使用MvvmLight和Xamarin.iOS将属性绑定到ViewModel

Question

I've used MvvmLight for a long time now which fits perfectly my needs for my Windows and Windows Phone developments, but I struggle with the new Xamarin.iOS binding features introduced in version 5.

I've checked the Flowers sample, and tried to create a really simple binding that doesn't work as expected : the update action is executed only once ...

Here the code of the view controller :

 public partial class MainViewController : UIViewController
{
    private MainViewModel ViewModel { get; set; }

    public MainViewController()
        : base("MainViewController", null)
    {
        this.ViewModel = new MainViewModel();
    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();

        this.SetBinding(() => this.ViewModel.IsUpdated).WhenSourceChanges(() =>
            {
                this.updateLabel.Text = this.ViewModel.IsUpdated ? "It's okay !" : "Nope ...";
            });

        this.updateButton.SetCommand("TouchUpInside", this.ViewModel.UpdateCommand);

    }
}

The generated partial class declaration with two interface elements:

[Register ("MainViewController")]
partial class MainViewController
{
    [Outlet]
    MonoTouch.UIKit.UIButton updateButton { get; set; }

    [Outlet]
    MonoTouch.UIKit.UILabel updateLabel { get; set; }

    void ReleaseDesignerOutlets ()
    {
        if (updateLabel != null) {
            updateLabel.Dispose ();
            updateLabel = null;
        }

        if (updateButton != null) {
            updateButton.Dispose ();
            updateButton = null;
        }
    }
}

And the associated ViewModel :

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        this.UpdateCommand = new RelayCommand(() =>
            {
                this.IsUpdated = !this.IsUpdated;
            });
    }

    private bool isUpdated;

    public bool IsUpdated
    {
        get { return this.isUpdated; }
        set
        {
            this.Set<bool>(ref this.isUpdated, value);
        }
    }

    public RelayCommand UpdateCommand { get; private set; }
}

Anybody have a working example, and some explanations ?

Answer 1

You have to store the Binding created in SetBinding in your ViewController or it goes away as soon as you leave the scope of ViewDidLoad. In the Flowers example, that code only runs during the Loading of the view. It's never run due to the change of a value.

public partial class MainViewController : UIViewController
{
    private Binding<bool, bool> _isUpdatedBinding;
    private MainViewModel ViewModel { get; set; }

    public MainViewController()
        : base("MainViewController", null)
    {
        this.ViewModel = new MainViewModel();
    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();

        _isUpdatedBinding = this.SetBinding(() => this.ViewModel.IsUpdated).WhenSourceChanges(() =>
            {
                this.updateLabel.Text = this.ViewModel.IsUpdated ? "It's okay !" : "Nope ...";
            });

        this.updateButton.SetCommand("TouchUpInside", this.ViewModel.UpdateCommand);

    }
}

I believe these changes should fix your issue.