maui-custom-handlers

name

maui-custom-handlers

description

Implement/migrate MAUI visual handlers. USE FOR: `EntryHandler.Mapper.AppendToMapping`, scoped `BorderlessEntry` type guards, unique mapper keys, repeated mapper callbacks, renderer-to-handler migration, `ConnectHandler`/`DisconnectHandler` cleanup, `PropertyMapper`, `CommandMapper`, custom `ViewHandler`, platform partial handlers, conflicting partial base classes, `CreatePlatformView`, native namespace leaks, `ConfigureMauiHandlers`. DO NOT USE FOR: non-visual platform APIs, full Xamarin migration, native SDK bindings, or backend code.

MAUI Custom Handlers

Use this skill when a MAUI visual element needs native platform behavior. Keep handlers focused on views. Route non-visual APIs to platform services and route large native SDK surfaces to slim bindings.

Response Checklist

• Use handler language explicitly: AppendToMapping, PropertyMapper, CommandMapper, and ViewHandler.
• For renderer migration, map subscription/setup to ConnectHandler and cleanup to DisconnectHandler.
• Keep mapper changes scoped to the intended control type, not all controls.

Decision Tree

Need	Prefer
Tweak an existing control for the whole app	EntryHandler.Mapper.AppendToMapping in MauiProgram.cs (or the matching concrete control handler mapper)
Tweak only specific control instances	Subclass the control and guard mapper logic with if (view is MyControl)
Add a reusable cross-platform control	Custom ViewHandler<TVirtualView,TPlatformView>
Map bindable properties to native properties	PropertyMapper entries
Invoke actions such as play/pause/scroll	CommandMapper entries
Use camera, sensors, payment, health, or background APIs	Platform service through DI
Wrap a third-party Android/iOS/macOS SDK	Slim binding plus a platform service facade

Workflow

1. Inspect the target frameworks and existing UI architecture.

2. Define the cross-platform view API first: bindable properties, commands, and events that make sense to app code.

3. Choose global mapper customization, subclass-scoped mapper customization, or a full custom handler.

4. Put native implementation in partial handler files or #if guarded blocks for the exact platforms supported by the project.

5. Register handlers in MauiProgram.cs:

   builder.ConfigureMauiHandlers(handlers =>
   {
       handlers.AddHandler<CameraPreview, CameraPreviewHandler>();
   });
   

6. Use ConnectHandler to subscribe native events and allocate native resources.

7. Use DisconnectHandler to unsubscribe, stop timers, clear delegates, and dispose only native objects owned by the handler. Call base.DisconnectHandler(platformView) as the final statement so the base handler clears its own state.

8. Build each targeted platform and verify the behavior with UI automation or DevFlow when available.

Scoped Mapper Customization

For an existing MAUI control, avoid an unscoped global mapper when only one control instance should change:

public class BorderlessEntry : Entry
{
}

EntryHandler.Mapper.AppendToMapping("Borderless", (handler, view) =>
{
    if (view is not BorderlessEntry)
        return;

#if ANDROID
    handler.PlatformView.Background = null;
#elif IOS || MACCATALYST
    handler.PlatformView.BorderStyle = UIKit.UITextBorderStyle.None;
#elif WINDOWS
    handler.PlatformView.BorderThickness = new Microsoft.UI.Xaml.Thickness(0);
#endif
});

Use a unique mapping key. Do not repeatedly append the same mapping from page constructors; register once during app startup or from a guarded initialization path.

Custom Handler Shape

public class MeterView : View
{
    public static readonly BindableProperty ValueProperty =
        BindableProperty.Create(nameof(Value), typeof(double), typeof(MeterView), 0d);

    public double Value
    {
        get => (double)GetValue(ValueProperty);
        set => SetValue(ValueProperty, value);
    }
}

public partial class MeterViewHandler
{
    public static readonly IPropertyMapper<MeterView, MeterViewHandler> Mapper =
        new PropertyMapper<MeterView, MeterViewHandler>(ViewHandler.ViewMapper)
        {
            [nameof(MeterView.Value)] = MapValue
        };

    public MeterViewHandler() : base(Mapper)
    {
    }

    public static partial void MapValue(MeterViewHandler handler, MeterView view);
}

Implement CreatePlatformView, ConnectHandler, DisconnectHandler, and mapping partials per platform. Put the ViewHandler<TVirtualView, TPlatformView> base class on the platform partial so native types stay out of shared files:

// Platforms/Android/MeterViewHandler.android.cs
public partial class MeterViewHandler : ViewHandler<MeterView, Android.Widget.FrameLayout>
{
    protected override Android.Widget.FrameLayout CreatePlatformView() => new(Context);

    public static partial void MapValue(MeterViewHandler handler, MeterView view)
    {
        // Update handler.PlatformView from view.Value.
    }
}

Property and Command Mappers

• Use PropertyMapper for state that should update when a bindable property changes.
• Use CommandMapper for imperative requests such as Play, Pause, ScrollTo, or Reload.
• Mapper methods should be idempotent. They may be called more than once.
• Validate handler.PlatformView and handler.VirtualView assumptions through types, not broad try/catch blocks.

Renderer Migration Notes

When replacing Xamarin.Forms renderers:

1. Move renderer logic that creates native controls into CreatePlatformView.
2. Move OnElementChanged subscriptions into ConnectHandler.
3. Move renderer cleanup into DisconnectHandler, and end with base.DisconnectHandler(platformView).
4. Replace OnElementPropertyChanged switch statements with PropertyMapper entries.
5. Replace renderer actions with CommandMapper entries or view methods that call Handler?.Invoke.

Validation Checklist

• The handler is registered in one startup location.
• Mapper keys are unique and scoped when customization should not be global.
• Native event subscriptions are unsubscribed in DisconnectHandler.
• No platform namespace leaks into shared code unintentionally.
• The implementation builds for every target framework it is included in.
• Non-visual SDK work is not hidden in a handler.