How do I handle errors when binding custom views in MvvmCross

Understanding MvvmCross Data Binding

MvvmCross uses a powerful data binding system that allows you to connect your views to view models. This system is based on the MVVM (Model-View-ViewModel) pattern, where the view model acts as an intermediary between the view and the model. MvvmCross supports both one-way and two-way bindings, allowing data to flow from the view model to the view and vice versa.

Implementing Custom Bindings

When dealing with custom views that MvvmCross does not support out of the box, you need to create custom target bindings. Here's how you can do it:

1. Create a Target Binding Class: You need to create a class that inherits from `MvxConvertingTargetBinding` or one of its subclasses like `MvxPropertyInfoTargetBinding`. This class will define how to bind a specific property of your custom view.

csharp
   public class MyViewMyPropertyTargetBinding : MvxPropertyInfoTargetBinding
   {
       // Implementation details
   }
   

2. Override Necessary Methods: You must override methods like `SetValueImpl` to set the value on your view and `SubscribeToEvents` to listen for changes if you're implementing a two-way binding.

csharp
   protected override void SetValueImpl(object target, object value)
   {
       var view = target as MyView;
       if (view == null) return;
       view.MyProperty = (string)value;
   }

   public override void SubscribeToEvents()
   {
       var myView = View;
       if (myView == null) return;
       myView.MyPropertyChanged += HandleMyPropertyChanged;
   }

   private void HandleMyPropertyChanged(object sender, EventArgs e)
   {
       var myView = View;
       if (myView == null) return;
       FireValueChanged(myView.MyProperty);
   }
   

3. Register the Custom Binding: In your `Setup.cs` file, you need to register your custom binding by overriding the `FillTargetFactories` method.

csharp
   protected override void FillTargetFactories(IMvxTargetBindingFactoryRegistry registry)
   {
       base.FillTargetFactories(registry);
       registry.RegisterPropertyInfoBindingFactory(
           typeof(MyViewMyPropertyTargetBinding),
           typeof(MyView), "MyProperty");
   }
   

Handling Errors

When handling errors in custom bindings, consider the following strategies:

- Check for Null References: Always ensure that the target view and its properties are not null before attempting to set values or subscribe to events.

- Use Try-Catch Blocks: Wrap code that might throw exceptions in try-catch blocks to handle and log errors gracefully.

- Implement Dispose Properly: Ensure that event handlers are properly unsubscribed in the `Dispose` method to prevent memory leaks.

- Use MvvmCross Tracing: MvvmCross provides tracing capabilities that can help diagnose binding issues. Use `MvxBindingTrace.Trace` to log errors or warnings.

- Test Thoroughly: Thoroughly test your custom bindings under different scenarios to catch any potential issues early.

Example of Error Handling

Here's an example of how you might handle errors in a custom binding:

csharp
protected override void SetValueImpl(object target, object value)
{
    try
    {
        var view = target as MyView;
        if (view == null) return;
        view.MyProperty = (string)value;
    }
    catch (Exception ex)
    {
        MvxBindingTrace.Trace(MvxTraceLevel.Error, "Error setting value on MyView", ex);
    }
}

public override void SubscribeToEvents()
{
    try
    {
        var myView = View;
        if (myView == null) return;
        myView.MyPropertyChanged += HandleMyPropertyChanged;
    }
    catch (Exception ex)
    {
        MvxBindingTrace.Trace(MvxTraceLevel.Error, "Error subscribing to events on MyView", ex);
    }
}

By following these guidelines, you can effectively handle errors when binding custom views in MvvmCross, ensuring robust and reliable data binding in your applications.