Callbacks with array

Question

I am trying to setup the following method to perform the success block once all the show.getVideosForShow() success blocks have been performed and all videos appended. Note: show.getVideosForShow() is asynchronous and could take a few seconds to get a result. Could someone please provide some assistance?

private func getNextVideoRecommendations(success: ([Video]) -> ()) {
    var relatedVideos = [Video]()
    if let relatedShows = self.videoCurrentlyPlaying?.show?.getShowsWithSameGenre(fetchLimit: 3) {
        for show in relatedShows {
            show.getVideosForShow(tvSeason: nil, longForm: true, sortType: VideoSort.Latest, success: { (videos: [Video]) in
                print("Found Related Show: \(show.title)")
                if videos.count > 0 {
                    relatedVideos.append(videos[0])
                }
            })
        }
        print("Finished all operations")
        success(relatedVideos)
    }
}

It looks good provide more details what you want to achieve & is there any tableview or collection structure you are using ? — Muhammad Ali
– Muhammad Ali, Commented May 6, 2016 at 5:38
I want success(relatedVideos) to be called when all getVideosForShow sucessblocks have been completed. Currently it will call the success(relatedVideos) block immediately. Yes the results will be used to populate a collection view later on, however that is unrelated to this issue. — Smashkon
– Smashkon, Commented May 6, 2016 at 5:46
then you have to use a completionhandler for getNextVideoRecommendations method and call success(relatedvideos) from where you are call getNextVideoRecommendations — Muhammad Ali
– Muhammad Ali, Commented May 6, 2016 at 5:48
Or a workaround will be to use for loop like for (index, show) in relatedShows { & check for condition if index == relatedShows.count-1 at the last of for loop to call success(relatedVideos) — Muhammad Ali
– Muhammad Ali, Commented May 6, 2016 at 5:50
The completionhandler is success(). See method signature. The problem is, how do you know when all the getVideosForShow completionHandlers (success block) has been called? — Smashkon
– Smashkon, Commented May 6, 2016 at 5:53

jtbandes · Accepted Answer · 2016-05-06 06:40:22Z

4

This is a good use case for dispatch groups, which allow you to submit another block once all your operations have finished:

private func getNextVideoRecommendations(success: ([Video]) -> ()) {
    var relatedVideos = [Video]()
    if let relatedShows = self.videoCurrentlyPlaying?.show?.getShowsWithSameGenre(fetchLimit: 3) {
        let group = dispatch_group_create()
        for show in relatedShows {
            dispatch_group_enter(group)  // start tracking one unit of work
            show.getVideosForShow(tvSeason: nil, longForm: true, sortType: VideoSort.Latest, success: { (videos: [Video]) in
                print("Found Related Show: \(show.title)")
                if videos.count > 0 {
                    relatedVideos.append(videos[0])
                }
                dispatch_group_leave(group)  // finish one unit of work
            })
        }
        dispatch_group_notify(group, dispatch_get_main_queue()) {  // and when done...
            print("Finished all operations")
            success(relatedVideos)
        }
    }
}

answered May 6, 2016 at 6:40

jtbandes

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5 Comments

Smashkon Over a year ago

Perfect! Thanks so much. Just for future reference, is it possible to do this using NSOperationQueue as well?

jtbandes Over a year ago

Yes, you can use dependencies between operations to make your final operation execute only once all the others have finished.

Smashkon Over a year ago

Would you be able to send me a code snippet for that if you have a chance please? I tried it myself using dependencies but I'm pretty sure did it wrong

jtbandes Over a year ago

If you're having trouble and can show what you've tried, you might want to ask a different question. The basics are probably NSBlockOperation to execute the final success callback, and using addDependency before you put it in an operation queue.

Smashkon Over a year ago

I tried creating block operations for the "Finished all operations" and then created block operations for inside the success block of show.getVideosForShow. But It doesn't quite work

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