Xamarin Forms - Android 上的进度条未更新。 在 UWP 上工作正常

Question

I'm using a progress bar to show the progress of some CPU heavy work. I create an array so that each element represents completion of one of the iterations of the work, and can be accessed thread safe without impacting performance, and then I use a 100ms Timer to calculate how many iterations have completed, and display that progress on the ProgressBar. I do all the heavy work in a Task.Run so it should be running on another thread and not locking the UI. When I test it on Windows, it works exactly as expected, the progress bar scrolls across the screen. When I test on Android, both on the Emulator as well as my 8 core physical device, it just stays in the first position until the work is done. I even tested increasing the iterations of the work considerably to make sure it takes longer, and same result. It never updates. I'm not really sure what I'm doing wrong here.

Here's what I'm doing:

var CompletedProgress = new int[Iterations];

var ProgressTimer = new Timer(100) { AutoReset = true };
ProgressTimer.Elapsed += async (se, ee) => await Device.InvokeOnMainThreadAsync(() => BreakdownProgress.Progress = CompletedProgress.Sum() / (double)Iterations);
ProgressTimer.Start();

Task.Run(() =>
{ 
    AllResults[0] = GenerateDetails(show, Adjustments, Numbers);
    CompletedProgress[0] = 1;

    Parallel.For(1, Iterations, i =>
    {
        var OrderedNumbers = Numbers.OrderBy(x => Random.NextDouble()).ToArray();
        AllResults[i] = GenerateDetails(show, Adjustments, OrderedNumbers);
        CompletedProgress[i] = 1;
    });
});

I also tried using a Binded property, and calling OnPropertyChanged from the timer, and it was the same result. I also tried stopping the timer before doing its work and then restarting it as well, in case the timer was running too quickly. Also tried Device.BeginInvokeOnMainThread instead of Device.InvokeOnMainThreadAsync.

Edit: I have actually noticed the progress bar go up occasionally, but not usually, and whenever it does move at the start, it still always gets stuck. I can still tap my toolbar menu items so I don't think the UI thread is overloaded.

Edit 2: Tried updating a label instead of a progress bar and I get the same issue, so the issue isn't with the progressbar itself. Even displaying just a simple counter has the same problem on the label.

Edit 3: Changing Parallel.For to regular for works fine. It's just much slower. So Parallel,For is somehow blocking the UI updating for the progress bar, and the label example I had. but somehow it's not blocking the toolbar items from displaying just fine. This is so confusing.

Answer 1

I figured it out. It appears I was right that the UI thread wasn't being blocked, but instead I believe the thread pool was being overloaded and the threads used by the timer were being delayed too long because of all of the threads used by Parallel.For, so I was able to solve it with this:

Parallel.For(1, Iterations, new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount - 1 }, async i =>
{
            var OrderedNumbers = Numbers.OrderBy(x => Random.NextDouble()).ToArray();
            AllResults[i] = GenerateDetails(show, Adjustments, OrderedNumbers);
            CompletedProgress[i] = 1;
});

Unfortunately this does slow the process slightly, but it does bring the animation back. If there's another way to still use the full CPU to do this, without blocking the timer thread, let me know in a comment below.