How do you implement One-Time binding in MvvmCross for language files

Understanding One-Time Binding

In MvvmCross, OneTime binding transfers values from the ViewModel to the View only once, when the binding source is set. It doesn't monitor changes and doesn't perform any updates unless the binding source itself is reset[2]. This makes it ideal for language files, as once a language is selected, it's uncommon for the user to change it frequently.

Steps to Implement One-Time Binding for Language Files

1. Prepare Language Files: Use `.resx` files to store your localized text. These files should be placed in the appropriate folders for each language (e.g., `Resources/en-US/Strings.resx` for English).

2. Access Language Files in ViewModel: You can access the localized text in your ViewModel by using the `IMvxLanguageBinder` or directly accessing the resource files. However, for one-time binding, you typically want to load the text once and store it in a property.

3. Create a Property for the Localized Text: In your ViewModel, create a property that holds the localized text. This property will be the source for your one-time binding.

csharp
    public class MyViewModel : MvxViewModel
    {
        private string _localizedText;
        
        public string LocalizedText
        {
            get => _localizedText;
            set => SetProperty(ref _localizedText, value);
        }

        public MyViewModel()
        {
            // Load localized text from resources
            LocalizedText = Mvx.IoCProvider.Resolve().GetLocalizedText("MyLocalizedString");
        }
    }
    

4. Bind the Property in the View: In your View (e.g., an Android layout file), you can bind the `LocalizedText` property using the OneTime binding mode. However, MvvmCross doesn't directly support setting the binding mode to OneTime in XML for Android views. Instead, you can achieve similar behavior by ensuring that the binding is only updated once when the ViewModel is loaded.

For Android, you might need to create a custom binding or handle it programmatically. Here's an example of how you might bind it programmatically:

csharp
    public class MyView : MvxActivity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.MyLayout);

            var textView = FindViewById(Resource.Id.myTextView);
            var viewModel = ViewModel as MyViewModel;

            // One-time binding equivalent
            textView.Text = viewModel.LocalizedText;
        }
    }
    

For platforms like iOS or UWP where you use XAML, you can set the binding mode directly in XAML if you're using a framework that supports it (like .NET MAUI).

5. Custom Binding for One-Time Behavior: If you need more control over when the binding updates, you can create a custom binding that only updates once. This involves subclassing `MvxTargetBinding` and overriding the `SetValueImpl` method to only update the target once[4].

Conclusion

Implementing one-time binding in MvvmCross for language files involves loading the localized text into a ViewModel property and then binding it to the View. While MvvmCross doesn't directly support OneTime binding in XML for Android, you can achieve similar behavior programmatically or by creating custom bindings. This approach ensures that the localized text is updated only once when the ViewModel is loaded, which is suitable for static data like language files.