使用ViewModel（MVVM）在当前视图的子UserControl中设置属性

Question

I have a main View called MainContentView.xaml in wich I have set the DataContext to a ViewModel called MainContentViewModel.cs . Here I can update the values that appear in the UI, just as a normal MVVM would work.
It looks like this:

<UserControl x:Class="namespace:MyProject.View.Home.MainContentView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:UserTools="clr-namespace:MyProject.View.Tools"
             xmlns:ViewModel="clr-namespace:MyProject.ViewModel.Home"
             mc:Ignorable="d" >
    <UserControl.DataContext>
        <ViewModel:MainContentViewModel />
    </UserControl.DataContext>

    <Grid x:Name="mainGrid">
        <!-- Normal objects -->
        <StackPanel>
            <Label Text="Im a label" />
            <TextBox />
        </StackPanel>
        <!-- My Custom Object -->
        <UserTools:MyCustomUserControl x:Name="myUserControl" />
    </Grid>
</UserControl>

And my MainContentViewModel.cs looks like this:

public class MainContentViewModel : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged;

    private string myLabel = "";
    public string MyLabel {
        get { return this.myLabel; }
        set {
            this.myLabel = value;
            OnPropertyChangedEvent("MyLabel");
        }
    }

    public MainContentViewModel() {
        MyLabel = string.Format("my label text");
    }

    protected virtual void OnPropertyChangedEvent(string _propertyName) {
        var _handler = this.PropertyChanged;
        if(_handler != null) { _handler(this, new PropertyChangedEventArgs(_propertyName)); }
    }
}

Now I want to set some properties in the MyCustomUserControl.xaml through it's own MyCustomUserControlViewModel.cs .
My MyCustomUserControl.xaml looks like this:

<UserControl x:Class="MyProject.View.Tools.MyCustomUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:ViewModel="clr-namespace:MyProject.ViewModel.Tools"
             mc:Ignorable="d" >
    <UserControl.DataContext>
        <ViewModel:MyCustomUserControlViewModel />
    </UserControl.DataContext>
    <Grid x:Name="mainGrid">
        <Label Content="{Binding myCustomLabel}" />
    </Grid>
</UserControl>

And my MyCustomUserControlViewModel.cs looks like this:

public class MyCustomUserControlViewModel : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged;

    private string myCustomLabel = "";
    public string MyCustomLabel {
        get { return this.myCustomLabel; }
        set {
            this.myCustomLabel = value;
            OnPropertyChangedEvent("MyCustomLabel");
        }
    }

    public MyCustomUserControlViewModel() {
        MyCustomLabel = string.Format("my custom label text");
    }

    protected virtual void OnPropertyChangedEvent(string _propertyName) {
        var _handler = this.PropertyChanged;
        if(_handler != null) { _handler(this, new PropertyChangedEventArgs(_propertyName)); }
    }
}

Now I want to, from the MainContentViewModel.cs , update the MyCustomLabel property on the MyUserCustomUserControl.xaml , and I suppose I have to do it through the MyCustomUserControlViewModel.cs so I can keep the MVVM pattern.

I've tried something like this: In the MainContentViewModel.cs

private MyCustomUserControlViewModel _viewModel = new MyCustomUserControlViewModel ();
(...)
public MainContentViewModel() {
    (...)
    _viewModel.SetMyCustomLabel("my new custom label text");
}

And in MyCustomUserControlViewModel.cs

public void SetMyCustomLabel(string _text) {
    MyCustomLabel = _text;
}

But this does not work. I'm guessing it's because I'm instantiating another MyCustomUserControlViewModel.cs object.

So, how can I do this?

UPDATED AND WORKING (Thank you Sheridan)
I picked up one of Sheridan's solutions and ended up with this working solution. In my MainContentView.xaml :

(...)
<UserTools:MyCustomUserControl DataContext="{Binding ChildViewModel, Mode=OneWay}" />

In my MainContentViewModel.cs :

private MyCustomUserControlViewModel childViewModel = new MyCustomUserControlViewModel();
public MyCustomUserControlViewModel ChildViewModel {
    get { return childViewModel; }
    private set { childViewModel = value; OnPropertyChangedEvent("ChildViewModel"); }
}

And then I can do this:

public MainContentViewModel() {
    (...)
    ChildViewModel.SetMyCustomLabel("my new custom label text");
}

Answer 1

If your parent view model had access to the actual child view model instance that is used (which it probably doesn't because of the way that you instantiate your child view model), then you could have simply done this:

public MainContentViewModel() {
    (...)
    _viewModel.CustomLabel  = "my new custom label text";
}

1.

One solution would be to remove the child view model from the child view to the parent view model and then to display the child view using a ContentControl and a DataTemplate :

<DataTemplate DataType="{x:Type ViewModels:MyCustomUserControlViewModel}">
    <Views:MyCustomUserControlView />
</DataTemplate>

...

In MainContentViewModel:

public MyCustomUserControlViewModel ChildViewModel
{
    get { return childViewModel; }
    private set { childViewModel = value; NotifyPropertyChanged("ChildViewModel"); }
}

...

<Grid x:Name="mainGrid">
    <!-- Normal objects -->
    <StackPanel>
        <Label Text="Im a label" />
        <TextBox />
    </StackPanel>
    <!-- My Custom Object -->
    <ContentControl x:Name="myUserControl" Content="{Binding ChildViewModel}" />
</Grid>

Doing this would enable you to call the property simply, as shown in my first example.

2.

One alternative would be to move the property to the parent view model and bind to it directly from the child UserControl :

<Grid x:Name="mainGrid">
    <Label Content="{Binding DataContext.myCustomLabel, RelativeSource={RelativeSource 
        AncestorType={x:Type Views:MainContentView}}}" />
</Grid>

---

UPDATE >>>

3.

Ok, I've just had another idea... you could add another property into the parent view model and data bind to the child view model like this maybe:

<UserTools:MyCustomUserControl DataContext="{Binding ChildViewModel, 
Mode=OneWayToSource}" x:Name="myUserControl" />

This is just a guess, but it might be able to hook onto the child view model like this... then you could do this in the parent view model:

if (ChildViewModel != null) ChildViewModel.CustomLabel = "my new custom label text";

4.

Actually... I've just had another idea. You could do bind from a property in your parent view model through to the UserControl if you add a DependencyProperty to it:

<UserTools:MyCustomUserControl CustomLabel="{Binding CustomLabelInParentViewModel}" 
x:Name="myUserControl" />

You could simply remove the property from the child view model in this case.