Why app is blocked by semaphore?

Question

I have the following function that suppose to return [CIImage] for my purpose - displaying some metadata of photos in tableView.

func getCIImages() -> [CIImage] {
    var images = [CIImage]()
    let assets = PHAsset.fetchAssetsWithMediaType(.Image, options: nil)

    for i in 0..<assets.count {
        guard let asset = assets[i] as? PHAsset else {fatalError("Cannot cast as PHAsset")}  
        let semaphore = dispatch_semaphore_create(0)

        asset.requestContentEditingInputWithOptions(nil) { contentEditingInput, _ in
            //Get full image
            guard let url = contentEditingInput?.fullSizeImageURL else {return}
            guard let inputImage = CIImage(contentsOfURL: url) else {return}
            images.append(inputImage)
            dispatch_semaphore_signal(semaphore)
        }
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
    }
    return images
}

but it stucks in semaphore wait and didn't go further. I have walked through many tutorials but other variants of GCD don't works. I think it's because of simulator, I don't know, can't test on real device. Please help.

@vadian because i want to display some metadata of `CIImages` in `tableView` and it is can be done by synchronous manner with one `tableView.reloadData()` call. — Andrey M., Aug 23 '16 at 10:48

Silmaril · Accepted Answer · 2016-08-23T12:27:19.990

6

guards inside requestContentEditingInputWithOptions callback closure prevents signal sent to semaphore. In such cases (when you need cleanup actions) it is good to use defer. In your case:

asset.requestContentEditingInputWithOptions(nil) { contentEditingInput, _ in
    defer { dispatch_semaphore_signal(semaphore) }

    //Get full image
    guard let url = contentEditingInput?.fullSizeImageURL else {return}
    guard let inputImage = CIImage(contentsOfURL: url) else {return}
    images.append(inputImage)
}

UPDATE

Apart from cleanup bug there is another one. Completion closure of requestContentEditingInputWithOptions called on main thread. Which means that if you blocking main thread with semaphore: completion closure is blocked form executing as well. To fix blocked semaphore issue you need call getCIImages on a different thread than main.

Anyway making asynchronous things synchronous is wrong. You should think of different approach.

edited Aug 23 '16 at 12:27

answered Aug 22 '16 at 16:27

Silmaril

4,241
20
22

2

Exactly the case why `defer` has been introduced! – Sulthan Aug 22 '16 at 16:29
None one of the `guard` does not calls `else` statement. This code produces the same behavior like my code - programm freezes – Andrey M. Aug 23 '16 at 05:41
@Silmaril Even if I make the call asynchronous how I can return the all images? – Andrey M. Aug 24 '16 at 09:41
1

@AndreyM. It depends on your requirements. But I created a simple example for you: https://gitlab.com/snippets/24749. Hope this will be enough to understand the idea. – Silmaril Aug 24 '16 at 11:04

Why app is blocked by semaphore?

1 Answers1