davidortinau/maui-skills/maui-custom-handlers

.NET MAUI Custom Handlers

Decision: Customize Existing vs. Create New

⚠️ 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.

Mapper Method Selection

PropertyMapper vs. CommandMapper

⚠️ Don't put property sync logic in CommandMapper — it won't re-run when

the property changes, leading to stale native views.

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