MvvmCross supports data binding in views by providing a robust mechanism to link views with their corresponding view models. This is achieved through a combination of fluent syntax for binding setup and the use of target bindings to define how properties are bound between the view and the view model.
Core Concepts
1. Data Binding Modes: MvvmCross supports several data binding modes, including One-Time, One-Way, and Two-Way.
- One-Time binding transfers data from the view model to the view only once, without monitoring changes.
- One-Way binding updates the view when the view model changes, but does not update the view model when the view changes.
- Two-Way binding updates both the view and the view model when either changes, which is the most commonly used mode.
2. Fluent Syntax: MvvmCross uses a fluent syntax to create bindings. This involves specifying the target view property and the source view model property using methods like `For` and `To`. For example:
csharp
var set = this.CreateBindingSet();
set.Bind(someView)
.For(view => view.ViewProperty)
.To(viewModel => viewModel.ViewModelProperty);
set.Apply();
The `Apply` method is crucial as it activates the bindings.
3. Target Bindings: For custom views or properties that require special handling, MvvmCross allows developers to create custom target bindings. These bindings define how to set and get values for a specific view property and how to handle change events. This is particularly useful for views that do not support two-way binding out of the box[4][7].
4. ViewModel and View Association: MvvmCross uses naming conventions or attributes like `MvxViewFor` to associate views with their view models. This simplifies the process of linking views to their respective view models[3][9].
Implementation Details
- C# Properties: In the view model, properties are typically implemented with a private backing field and a public getter and setter. The setter often includes a call to `RaisePropertyChanged` to notify the view of changes[1].
- Custom Bindings: When creating custom bindings, developers need to define a target binding class that inherits from `MvxPropertyInfoTargetBinding` or similar classes. This class specifies how to set and get values for a view property and how to subscribe to change events[4].
- Platform Differences: While the core binding concepts remain consistent across platforms, the implementation details can vary. For example, on iOS, bindings are typically defined in C# code, whereas on Android, both C# and AXML can be used[3].
Benefits and Best Practices
- Separation of Concerns: Data binding helps maintain a clear separation between the view and the view model, making the code more manageable and easier to maintain.
- Consistency: Using data binding consistently across the application ensures that the UI reflects the state of the view model accurately and in real-time.
- Reusability: Custom target bindings can be reused across different views, reducing code duplication and improving maintainability.
Overall, MvvmCross provides a flexible and powerful data binding system that supports complex UI scenarios while maintaining the principles of the MVVM architecture.
Citations:[1] https://www.mvvmcross.com/documentation/fundamentals/data-binding
[2] https://yanxiaodi.gitbook.io/xamarin-mvvmcross-handbook/using-mvvmcross-to-create-your-first-xamarin-app/creating-the-ios-project/understanding-the-data-binding-syntax
[3] https://learn.microsoft.com/en-us/archive/msdn-magazine/2014/august/windows-phone-build-mvvm-apps-with-xamarin-and-mvvmcross
[4] https://www.mvvmcross.com/documentation/advanced/custom-data-binding
[5] https://stackoverflow.com/questions/28798209/mvvmcross-binding-your-datamodel-to-your-viewmodel
[6] https://stackoverflow.com/questions/25460030/when-to-use-mvvmcross-data-bindings
[7] https://blog.ostebaronen.dk/2018/01/mvvmcross-binding-target.html
[8] https://learn.microsoft.com/en-us/dotnet/maui/xaml/fundamentals/mvvm?view=net-maui-9.0
[9] https://www.mvvmcross.com/documentation/tutorials/tipcalc/a-note-about-views-and-viewmodels