Xamarin.Forms: Reopening application best pratices

If you have experienced that reopening your application from the phone’s app drawer or launcher is fails because it crashes your application, or displays the first set main page again instead of displaying the latest one, keep reading this article.

If you have crashes, or malfunctions, you are probably initializing something in the constructor of the App.xaml.cs, or in the overridden method called ‘OnStart’ like this:

        public App()
        {
            InitializeComponent();
            InitalizeOnlyOnceClass.Initalize();
        }

        protected override void OnStart()
        {
            AnAnotherInitalizeOnlyOnceClass.Initalize();
        }

Imaginary InitalizeOnlyOnceClass’s Initialize method can be called only once, for the second call, it throws an exception. After you have started your program, sent to background, and bringing it back throws the second Initalization’s exception. This is because the App class gets constructed again when reopening application from the drawer.

To handle this, you should make a boolean for the application’s initialization progress.

        private static bool isInitalized;

        public App()
        {
            InitializeComponent();

            if(isInitalized == false)
            {
                InitalizeOnlyOnceClass.Initalize();
            }
        }

If you receive a blank white screen, you are probably forget to set the Main Page

Make sure, that the Application.Current.MainPage always gets a value.
If you have implemented the Initialization by your self, or an another way, make sure you have handled correctly the else statement also.
Even if the application has been initialized once, the Application.Current.MainPage have to be set always when reconstructing the App.

Continue with the last page opened in the application

If you want to continue always with the last page opened, you need to store the last page always, when you are navigating from one to an another.

You can store the last page with making a class used for navigation, like this:

    public static class SimpleNavigationLogic
    {
        private static Xamarin.Forms.Page lastNavigatedPage;

        public static void ChangeMainPage(Xamarin.Forms.Page pageToSet)
        {
            Xamarin.Forms.Device.BeginInvokeOnMainThread(() =>
            {
                Application.Current.MainPage = pageToSet;
            });
            lastNavigatedPage = pageToSet;
        }

        public static void RestoreMainPage()
        {
            Xamarin.Forms.Device.BeginInvokeOnMainThread(() =>
            {
                Application.Current.MainPage = lastNavigatedPage;
            });
        }
    }

Than you can make the initialization for your application like this:

        public App()
        {
            InitializeComponent();

            if(!isInitalized)
            {
                InitalizeOnlyOnceClass.Initalize();
                SimpleNavigationLogic.ChangeMainPage(new AwesomeMainPage());
            }
            else
            {
                SimpleNavigationLogic.RestoreMainPage();
            }
        }

Xamarin: How .NET MAUI will change the life of Xamarin.Forms development

.NET 6 and .NET MAUI is on the sill, but how will it affect the development of Xamarin applications?

For today, I have brought you a video by James Montemagno, a Xamarin developer working at Microsoft.

Follow James on the following platforms:

🌱 https://montemagno.com
🎙 https://mergeconflict.fm, https://blunders.fm, https://nintendodispatch.com
🦜 https://twitter.com/jamesmontemagno
🎥 https://youtube.com/jamesmontemagno

Xamarin.Android : Google Play app target level error solution

If you are uploading an application to the Google Play, and it gives the following error:

Your app currently targets API level 28 and must target at least API  level 29 to ensure it is built on the latest APIs optimized for security  and performance. Change your app's target API level to at least 29 

You have to change the target framework and the target android version in your androidmanifest.xml or in the UI editor of the manifest. To change it, set the Target Framework in the properties tab of your android application

Changing target framework

Then Navigate to Android Manifest option, and set target android version

Setting Target Android version

If you are re archive your application, it will upload successfully.

