What are the common issues with two-way binding in MvvmCross and how can they be resolved

1. Lack of Change Notification**

One of the primary issues with two-way binding in MvvmCross is that the framework needs to be notified when the UI value changes. For example, when using a `UITextView`, MvvmCross does not inherently know when the text has changed. To resolve this, you can add an event handler to notify the ViewModel of changes.

**Solution: Implement an event in your custom view or cell that fires whenever the text changes. For instance, you can add a `FieldDescriptionChanged` event to your custom cell class and fire it whenever the `UITextView` text changes. This event should be subscribed to by the binding mechanism to update the ViewModel.

csharp
public event EventHandler FieldDescriptionChanged;

// In your UITextView's TextChanged event handler:
private void FieldDescriptionLabel_TextChanged(object sender, EventArgs e)
{
    FieldDescriptionChanged?.Invoke(this, EventArgs.Empty);
}

2. Missing TargetBinding**

MvvmCross uses `TargetBinding` to define how a View can bind in modes other than one-way. If you're trying to bind a custom view or a control that doesn't support two-way binding out of the box (like `UITextView`), you need to create a custom `TargetBinding`.

**Solution: Create a custom `TargetBinding` class that describes how to bind your specific property and listens for the appropriate change events. This involves implementing `IMvxTargetBinding` and overriding the necessary methods to handle setting and getting the value, as well as subscribing to change events.

csharp
public class CustomTextViewTargetBinding : MvxTargetBinding
{
    private UITextView _textView;

    public CustomTextViewTargetBinding(UITextView textView)
    {
        _textView = textView;
        _textView.TextChanged += OnTextViewTextChanged;
    }

    private void OnTextViewTextChanged(object sender, EventArgs e)
    {
        FireValueChanged(_textView.Text);
    }

    public override void SetValue(object value)
    {
        _textView.Text = (string)value;
    }

    public override Type TargetType => typeof(string);

    protected override void Dispose(bool isDisposing)
    {
        if (isDisposing)
        {
            _textView.TextChanged -= OnTextViewTextChanged;
        }
        base.Dispose(isDisposing);
    }
}

3. Mono Linker Issues**

When building Xamarin apps, the Mono Linker can sometimes strip out code that is used via reflection, which MvvmCross relies on for bindings. This can lead to unexpected behavior or crashes.

**Solution: Use the `LinkerPleaseInclude` class to ensure that the necessary types and members are not stripped by the linker. This involves adding a dummy class that references the types and methods used by MvvmCross bindings.

csharp
public class LinkerPleaseInclude
{
    public void Include()
    {
        // Include your custom view types and any other necessary classes here.
        var type = typeof(CustomTextViewTargetBinding);
    }
}

4. Incorrect Binding Setup**

Sometimes, the issue might be as simple as setting up the binding incorrectly. Ensure that you're using the correct binding mode (`TwoWay`) and that both the source and target properties are correctly specified.

**Solution: Double-check your binding setup. Use fluent bindings or Swiss/Tibet binding expressions correctly, ensuring that the `Mode` is set to `TwoWay` if necessary.

csharp
this.CreateBinding(_cells)
    .For(cll => cll.FieldDescription)
    .To((TimesheetViewModel vm) => vm.AccountID)
    .Mode(MvxBindingMode.TwoWay)
    .Apply();

By addressing these common issues, you can effectively troubleshoot and resolve problems with two-way binding in MvvmCross.