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arraysswiftloopsgrand-central-dispatch

My function returns empty array in swift and parse

HI there I am writing a function to get data from my parse server in swift everything is working alright and the data is getting read well. but when I try to return the array it returns me an empty array. I also added a print in the "Get data in background" and there, the array was getting full.So the problem is not getting the data.

public func getthearray() -> Array<UIImage>
{
    let user = PFUser.current()
    let array = user?["Photos"] as! Array<PFFileObject>
    var imagearray = [UIImage]()

    for x in array
    {
        x.getDataInBackground
        { (dataa, error) in

            var img = UIImage(data: dataa!)
            imagearray.append(img!)
        }
    }

    return imagearray
}
Solution

  • The reason the function returns an empty array is the fact that the code in the x.getDataInBackground completion gets called asynchronously, and before any image will get added to the array, the faction will already return.

    Here is the solution (in 2 versions), which is your function slightly refactored and rewritten with completion handler and Grand Central Dispatch

    Approach 1. Semaphores

    public func getTheArray(_ completion: @escaping ([UIImage]) -> Void) {
    let user: PFUser = .current()
    let array = user?["Photos"] as? [PFFileObject] ?? []
    var result = [UIImage]()

    let semaphore = DispatchSemaphore(value: 0)
    // dispatch to a separate thread which we can safely occupy and block
    // while waiting for the results from async calls.
    DispatchQueue.global().async {
        for file in array {
            file.getDataInBackground { (data, error) in
                if let data = data, let image = UIImage(data: data) {
                    result.append(image)
                }
                // the result have arrived, signal that we are ready
                // to move on to another x
                semaphore.signal()
            }

            // wait until the `signal()` is called, thread is blocked
            semaphore.wait()
        }
        // dispatch to main queue where we are allowed to perform UI updates
        DispatchQueue.main.async {
            // call the completion, but we are doing it only once,
            // when the function is finished with its work
            completion(result)
        }
    }
}

    Approach 2. Dispatch Groups

    public func getTheArray(_ completion: @escaping ([UIImage]) -> Void) {
    let user: PFUser = .current()
    let array = user?["Photos"] as? [PFFileObject] ?? []
    var result = [UIImage]()

    let group = DispatchGroup()
    for file in array {
        group.enter()
        file.getDataInBackground { (data, error) in
            if let data = data, let image = UIImage(data: data) {
                result.append(image)
            }
            group.leave()
        }
    }

    // notify once all task finish with `leave()` call.
    group.notify(queue: .main) {
        // call the completion, but we are doing it only once,
        // when the function is finished with its work
        completion(result)
    }
}

    Usage

    and you would call it like this

    getTheArray { result in
    // do what you want with result which is your array of UIImages
}

    Documentation:

    Relevant blogposts: