如何在Xamarin Forms中将子视图的属性绑定到当前视图模型的属性

Question

I'm trying to build a custom numpadcontrol in xamarin forms and I think I am getting something totally wrong here because the only result of my work are confusing exceptions.

This is my numpad-views xaml:

    <?xml version="1.0" encoding="UTF-8"?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="XFormsControls.Numpad">
  <ContentView.Content>
        <Grid>
            <Label BackgroundColor="Silver" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="4" Text="{Binding Value}" HorizontalTextAlignment="End" />
            <Button Grid.Row="1" Grid.RowSpan="2" Grid.Column="0" Text="+1" />
            <Button Grid.Row="3" Grid.RowSpan="2" Grid.Column="0" Text="-1" />

            <Button Grid.Row="1" Grid.Column="1" Text="1" />
            <Button Grid.Row="1" Grid.Column="2" Text="2" />
            <Button Grid.Row="1" Grid.Column="3" Text="3" />

            <Button Grid.Row="2" Grid.Column="1" Text="4" />
            <Button Grid.Row="2" Grid.Column="2" Text="5" />
            <Button Grid.Row="2" Grid.Column="3" Text="6" />

            <Button Grid.Row="3" Grid.Column="1" Text="7" />
            <Button Grid.Row="3" Grid.Column="2" Text="8" />
            <Button Grid.Row="3" Grid.Column="3" Text="9" />

            <Button Grid.Row="4" Grid.Column="1" Text="Trash" />
            <Button Grid.Row="4" Grid.Column="2" Text="0" />
            <Button Grid.Row="4" Grid.Column="3" Text="Ok" />
        </Grid>
  </ContentView.Content>
</ContentView>

and my numpad.xaml.cs:

public partial class Numpad : ContentView
{
    private double? _value { get; set; }
    public double? Value
    {
        get
        {
            return _value;
        }
        set
        {
            if (_value != value)
            {
                _value = value;
                OnPropertyChanged();
            }
        }
    }

    public Numpad()
    {

        InitializeComponent();
        BindingContext = this;
    }
}

I want to be able to load this control from any other page or view and just bind the control's Value property to any property of a viewmodel. So I tried to make the Value property of my control a Bindable Property like this:

        public static readonly BindableProperty valueProperty = BindableProperty.Create(
                                                        propertyName: "Value",
                                                        returnType: typeof(double),
                                                        declaringType: typeof(Numpad),
                                                        defaultValue: 0,
                                                        defaultBindingMode: BindingMode.TwoWay);

    public double Value
    {
        get { return (double)GetValue(valueProperty); }
        set
        {
            SetValue(valueProperty, value);
        }
    }

and tried to bind it to a property of the containing Page in XAML like this:

<local:Numpad Value="{Binding selectedValue}"></local:Numpad>

the selectedValue property of my ContentPage is defined like that (in codebehind):

private double _selectedValue = 25;
public double selectedValue
{
    get
    {
        return _selectedValue;
    }
    set
    {
        _selectedValue = value;
        OnPropertyChanged();
    }
}

On my UWP-Localmachine debugging it throws a "System.Reflection.TargetInvocationException" (thrown by the the target of an invocation) ??dafuq?

Or this exception when I run on iOS Simulator:

I tried for hours now and then i thought to turn things around and make my "selectedValue" of the parent class a bindable property, that didn't work either. It also makes no sense to me to turn it around because I can bind to any property of a viewmodel without making them bindable properties, or am I wrong here?

I am new to Xamarin.Forms and I am coming from technologies like Angular 2, where stuff like that is just straight forward.

Please Help

(at the moment I am at phase two of the development process )

I have uploaded the Code to my Git-Repo

Answer 1

Declaring Default value could've caused the problem other than that i don't see any difference. Just try to change the declaration as below, hope that helps!

private static double defaultValue = 0;

        public static readonly BindableProperty valueProperty = BindableProperty.Create(
            propertyName: "Value",
            returnType: typeof(double),
            declaringType: typeof(Numpad),
            defaultValue: defaultValue,
            defaultBindingMode: BindingMode.TwoWay);

I don't get any exceptions like you mentioned except XAML parse exception

Answer 2

So two things were problematic here, first, what @Dinesh kumar pointed out, I did not declare the binding property with the same name as the actual property, I wrote valueProperty instead of ValueProperty.

The second mistake was to set the BindingContext inside of the constructor of my numpad. I removed it and set x:name="this" on the top of my numpad.xaml. Thanks to @AdamMeaney in the Xamarin Forum

He also posted a very usefull link to a example of how to build custom user controls with xamarin.forms.