--- 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: ```csharp // ❌ 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. ```csharp // ❌ Subscribes but never unsubscribes — leaks entry.HandlerChanged += (s, e) => { #if ANDROID ((Entry)s!).Handler!.PlatformView.As()! .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` base-class generic resolves correctly per platform. ```csharp // ✅ 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 | 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. --- ## Checklist — New Handler - [ ] Cross-platform control inherits `View` (or appropriate base) - [ ] Shared handler file has conditional `using PlatformView = ...` aliases - [ ] Handler inherits `ViewHandler` - [ ] `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` --- Source attribution: davidortinau Original project: https://github.com/davidortinau/maui-skills/blob/79bb4bfd8f671ed0dd97bdf92a36430928e8d984/plugins/maui-skills/skills/maui-custom-handlers/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice MIT License Copyright (c) 2026 David Ortinau Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Mega AI Hub resource metadata Creator / publisher: davidortinau Original project: https://github.com/davidortinau/maui-skills/blob/79bb4bfd8f671ed0dd97bdf92a36430928e8d984/plugins/maui-skills/skills/maui-custom-handlers/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Product Management Roles: Frontend Developer, Mobile Developer, Product Manager Specialties: API, Frontend, Mobile, Remotion Tools: .Net Maui Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 3e600b8fbe3ae5df0cc9cac7a7425ce63962eb5c1127f7075f7dc8dd004a7f73