View presenters in MvvmCross play a crucial role in managing how view models are presented within an application. They act as intermediaries between the view model layer and the actual UI, allowing for flexible and platform-specific presentation of views.
Role of View Presenters
1. Navigation and Presentation: View presenters handle requests from view models to navigate to new views or change the presentation of existing ones. They decide how to display these views based on the platform's capabilities and the application's requirements[1][2].
2. Platform-Specific Implementation: Since presenters deal directly with the view layer, they are inherently platform-specific. This allows developers to customize the presentation based on the device type, such as using tabs on iOS and Android but a pivot view on Windows Phone[1].
3. Customization and Flexibility: Presenters enable developers to implement different presentation strategies for various screen sizes or orientations. For example, on a small phone screen, a list and details might be shown in separate screens, while on a tablet, they could be displayed side-by-side using a split view[1].
4. Use of Presentation Values: Presenters can receive additional information through `PresentationValues`, which are part of the `MvxViewModelRequest`. This allows view models to pass hints to the presenter about how the view should be presented, such as clearing the navigation stack after a login operation[7].
5. Default and Custom Presenters: MvvmCross provides default presenters for each platform, but developers can also create custom presenters to meet specific application needs. Custom presenters can override default behavior to implement unique navigation patterns or presentation styles[2][4].
Key Methods of View Presenters
- `Show(MvxViewModelRequest request)`: This method is called when `ShowViewModel()` is invoked from a view model. It constructs and displays the corresponding view based on the request details[1].
- `ChangePresentation(MvxPresentationHint hint)`: This method allows for presentation changes that don't involve showing a new view model, such as closing a view or changing its presentation mode[1].
Benefits of Using View Presenters
- Decoupling: Presenters help decouple the view model logic from the UI presentation, allowing for cleaner and more maintainable code.
- Cross-Platform Consistency: Despite platform-specific implementations, presenters ensure that view models behave consistently across different platforms.
- Customization: They provide a flexible way to adapt the UI to different devices and screen sizes without altering the view model logic.
In summary, view presenters in MvvmCross are essential for managing how views are presented across different platforms, offering flexibility, customization, and a clean separation of concerns between the view model and UI layers.
Citations:[1] https://gregshackles.com/presenters-in-mvvmcross-a-primer/
[2] https://www.mvvmcross.com/documentation/getting-started/mvvmcross-overview
[3] https://learn.microsoft.com/en-us/archive/msdn-magazine/2011/december/mvpvm-design-pattern-the-model-view-presenter-viewmodel-design-pattern-for-wpf
[4] https://www.mvvmcross.com/mvvmcross-5-release/
[5] https://www.mvvmcross.com/documentation/platform/ios/ios-view-presenter
[6] https://softwareengineering.stackexchange.com/questions/60774/model-view-presenter-implementation-thoughts
[7] https://gregshackles.com/presenters-in-mvvmcross-using-presentation-values/
[8] https://www.mvvmcross.com/documentation/platform/xamarin.forms/xamarin-forms-view-presenter
[9] https://www.gatling.xyz/explaining-model-view-presenter/
[10] https://www.mvvmcross.com/documentation/platform/android/android-view-presenter