Binding a button to a different view model

Question

I have a button in View "A" which already has a bindingSet attached to it (it binds to ViewModel "A"). I have button though which needs to be bound to ViewModel "B".

What is the best way to do this?

Stuart Stuart · Accepted Answer · 2015-01-19T15:03:05

Your ViewModel is your Model for your View.

If that ViewModel is made up of parts, then that can be done by aggregation - by having your ViewModel made up of lots of sub-models - e.g:

// simplified pseudo-code (add INPC to taste)
public class MyViewModel
{
   public MainPartViewModel A {get;set;}
   public SubPartViewModel B {get;set;}
   public string Direct {get;set;}
}

With this done, then a view component can be bound to direct sub properties as well as sub properties of sub view models:

set.Bind(button).For("Title").To(vm => vm.Direct);     
set.Bind(button).For("TouchUpInside").To(vm => vm.A.GoCommand);     
set.Bind(button).For("Hidden").To(vm => vm.B.ShouldHideThings);

As long as each part supports INotifyPropertyChanged then data-binding should "just work" in this situation.

If that approach doesn't work for you... In mvvmcross, you could set up a nested class within the View that implemented IMvxBindingContextOwner and which provided a secondary binding context for your View... something like:

 public sealed class Nested : IMvxBindingContextOwner, IDisposable {

     public Nested() { _bindingContext = new MvxBindingContext(); }

     public void Dispose() {
        _bindingContext.Dispose();
     }

     private MvxBindingContext _bindingContext;
     public IMvxBindingContext BindingContext { get { return _bindingContext; } }
     public Thing ViewModel {
        get { return (Thing)_bindingContext.DataContext; }
        set { _bindingContext.DataContext = value; }
     }
 }

This could then be used as something like:

 _myNested = new Nested();
 _myNested.ViewModel = /* set the "B" ViewModel here */
 var set2 = _myNested.CreateBindingSet<Nested, Thing>();
 // make calls to set2.Bind() here
 set2.Apply();

Notes:

I've not run this pseudo-code, but it feels like it should work...
to get this fully working, you will also want to call Dispose on the Nested when Dispose is fired on your View
given that Views and ViewModels are normally written 1:1 I think this approach is probably going to be harder to code and to understand later.

Binding a button to a different view model

1 Answers