Xamarin.Forms: Bindable Property snippet for Visual Studio

Use bprop tab tab to generate commands in ViewModels with this snippet

The result of the snippet

Save the snippet file as {name}.snippet in Visual Studio’s folder: C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC#\Snippets\1033\Visual C#

<?xml version="1.0" encoding="utf-8" ?>
<CodeSnippets  xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
	<CodeSnippet Format="1.0.0">
		<Header>
			<Title>Bindable Property</Title>
			<Shortcut>bprop</Shortcut>
			<Description>Xamarin BindableProperty declaration code snippet</Description>
			<Author>banditoth.hu</Author>
			<SnippetTypes>
				<SnippetType>Expansion</SnippetType>
			</SnippetTypes>
		</Header>
		<Snippet>
			<Declarations>
				<Literal>
					<ID>PropertyName</ID>
					<ToolTip>The property's name</ToolTip>
					<Default>Foo</Default>
				</Literal>
				<Literal>
					<ID>Type</ID>
					<ToolTip>The type of the property</ToolTip>
					<Default>object</Default>
				</Literal>
				<Literal>
					<ID>DefaultValue</ID>
					<ToolTip>The default value of the property</ToolTip>
					<Default>null</Default>
				</Literal>
				<Literal>
					<ID>BindingMode</ID>
					<ToolTip>The binding mode of the property</ToolTip>
					<Default>TwoWay</Default>
				</Literal>
				<Literal>
					<ID>PropertyChangedHandler</ID>
					<ToolTip>The property changed handler method</ToolTip>
					<Default>null</Default>
				</Literal>
				<Literal>
					<ID>PropertyChangingHandler</ID>
					<ToolTip>The property changing handler method</ToolTip>
					<Default>null</Default>
				</Literal>
			</Declarations>
			<Code Language="csharp">
				<![CDATA[
        public static readonly BindableProperty $PropertyName$Property = BindableProperty.Create(
                                        propertyName: nameof($PropertyName$),
                                        returnType: typeof($Type$),
                                        declaringType: typeof(View),
                                        defaultValue: $DefaultValue$,
                                        defaultBindingMode: BindingMode.$BindingMode$,
                                        propertyChanged: $PropertyChangedHandler$,
					propertyChanging: $PropertyChangingHandler$);

        public $Type$ $PropertyName$
        {
            get { return ($Type$)GetValue($PropertyName$Property); }
            set { SetValue($PropertyName$Property, value); }
        }
		$end$]]>
			</Code>
		</Snippet>
	</CodeSnippet>
</CodeSnippets>

Xamarin.Forms: Lazy loaded Command snippet for Visual Studio

Use xamcomm tab tab to generate commands in ViewModels with this snippet

IntelliSense recommendation

Snippet in work

Save the snippet file as {name}.snippet in Visual Studio’s folder: C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC#\Snippets\1033\Visual C#

<?xml version="1.0" encoding="utf-8" ?>
<CodeSnippets  xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
	<CodeSnippet Format="1.0.0">
		<Header>
			<Title>Xamarin Lazy Command</Title>
			<Shortcut>xamcomm</Shortcut>
			<Description>Xamarin Command declaration code snippet for MVVM design pattern</Description>
			<Author>banditoth.hu</Author>
			<SnippetTypes>
				<SnippetType>Expansion</SnippetType>
			</SnippetTypes>
		</Header>
		<Snippet>
			<Declarations>
				<Literal>
					<ID>BackFieldName</ID>
					<ToolTip>Backfield Name</ToolTip>
					<Default>_backfieldname</Default>
				</Literal>
				<Literal>
					<ID>CommandName</ID>
					<ToolTip>Command name</ToolTip>
					<Default>CommandName</Default>
				</Literal>
				<Literal>
					<ID>ActionToExecute</ID>
					<ToolTip>Action to execute</ToolTip>
					<Default>() => { return; /*TODO: Implement logic for this Command*/ }</Default>
				</Literal>
        <Literal>
          <ID>ActionCanExecute</ID>
          <ToolTip>Action to determine can execute</ToolTip>
          <Default>() => true</Default>
        </Literal>
			</Declarations>
			<Code Language="csharp">
			<![CDATA[
	private Command $BackFieldName$Command;

	public Command $CommandName$Command
	{
		get { return $BackFieldName$Command ?? ($BackFieldName$Command = new Command($ActionToExecute$,$ActionCanExecute$)); }
	}
	$end$]]>
			</Code>
		</Snippet>
	</CodeSnippet>
</CodeSnippets>