Add Main Thread Detection

[craylward]

Use Case: Obtaining the main thread for Xamarin Forms UI navigation from a method that may or may not be running in the background thread.

Part of: Platform

API:

- bool IsMainThread { get; }

void NavigationMethod()
{
   if (Platform.IsMainThread)
   {
      Navigate();
   }
   else
   {
      Device.BeginInvokeOnMainThread(Navigate());
   }
}

Android: Looper.MyLooper == MainLooper
iOS: NSThread.Current.IsMainThread

[jamesmontemagno]

Yes, I like this! We actually have:

Platform.BeginInvokeOnMainThread(Navigate);

https://github.com/xamarin/Essentials/blob/master/Xamarin.Essentials/Platform/Platform.shared.cs#L8

This API should live inside of "Platform" and simply be

bool IsMainThread { get; }

This naming will match iOS which is good stuff.

[mattleibow]

We should literally be able to expose what we do in the platform code as a property. eg: https://github.com/xamarin/Essentials/blob/master/Xamarin.Essentials/Platform/Platform.android.cs#L61

This looks like a quick way to get involved in the project... hint... hint...