation won't be executed on the main queue, though.

NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    // This gets called when all the other operations have finished
}];

for (...) {
    NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        ...
    }];

    [completionOperation addDependency:operation];
    [operationQueue addOperation:operation];
}

[operationQueue addOperation:completionOperation];

edited Aug 24 '17 at 19:36

answered Aug 24 '17 at 19:26

Charles Srstka

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How does NSOperationQueue it will only use the last call to the method? – user2924482 Aug 24 '17 at 19:33
I added an `NSOperationQueue`-based example as an edit. – Charles Srstka Aug 24 '17 at 19:37
From your both examples. All the calls to the method would be executed. Looking into your example implementation how does the last call is the only one executed ? – user2924482 Aug 24 '17 at 20:45
Put the part that you only want executed once in the `dispatch_queue_notify` block (for the first example) or `completionOperation` (for the second one). The code you put there will only be executed once, after everything else has completed. – Charles Srstka Aug 24 '17 at 21:52
But my point is all the calls are being executed, none of the are been cancel. What I want is just to execute the very last call in the queue – user2924482 Aug 24 '17 at 22:08
You can't do that, because as soon as you call `dispatch_async`, the first block may have already started executing. If you use `NSOperationQueue`, you can send the `-cancel` method to the individual `NSOperation` objects, but this will only prevent them from starting; if they've already started, it won't stop them, and there's not really a good way to do that. Most likely, you will need to find a different way to do what you're trying to do. The method I outlined would ensure that the completion block would only run once, at the end. What exactly is it that you're trying to do? – Charles Srstka Aug 24 '17 at 22:13
What I'm trying to do is get all the calls to this method and just execute the last one – user2924482 Aug 24 '17 at 22:16
1

What are you trying to accomplish by doing that? – Charles Srstka Aug 24 '17 at 22:38

Objective-C: Waiting for the last call in method

1 Answers1