Xamarin BindableProperty in custom control

Question

I am using a bindable property in a custom control in order to set a property from the xaml code. However, it seems like my property always will get the default value that I've specified for the bindable property.

My xaml code:

<controls:MyView ID="4" />

My code behind:

public partial class MyView : ContentView
{
    public static readonly BindableProperty IDProperty = BindableProperty.Create(
                                    nameof(ID),
                                    typeof(int),
                                    typeof(MyView),
                                    15);
    public int ID
    {
        get
        {
             return (int)GetValue(IDProperty);
        }
        set
        {
            SetValue(IDProperty, value);
        }
    }

    private MyViewViewModel viewModel;

    public MyView()
    {
        InitializeComponent();
        viewModel = new MyViewViewModel() {};
        BindingContext = viewModel; 
    }
}

I expect that my property should get value 4 in this example, but it always get the default value 15. Should the property be set in the constructor or later?

What am I doing wrong?

Can you share all the code of your control and especially constructor? Maybe you set the BindingContext incorrectly ... — EvZ
Why do you have a ViewModel embedded into your CustomControl? — EvZ
It's not working because the BindingContext is pointing to a MyViewViewModel object. If you want to make it works change BindingContext = viewModel; by BindingContext = this. BTW, the MyViewViewModel field is not necessary. I hope it helps. — dbvega

EvZ EvZ · Accepted Answer · 2018-05-11T09:03:29

Why do you embed a ViewModel inside your custom control?

It is weird and even wrong. The idea behind a custom control is that you could reuse and bind it to the parent's ViewModel. Think of a simple Button control, it is reusable by simple placing it on the screen and setting the BindableProperties like Text, Command and etc. It is working because it's BindingContext by default is the same as it's parent.

In your case you sort of isolate your control from any modifications, since you set the BindingContext to a private custom ViewModel class. You have to rethink your solution.

It should be as simple as:

public partial class MyView : ContentView
{
    public static readonly BindableProperty IDProperty = BindableProperty.Create(
                                    nameof(ID),
                                    typeof(int),
                                    typeof(MyView),
                                    15);
    public int ID
    {
        get => (int)GetValue(IDProperty);
        set => SetValue(IDProperty, value);
    }

    public MyView()
    {
        InitializeComponent();
    }
}

Xamarin BindableProperty in custom control

4 Answers