Wait for RunOnUIThread to finish and continue executing the rest of the task

Question

im developing an app for android via c#(xamarin.visual studio) , the problem is that i have some task to do that running in other threads , and when it should update the layout it should call Activity.RunOnUIThread , everything it's working well but the thread dont wait this method to finnish and continue executing the rest withuout waiting.

The question is : How to wait for RunOnUIThread to finish and after that continue executing rest of the commands of the task. ?

public void start(int threadCounter)
    {
        for (int i = 0; i < threadCounter; i++)
        {

            Thread thread1 = new Thread(new ThreadStart(RunScanTcp));
            thread1.Start();

        }

    }
    public void RunScanTcp()
    {

        int port;

        //while there are more ports to scan 
        while ((port = portList.NextPort()) != -1)
        {
            count = port;

            Thread.Sleep(1000); //lets be a good citizen to the cpu

            Console.WriteLine("Current Port Count : " + count.ToString());

            try
            {

                Connect(host, port, tcpTimeout);

            }
            catch
            {
                continue;
            }

            Activity.RunOnUiThread(() =>
            {
                mdata.Add(new data() { titulli = "Port : " + port, sekuenca = "Sequence : ", ttl = "Connection Sucessfull !", madhesia = "", koha = "Time : " });
                mAdapter.NotifyItemInserted(mdata.Count() - 1);
                if (ndaluar == false)
                {
                    mRecyclerView.ScrollToPosition(mdata.Count() - 1);
                }
            }); // in that point i want to wait this to finish and than continue below...
            Console.WriteLine("TCP Port {0} is open ", port);

        }

Answer 1

First of all you should avoid creating new Threads . In you case you must use ThreadPool.QueueUserWorkItem to enqueue the CPU bound operation. Then you could use a ManualResetEventSlim or TaskCompletionSource to synchronize the the UI thread and the Worker Thread.

Example:

// mre is used to block and release threads manually. It is
// created in the unsignaled state.

ManualResetEventSlim mre = new ManualResetEventSlim(false);

RunOnUiThread(() =>
{            
    // Update UI here.
    // Release Manual reset event.

    mre.Set();
});

// Wait until UI operations end.
mre.Wait();

---

In your specific case:

for (int i = 0; i < threadCounter; i++)
{
    ThreadPool.QueueUserWorkItem(RunScanTcp);
}

private void RunScanTcp(object stateInfo) 
{
    // Do CPU bound operation here.
    var a = 100;
    while (--a != 0)
    {
        // mre is used to block and release threads manually. It is
        // created in the unsignaled state.
        ManualResetEventSlim mre = new ManualResetEventSlim(false);

        Activity.RunOnUiThread(() =>
        {
            // Update UI here.

            // Release Manual reset event.
            mre.Set();
        });

        // Wait until UI operation ends.
        mre.WaitOne();
    }
}

If you prefer to use TaskCompletionSource you could use an alternative approach :

private async void RunScanTcp(object stateInfo)
{
    // Do CPU bound operation here.
    var a = 100;
    while (--a != 0)
    {
        // using TaskCompletionSource
        var tcs = new TaskCompletionSource<bool>();

        RunOnUiThread(() =>
        {
            // Update UI here.

            // Set result
            tcs.TrySetResult(true);
        });

        // Wait until UI operationds.
        tcs.Task.Wait();
    }
}

Answer 2

You could use Monitor.Wait and Monitor.Pulse with a shared myLock object to wait for the UI execution.

Activity.RunOnUiThread(() =>
{
    mdata.Add(new data() { titulli = "Port : " + port, sekuenca = "Sequence : ", ttl = "Connection Sucessfull !", madhesia = "", koha = "Time : " });
    mAdapter.NotifyItemInserted(mdata.Count() - 1);
    if (ndaluar == false)
    {
        mRecyclerView.ScrollToPosition(mdata.Count() - 1);
    }
    lock(myLock) Monitor.Pulse(myLock)
});
lock(myLock) Monitor.Wait(myLock)
Console.WriteLine("TCP Port {0} is open ", port);

Answer 3

For those interested in an async / await solution, there is Stephen Cleary's AsyncManualResetEvent , eg:

var mre = new AsyncManualResetEvent();
this.context.RunOnUiThread(() =>
{
    // Do awesome UI stuff
    mre.Set();
});
await mre.WaitAsync();