Xamarin BeginInvokeOnMainThread without Xamarin.Forms

Question

Sorry for what I'm sure will turn out to be a very stupid question..

I am using Android UI not Xamarin Forms for the presentation layer in my Xamarin app, but I want to use the Activity.RunOnUIThread (from Android), which all Xamarin docs suggests is Device.BeginInvokeOnMainThread (from the Xamarin.Forms) project. Obviously I don't have this available since I don't have a reference on the xamarin.forms project.

Where do I find the run-on-ui-thread mechanism in Xamarin if I don't want to use Forms?

Answer 1

Android:

Android Activitys have a RunOnUiThread method that you can use:

RunOnUiThread  ( () => {
    // manipulate UI controls
});

Ref: https://developer.xamarin.com/api/member/Android.App.Activity.RunOnUiThread/p/Java.Lang.IRunnable/

iOS:

InvokeOnMainThread (delegate {  
    // manipulate UI controls
});

Answer 2

If you want to do this from your PCL / shared code and everywhere else in your project. You have two options doing this.

Cross platform way, using the native mechanisms

• add this to PCL

  public class InvokeHelper
  {
      public static Action<Action> Invoker;
  
      public static void Invoke(Action action)
      {
          Invoker?.Invoke(action);
      }
  }
  

• add this to iOS (e.g. AppDelegate)

  public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
  {
      // ...
  
      InvokeHelper.Invoker = InvokeOnMainThread;
      return true;
  }
  

• add this to Android (e.g. your application class)

  [Application]
  class MainApplication : Application
  {
      protected MainApplication(IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer)
      {
      }
  
      public override void OnCreate()
      {
          base.OnCreate();
          InvokeHelper.Invoker = (action) =>
          {
              var uiHandler = new Handler(Looper.MainLooper);
              uiHandler.Post(action);
          };
      }
  }
  

And then you can call from your shared code

InvokeHelper.Invoke(() => DoSomething("bla"));

Complete cross platform way

You can implement InvokeHelper cross platform, too.

public class InvokeHelper
{
    // assuming the static initializer is executed on the UI Thread.
    public static SynchronizationContext mainSyncronisationContext = SynchronizationContext.Current;

    public static void Invoke(Action action)
    {
        mainSyncronisationContext?.Post(_ => action(), null);
    }
}

Answer 3

Here you have an example obtained from the official documentation:

public class ThreadDemo : Activity
{
  TextView textview;

  protected override void OnCreate (Bundle bundle)
  {
      base.OnCreate (bundle);
      // Create a new TextView and set it as our view
      textview = new TextView (this);
      textview.Text = "Working..";
      SetContentView (textview);
      ThreadPool.QueueUserWorkItem (o => SlowMethod ());
  }

  private void SlowMethod ()
  {
      Thread.Sleep (5000);
      RunOnUiThread (() => textview.Text = "Method Complete");
  }
}

Basically, if you want to run more than one line of code, you can do this:

RunOnUiThread(()=>{
  MethodOne();
  MethodTwo();
});

source