Thread Safe Read/Write Array access with Concurrent Dispatch Group

Question

I'm developing an Photo editing application. I'm giving my users the option to import multiple assets to the edit View Controller. I'm creating a Dispatch Group for all the "PHAsset->Asset fetching" calls.
Each time the PHAsset fetch request completion block called, I need to append the fetched asset into an Mutable Array, and updating the UI (1/3 imported | 2/3 imported and so on) by the amount of the imported assets.
Because Swift Mutable Arrays aren't thread-safe, sometimes I'm getting a crash. I guess the completion handler called from multiple threads, causing some kind of a deadlock.

What would be the correct way to handle thread-safe read/write Array access with Concurrent Dispatch Group?

var assets = [Asset]()

func getAssetsFromPHAssets() {

    let dispatchGroup: DispatchGroup = DispatchGroup()

    for phAsset in phAssets {

        dispatchGroup.enter() // Enter group for each asset

        PHImageManager.default().requestPlayerItem(forVideo: phAsset, options: nil, resultHandler: { (item, info) in

            let asset = Asset(playerItem: item, position: 0)
            self.assets.append(asset)

            DispatchQueue.main.async {
                self.lblImport.text = "Importing \(self.assets.count)/3"
            }

            dispatchGroup.leave() // Leave group
        })
    }

    dispatchGroup.notify(queue: DispatchQueue.main, execute: {
            print("finished importing")
        })
}

When looking through the answers in the duplicate, the [answer by mooney](https://stackoverflow.com/a/40045439/1226963) is best. — rmaddy, Dec 21 '17 at 15:26
@rmaddy Thanks for replying. The comment on his answer looks skeptical about his solution. Can you confirm his answer — Roi Mulia, Dec 21 '17 at 15:40
I didn't actually test it but the approach is what you need. Create a concurrent queue. Use regular sync/async to read from your array and use a barrier sync/async to write to the array. This allows multiple concurrent readers but ensures a write is done on its own and blocks and concurrent reads or writes. — rmaddy, Dec 21 '17 at 15:42
@rmaddy Thanks, I've implemented the answer you have linked. Now I need to think about how to test it lol. One small question, does the dispatchGroup.leave(), should be at the current location it is, or should I wrap it inside async/sync call as well? — Roi Mulia, Dec 21 '17 at 16:33
Your calls to `leave` are fine. There's no need to move the call to `leave` inside the same block that updates the label. — rmaddy, Dec 21 '17 at 16:36

Andreas Oetjen · Answer 1 · 2017-12-21T14:24:35.363

-1

I would suggest adding the new Asset in the main thread:

let asset = Asset(playerItem: item, position: 0)

DispatchQueue.main.async {
    self.assets.append(asset)
    self.lblImport.text = "Importing \(self.assets.count)/3"
}

Maybe - but this depends on what you are doing in the notify closure - you should also move the dispatchGroup.leave() call into the main queue.

edited Dec 21 '17 at 14:24

answered Dec 21 '17 at 14:04

Andreas Oetjen

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Thread Safe Read/Write Array access with Concurrent Dispatch Group

What would be the correct way to handle thread-safe read/write Array access with Concurrent Dispatch Group?

1 Answers1