maui-custom-handlers

name

maui-custom-handlers

description

Guide for creating custom .NET MAUI handlers, customizing existing handlers with property mappers, and implementing platform-specific native views. Covers PrependToMapping/ModifyMapping/AppendToMapping, PropertyMapper, CommandMapper, partial handler classes, and handler registration. USE FOR: "custom handler", "PropertyMapper", "AppendToMapping", "PrependToMapping", "ModifyMapping", "CommandMapper", "platform-specific rendering", "native view", "handler registration", "custom control renderer". DO NOT USE FOR: platform API calls without custom controls (use maui-platform-invoke), data binding (use maui-data-binding), or gesture handling (use maui-gestures).

.NET MAUI Custom Handlers

Decision: Customize Existing vs. Create New

Scenario	Approach
Change how a built-in control looks/behaves on one platform	Customize — use AppendToMapping / PrependToMapping
Need the change on only some instances of a control	Customize — subclass the control + type-check in mapper
Need a completely new cross-platform control with native backing	Create new handler with partial classes

⚠️ Prefer AppendToMapping over ModifyMapping. ModifyMapping replaces the default mapper action entirely — if the framework adds behaviour in a future release, your override silently drops it.

---

Gotchas & Common Mistakes

Mapper customizations are global

Every instance of the control is affected. Guard with a subclass check for instance-specific behaviour:

// ❌ Removes borders from EVERY Entry in the app
EntryHandler.Mapper.AppendToMapping("NoBorder", (handler, view) =>
{
#if ANDROID
    handler.PlatformView.Background = null;
#endif
});

// ✅ Only affects BorderlessEntry instances
EntryHandler.Mapper.AppendToMapping("NoBorder", (handler, view) =>
{
    if (view is not BorderlessEntry) return;
#if ANDROID
    handler.PlatformView.Background = null;
#endif
});

Unsubscribe native events in HandlerChanging

Failing to remove native event handlers causes memory leaks because the native view may outlive the managed wrapper.

// ❌ Subscribes but never unsubscribes — leaks
entry.HandlerChanged += (s, e) =>
{
#if ANDROID
    ((Entry)s!).Handler!.PlatformView.As<Android.Widget.EditText>()!
        .FocusChange += OnNativeFocusChange;
#endif
};

// ✅ Pair subscribe in HandlerChanged with unsubscribe in HandlerChanging
entry.HandlerChanged += OnHandlerChanged;
entry.HandlerChanging += OnHandlerChanging;

Partial class name/namespace mismatch

Namespace and class name must match exactly across the shared handler file and every platform file. A mismatch silently creates separate classes — no compiler error, just a handler that does nothing on that platform.

Conditional using placement

The using PlatformView = ... aliases must be at the top of the shared handler file (not the platform files) so the ViewHandler<TControl, TPlatformView> base-class generic resolves correctly per platform.

// ✅ Top of Handlers/VideoPlayerHandler.cs
#if ANDROID
using PlatformView = Android.Widget.VideoView;
#elif IOS || MACCATALYST
using PlatformView = AVKit.AVPlayerViewController;
#elif WINDOWS
using PlatformView = Microsoft.UI.Xaml.Controls.MediaPlayerElement;
#endif

Missing CreatePlatformView()

Each platform partial must override CreatePlatformView(). Omitting it produces a compile error — but the error message points at the base class, not your handler, making it confusing to debug.

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Mapper Method Selection

Method	Risk	Use when
AppendToMapping	Low — runs after default	Adding behaviour without breaking defaults
PrependToMapping	Low — runs before default	Setting initial state that the default can override
ModifyMapping	⚠️ High — replaces default	You intentionally want to suppress the framework's mapper logic

---

PropertyMapper vs. CommandMapper

Mapper	Purpose	Pattern
PropertyMapper	Sync a bindable property to the native view	Runs whenever the property value changes
CommandMapper	Fire-and-forget action from control → handler	Runs once per invocation, no return value

⚠️ Don't put property sync logic in CommandMapper — it won't re-run when the property changes, leading to stale native views.

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Checklist — New Handler

• Cross-platform control inherits View (or appropriate base)
• Shared handler file has conditional using PlatformView = ... aliases
• Handler inherits ViewHandler<TControl, PlatformView>
• PropertyMapper maps every bindable property
• Each platform partial overrides CreatePlatformView()
• Namespace + class name identical across all partial files
• Handler registered in MauiProgram.cs via ConfigureMauiHandlers
• Native event subscriptions cleaned up in HandlerChanging