Migrate a Xamarin.Forms custom renderer to a .NET MAUI handler

Article
12/20/2023

In Xamarin.Forms, custom renderers can be used to customize the appearance and behavior of a control, and create new cross-platform controls. Each custom renderer has a reference to the cross-platform control and often relies on INotifyPropertyChanged to send property change notifications. Rather than using custom renderers, .NET Multi-platform App UI (.NET MAUI) introduces a new concept called a handler.

Handlers offer many performance improvements over custom renderers. In Xamarin.Forms, the ViewRenderer class creates a parent element. For example, on Android, a ViewGroup is created which is used for auxiliary positioning tasks. In .NET MAUI, the ViewHandler class doesn't create a parent element, which helps to reduce the size of the visual hierarchy and improve your app's performance. Handlers also decouple platform controls from the framework. The platform control only needs to handle the needs of the framework. This is not only more efficient, but it's much easier to extend or override when required. Handlers are also suitable for reuse by other frameworks such as Comet and Fabulous. For more information about handlers, see Handlers.

In Xamarin.Forms, the OnElementChanged method in a custom renderer creates the platform control, initializes default values, subscribes to events, and handles the Xamarin.Forms element the renderer was attached to (OldElement) and the element that the renderer is attached to (NewElement). In addition, a single OnElementPropertyChanged method defines the operations to invoke when a property change occurs in the cross-platform control. .NET MAUI simplifies this approach, so that every property change is handled by a separate method, and so that code to create the platform control, perform control setup, and perform control cleanup, is separated into distinct methods.

The process for migrating a Xamarin.Forms custom control that's backed by custom renderers on each platform to a .NET MAUI custom control that's backed by a handler on each platform is as follows:

Create a class for the cross-platform control, which provides the control's public API. For more information, see Create the cross-platform control.
Create a partial handler class. For more information, see Create the handler.
In the handler class, create a PropertyMapper dictionary, which defines the Actions to take when cross-platform property changes occur. For more information, see Create the property mapper.
Create partial handler classes for each platform that create the native views that implement the cross-platform control. For more information, see Create the platform controls.
Register the handler using the ConfigureMauiHandlers and AddHandler methods in your app's MauiProgram class. For more information, see Register the handler.

Then, the cross-platform control can be consumed. For more information, see Consume the cross-platform control.

Alternatively, custom renderers that customize Xamarin.Forms controls can be converted so that they modify .NET MAUI handlers. For more information, see Customize controls with handlers.

Create the cross-platform control

To create a cross-platform control, you should create a class that derives from View:

namespace MyMauiControl.Controls
{
    public class CustomEntry : View
    {
        public static readonly BindableProperty TextProperty =
            BindableProperty.Create(nameof(Text), typeof(string), typeof(CustomEntry), null);

        public static readonly BindableProperty TextColorProperty =
            BindableProperty.Create(nameof(TextColor), typeof(Color), typeof(CustomEntry), null);

        public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set { SetValue(TextProperty, value); }
        }

        public Color TextColor
        {
            get { return (Color)GetValue(TextColorProperty); }
            set { SetValue(TextColorProperty, value); }
        }
    }
}

The control should provide a public API that will be accessed by its handler, and control consumers. Cross-platform controls should derive from View, which represents a visual element that's used to place layouts and views on the screen.

Create the handler

After creating your cross-platform control, you should create a partial class for your handler:

#if IOS || MACCATALYST
using PlatformView = Microsoft.Maui.Platform.MauiTextField;
#elif ANDROID
using PlatformView = AndroidX.AppCompat.Widget.AppCompatEditText;
#elif WINDOWS
using PlatformView = Microsoft.UI.Xaml.Controls.TextBox;
#elif (NETSTANDARD || !PLATFORM) || (NET6_0_OR_GREATER && !IOS && !ANDROID)
using PlatformView = System.Object;
#endif
using MyMauiControl.Controls;
using Microsoft.Maui.Handlers;

namespace MyMauiControl.Handlers
{
    public partial class CustomEntryHandler
    {
    }
}

The handler class is a partial class whose implementation will be completed on each platform with an additional partial class.

The conditional using statements define the PlatformView type on each platform. The final conditional using statement defines PlatformView to be equal to System.Object. This is necessary so that the PlatformView type can be used within the handler for usage across all platforms. The alternative would be to have to define the PlatformView property once per platform, using conditional compilation.

