Unified iOS TabbedViewHandler

[PureWeen]

Unified iOS TabbedViewHandler

Target: .NET 10

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Goal

Implement TabbedViewHandler for iOS, replacing the legacy TabbedRenderer. Learn from both the existing TabbedRenderer implementation and Shell's tab handling to build a unified handler that both TabbedPage and Shell can use.

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Background

• WinUI already has a shared tab handling approach
• iOS currently uses TabbedRenderer (in Microsoft.Maui.Controls.Handlers.Compatibility)
• TabbedViewHandler exists but throws NotImplementedException on iOS (see issue #29540)
• Shell has its own tab bar implementation that duplicates much of the same logic

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Rollout Strategy

1. Implement TabbedViewHandler for .NET 10

2. Gate behind AppContext switch - Wire TabbedPage to use the new handler when the switch is enabled

3. Switch disabled by default initially - Allows testing and validation without affecting existing apps

4. Enable for TabbedPage and Shell - Once validated, flip the switch to make it the default path

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Guidelines

1. Implement TabbedViewHandler.CreatePlatformView() on iOS - Currently throws NotImplementedException

2. Learn from TabbedRenderer - Extract the best patterns for UITabBarController management, tab icons, selected/unselected colors

3. Learn from Shell tab handling - Incorporate patterns that work well for Shell's tab bar

4. TabbedPage and Shell both use it - Same handler, different consumers

5. Keep handler internal initially - Flexibility to iterate without public API commitment

6. Align with WinUI patterns - Use WinUI's unified approach as the reference model

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Success Criteria

• TabbedViewHandler works on iOS (no more NotImplementedException)
• TabbedPage uses the new handler (when switch enabled)
• Shell can use the same handler for tabs (when switch enabled)
• All existing TabbedPage tests pass with switch enabled
• All existing Shell tab tests pass with switch enabled