android

Xamarin Forms: White screen between page push and pop solved

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If you are experiencing white screen when pushing to Navigation or Modal stack on Android, read on.

I’m not sure is this a bug in Xamarin Forms or not, but I guess this is, because it comes only in certain scenarios, and not always.

What is happening, how do you notice this error?

You have got a NavigationPage. You are pushing a new page to the navigationstack, and the page is not getting rendered, only a blank white screen displays.

If you are rotating the device, the page is getting rendered well.

My environment was:
Xamarin.Forms: 4.8 up to 5.0
Device: Samsung Galaxy A12
Visual Studio 2019 Professional with Xamarin.Android SDK 11.4.0.5

Solution

Always invoke the INavigation’s methods on the applications Main Thread. UI changes must go always on the UI thread of the application.

Create a class which wraps the INavigation presented by your Views. It’s handy to store a reference in this class to the Applications Current MainPage’s INavigation instance, so try to build your code to supply the actual INavigation Instance every time to this class when the application’s mainpage is set.

	public class NavigationDispatcher : INavigation
	{
		private INavigation _navigation;

		public IReadOnlyList<Page> ModalStack => _navigation?.ModalStack;

		public IReadOnlyList<Page> NavigationStack => _navigation?.NavigationStack;

		private void SetNavigation(INavigation navigation)
		{
			_navigation = navigation;
		}

		public void InsertPageBefore(Page page, Page before)
		{
			_ = Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(() =>
			  {
				  _navigation.InsertPageBefore(page, before);
			  });
		}

		public Task<Page> PopAsync()
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				return await _navigation.PopAsync();
			});
		}

		public Task<Page> PopAsync(bool animated)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				return await _navigation.PopAsync(animated);
			});
		}

		public Task<Page> PopModalAsync()
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				return await _navigation.PopModalAsync();
			});
		}

		public Task<Page> PopModalAsync(bool animated)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				return await _navigation.PopModalAsync(animated);
			});
		}

		public Task PopToRootAsync()
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PopToRootAsync();
			});
		}

		public Task PopToRootAsync(bool animated)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PopToRootAsync(animated);
			});
		}

		public Task PushAsync(Page page)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PushAsync(page);
			});
		}

		public Task PushAsync(Page page, bool animated)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PushAsync(page, animated);
			});
		}

		public Task PushModalAsync(Page page)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PushModalAsync(page);
			});
		}

		public Task PushModalAsync(Page page, bool animated)
		{
			return Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(async () =>
			{
				await _navigation.PushModalAsync(page, animated);
			});
		}

		public void RemovePage(Page page)
		{
			_ = Xamarin.Essentials.MainThread.InvokeOnMainThreadAsync(() =>
			  {
				  _navigation.RemovePage(page);
			  });
		}
	}

Remarks

Consider a check for the current thread in the methods body.
If they are being executed in the main thread, you won’t need to switch to the main again.

Bug is reported on Github: https://github.com/xamarin/Xamarin.Forms/issues/11993

Xamarin Forms: Close applications programmatically

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In some scenarios, we need to programmatically close our application. We can do it in our common code with Xamarin Forms too. Buuuuuut …….

Can we close an iOS application?

There is no API provided for gracefully terminating an iOS application.

In iOS, the user presses the Home button to close applications. Should your application have conditions in which it cannot provide its intended function, the recommended approach is to display an alert for the user that indicates the nature of the problem and possible actions the user could take — turning on WiFi, enabling Location Services, etc. Allow the user to terminate the application at their own discretion.

Do not call the exit function. Applications calling exit will appear to the user to have crashed, rather than performing a graceful termination and animating back to the Home screen.

Source: https://developer.apple.com/library/archive/qa/qa1561/_index.html

Close on Android from Forms code

There is one important place to implement closing apps from code: When you are handling back button presses by yourself.

Imagine, that you have a main screen, which is the root page. When a users presses the hardware back button you should display a confirmer, that says “Do you really want to close the application?”

If the user choose yes, you should make a call like this:

					DependencyService.Get<IPlatformSpecificService>().CloseApplication();

The definition of the IPlatformSpecificService should be look like this:

	public interface IPlatformSpecificService
	{
		void CloseApplication();
	}

And the Android implementation of the IPlatformSpecificService should be something like:

using Plugin.CurrentActivity;

[assembly: Dependency(typeof(DeviceSpecificService))]
namespace AnAwesomeCompany.AGreatProduct.Droid.DependencyServices
{
	public class PlatformSpecificService: IPlatformSpecificService
	{
		public void CloseApplication()
		{
			CrossCurrentActivity.Current.Activity.FinishAndRemoveTask();
		}
	}
}

Activity supports a lot of closing methods of the application. FinishAndRemoveTask will clear the app from the recents list too.

Take a look at official Android documentation at: https://developer.android.com/reference/android/app/Activity#finishAndRemoveTask()

Xamarin.Android: Missing aapt.exe

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When trying to deploy an application to simulator or device, Visual Studio gives the following error:

Cannot find `aapt.exe`. Please install the Android SDK Build-Tools package with the `C:\Program Files (x86)\Android\android-sdk\tools\android.bat` program.

To solve this problem you sould go to Visual Studio’s Tools then Android and select SDK Manager option.

Navigate to this option

It will give an error dialog, that the SDK tool files are corrupted. Click repair.

Click on Repair

The mentioned batch command in the error message is deprecated by the way.

Microsoft Windows [Version 10.0.19042.804]
(c) 2020 Microsoft Corporation. All rights reserved.

C:\Users\banditoth>cd C:\Program Files (x86)\Android\android-sdk\tools\

C:\Program Files (x86)\Android\android-sdk\tools>android.bat
**************************************************************************
The "android" command is deprecated.
For manual SDK, AVD, and project management, please use Android Studio.
For command-line tools, use tools\bin\sdkmanager.bat
and tools\bin\avdmanager.bat
**************************************************************************

Invalid or unsupported command ""

Supported commands are:
android list target
android list avd
android list device
android create avd
android move avd
android delete avd
android list sdk
android update sdk

C:\Program Files (x86)\Android\android-sdk\tools>