Create the property mapper

Each handler typically provides a property mapper, which defines what Actions to take when a property change occurs in the cross-platform control. The PropertyMapper type is a Dictionary that maps the cross-platform control's properties to their associated Actions.

Note

The property mapper is the replacement for the OnElementPropertyChanged method in a Xamarin.Forms custom renderer.

PropertyMapper is defined in .NET MAUI's generic ViewHandler class, and requires two generic arguments to be supplied:

The class for the cross-platform control, which derives from View.
The class for the handler.

The following code example shows the CustomEntryHandler class extended with the PropertyMapper definition:

public partial class CustomEntryHandler
{
    public static PropertyMapper<CustomEntry, CustomEntryHandler> PropertyMapper = new PropertyMapper<CustomEntry, CustomEntryHandler>(ViewHandler.ViewMapper)
    {
        [nameof(CustomEntry.Text)] = MapText,
        [nameof(CustomEntry.TextColor)] = MapTextColor
    };

    public CustomEntryHandler() : base(PropertyMapper)
    {
    }
}

The PropertyMapper is a Dictionary whose key is a string and whose value is a generic Action. The string represents the cross-platform control's property name, and the Action represents a static method that requires the handler and cross-platform control as arguments. For example, the signature of the MapText method is public static void MapText(CustomEntryHandler handler, CustomEntry view).

Each platform handler must provide implementations of the Actions, which manipulate the native view APIs. This ensures that when a property is set on a cross-platform control, the underlying native view will be updated as required. The advantage of this approach is that it allows for easy cross-platform control customization, because the property mapper can be modified by cross-platform control consumers without subclassing. For more information, see Customize controls with handlers.

Create the platform controls

After creating the mappers for your handler, you must provide handler implementations on all platforms. This can be accomplished by adding partial class handler implementations in the child folders of the Platforms folder. Alternatively you could configure your project to support filename-based multi-targeting, or folder-based multi-targeting, or both.

Filename-based multi-targeting is configured by adding the following XML to the project file, as children of the <Project> node:

<!-- Android -->
<ItemGroup Condition="$(TargetFramework.StartsWith('net8.0-android')) != true">
  <Compile Remove="**\*.Android.cs" />
  <None Include="**\*.Android.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
</ItemGroup>

<!-- iOS and Mac Catalyst -->
<ItemGroup Condition="$(TargetFramework.StartsWith('net8.0-ios')) != true AND $(TargetFramework.StartsWith('net8.0-maccatalyst')) != true">
  <Compile Remove="**\*.MaciOS.cs" />
  <None Include="**\*.MaciOS.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
</ItemGroup>

<!-- Windows -->
<ItemGroup Condition="$(TargetFramework.Contains('-windows')) != true ">
  <Compile Remove="**\*.Windows.cs" />
  <None Include="**\*.Windows.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
</ItemGroup>

For more information about configuring multi-targeting, see Configure multi-targeting.

Each platform handler class should be a partial class and derive from the generic ViewHandler class, which requires two type arguments:

The class for the cross-platform control, which derives from View.
The type of the native view that implements the cross-platform control on the platform. This should be identical to the type of the PlatformView property in the handler.

Important

The ViewHandler class provides VirtualView and PlatformView properties. The VirtualView property is used to access the cross-platform control from its handler. The PlatformView property, is used to access the native view on each platform that implements the cross-platform control.

Each of the platform handler implementations should override the following methods:

CreatePlatformView, which should create and return the native view that implements the cross-platform control.
ConnectHandler, which should perform any native view setup, such as initializing the native view and performing event subscriptions.
DisconnectHandler, which should perform any native view cleanup, such as unsubscribing from events and disposing objects. This method is intentionally not invoked by .NET MAUI. Instead, you must invoke it yourself from a suitable location in your app's lifecycle. For more information, see Native view cleanup.

Note

The CreatePlatformView, ConnectHandler, and DisconnectHandler overrides are the replacements for the OnElementChanged method in a Xamarin.Forms custom renderer.

Each platform handler should also implement the Actions that are defined in the mapper dictionaries. In addition, each platform handler should also provide code, as required, to implement the functionality of the cross-platform control on the platform. Alternatively, for more complex controls this can be provided by an additional type.

