How can I implement custom One-Time bindings in MvvmCross

1. Understanding One-Time Bindings

One-time bindings are useful for setting static values that do not change frequently, such as text from language files. They transfer data from the view model to the view once when the binding is set and do not monitor for changes.

2. Creating a Custom Target Binding

To create a custom one-time binding, you need to define a target binding class. This class will inherit from `MvxTargetBinding` or one of its subclasses, depending on your needs. For a simple one-time binding, you can use `MvxConvertingTargetBinding` as a base class.

Here's an example of how you might create a custom one-time binding for a `TextView` in Android:

csharp
public class MyTextViewOneTimeBinding : MvxConvertingTargetBinding
{
    public MyTextViewOneTimeBinding(TextView target) 
        : base(target)
    {
        DefaultMode = MvxBindingMode.OneTime;
    }

    protected override void SetValueImpl(object target, object value)
    {
        var textView = target as TextView;
        if (textView != null)
        {
            textView.Text = (string)value;
        }
    }

    // Since this is one-time, we don't need to subscribe to events
    public override void SubscribeToEvents()
    {
        // No subscription needed for one-time bindings
    }

    public override void Dispose(bool isDisposing)
    {
        base.Dispose(isDisposing);
        // No cleanup needed for one-time bindings
    }
}

3. Registering the Custom Binding

After defining the custom binding, you need to register it in your `Setup.cs` file. This involves overriding the `FillTargetFactories` method to add your custom binding:

csharp
protected override void FillTargetFactories(IMvxTargetBindingFactoryRegistry registry)
{
    base.FillTargetFactories(registry);
    registry.RegisterPropertyInfoBindingFactory(
        typeof(MyTextViewOneTimeBinding),
        typeof(TextView), "Text");
}

However, since you're implementing a custom binding for a specific scenario, you might not need to register it as a property info binding. Instead, you can directly use it in your view bindings.

4. Using the Custom Binding

To use your custom one-time binding in a view, you can bind it like this:

csharp
var textView = FindViewById(Resource.Id.myTextView);
set.Bind(textView).For("Text").To(vm => vm.MyStaticText).OneTime();

However, since MvvmCross does not directly support specifying the binding mode in the fluent API for custom bindings, you might need to adjust your approach slightly. You can use the `DefaultMode` property in your custom binding class to ensure it behaves as a one-time binding.

5. Handling Converters

If you need to use converters with your one-time binding, you can leverage the `MvxConvertingTargetBinding` class, which supports converters. You can implement the `SetValueImpl` method to handle conversions if necessary.

Conclusion

Implementing custom one-time bindings in MvvmCross involves creating a custom target binding class and registering it appropriately. By setting the `DefaultMode` to `MvxBindingMode.OneTime`, you ensure that the binding only updates once when the view is initialized. This approach is useful for static data that does not require continuous updates.