The following example shows the CustomEntryHandler implementation on Android:

#nullable enable
using AndroidX.AppCompat.Widget;
using Microsoft.Maui.Handlers;
using Microsoft.Maui.Platform;
using MyMauiControl.Controls;

namespace MyMauiControl.Handlers
{
    public partial class CustomEntryHandler : ViewHandler<CustomEntry, AppCompatEditText>
    {
        protected override AppCompatEditText CreatePlatformView() => new AppCompatEditText(Context);

        protected override void ConnectHandler(AppCompatEditText platformView)
        {
            base.ConnectHandler(platformView);

            // Perform any control setup here
        }

        protected override void DisconnectHandler(AppCompatEditText platformView)
        {
            // Perform any native view cleanup here
            platformView.Dispose();
            base.DisconnectHandler(platformView);
        }

        public static void MapText(CustomEntryHandler handler, CustomEntry view)
        {
            handler.PlatformView.Text = view.Text;
            handler.PlatformView?.SetSelection(handler.PlatformView?.Text?.Length ?? 0);
        }

        public static void MapTextColor(CustomEntryHandler handler, CustomEntry view)
        {
            handler.PlatformView?.SetTextColor(view.TextColor.ToPlatform());
        }
    }
}

CustomEntryHandler derives from the ViewHandler class, with the generic CustomEntry argument specifying the cross-platform control type, and the AppCompatEditText argument specifying the type of native control.

The CreatePlatformView override creates and returns an AppCompatEditText object. The ConnectHandler override is the location to perform any required native view setup. The DisconnectHandler override is the location to perform any native view cleanup, and so calls the Dispose method on the AppCompatEditText instance.

The handler also implements the Actions defined in the property mapper dictionary. Each Action is executed in response to a property changing on the cross-platform control, and is a static method that requires handler and cross-platform control instances as arguments. In each case, the Action calls methods defined on the native control.

Register the handler

A custom control and its handler must be registered with an app, before it can be consumed. This should occur in the CreateMauiApp method in the MauiProgram class in your app project, which is the cross-platform entry point for the app:

using Microsoft.Extensions.Logging;
using MyMauiControl.Controls;
using MyMauiControl.Handlers;

namespace MyMauiControl;

public static class MauiProgram
{
  public static MauiApp CreateMauiApp()
  {
    var builder = MauiApp.CreateBuilder();
    builder
      .UseMauiApp<App>()
      .ConfigureFonts(fonts =>
      {
        fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
        fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
      })
      .ConfigureMauiHandlers(handlers =>
      {
        handlers.AddHandler(typeof(CustomEntry), typeof(CustomEntryHandler));
      });

#if DEBUG
    builder.Logging.AddDebug();
#endif

    return builder.Build();
  }
}

The handler is registered with the ConfigureMauiHandlers and AddHandler method. The first argument to the AddHandler method is the cross-platform control type, with the second argument being its handler type.

Note

This registration approach avoids Xamarin.Forms' assembly scanning, which is slow and expensive.

Consume the cross-platform control

After registering the handler with your app, the cross-platform control can then be consumed:

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="clr-namespace:MyMauiControl.Controls"
             x:Class="MyMauiControl.MainPage">
    <Grid>
        <controls:CustomEntry Text="Hello world"
                              TextColor="Blue" />
    </Grid>
</ContentPage>

Native view cleanup

Each platform's handler implementation overrides the DisconnectHandler implementation, which is used to perform native view cleanup such as unsubscribing from events and disposing objects. However, this override is intentionally not invoked by .NET MAUI. Instead, you must invoke it yourself from a suitable location in your app's lifecycle. This could be when the page containing the control is navigated away from, which causes the page's Unloaded event to be raised.

An event handler for the page's Unloaded event can be registered in XAML:

<ContentPage ...
             xmlns:controls="clr-namespace:MyMauiControl.Controls"
             Unloaded="ContentPage_Unloaded">
    <Grid>
        <controls:CustomEntry x:Name="customEntry"
                              ... />
    </Grid>
</ContentPage>

The event handler for the Unloaded event can then invoke the DisconnectHandler method on its Handler instance:

void ContentPage_Unloaded(object sender, EventArgs e)
{
    customEntry.Handler?.DisconnectHandler();
}

Migrate a Xamarin.Forms custom renderer to a .NET MAUI handler

Create the cross-platform control

Create the handler

Create the property mapper

Create the platform controls

Register the handler

Consume the cross-platform control

Native view cleanup

See also

Feedback

Additional resources