Loaded event and data-binding in WPF

Question

Suppose we have a control with a dependency property:

 public class MyControl : ContentControl
{

    static MyControl()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyControl), new FrameworkPropertyMetadata(typeof(MyControl)));
    }


    void DoSomething(object sender, RoutedEventArgs e)
    {
        //...

    }

    public MyControl()
    {           
        this.Loaded += DoSomething;
    }

    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
        "Value", typeof(string), typeof(MyControl),
        new FrameworkPropertyMetadata(default(string),
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Journal, OnValueChanged));


    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        //some actions              
    }

    public string Value
    {
        get { return (string)GetValue(MyControl.ValueProperty); }
        set { SetValue(MyControl.ValueProperty, value); }
    }}

And somewhere in XAML we bind ValueProperty to some property Val of some ViewModel.

My question is, at the moment when Loaded event is fired, will the value of MyControl.Value be already set to the value of Val which it has or not? So, am I able to use Val in DoSomething method which is executed when MyControl is loaded or not?

Thank you in advance for any answers.

Answer 1

My question is, at the moment when Loaded event is fired, will the value of MyControl.Value be already set to the value of Val which it has or not?

You cannot rely on this. The Loaded event is not a 'data-binding-has-completed' event. Also, the value of a dependency property can change at any time and the place to handle any changes to your Value property is in the OnValueChanged callback. You could check whether the control has been loaded in the callback if you want to:

private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    MyControl ctrl = d as MyControl;
    if (ctrl.IsLoaded)
    {
        //...
    }
}

The point is that you should perform any action that depends upon the value of your dependency property in the callback and not in the Loaded event handler.

Answer 2

public class MyControl : ContentControl
{
    public MyControl()
    {           
        this.Loaded += (s,e) => DoSomething();
    }

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var c = (MyControl)d;
        if ( c.IsLoaded )
            c.DoSomething();              
    }

    // this will be called only if the control is loaded
    private void DoSomething()
    {
        var value = Value;
        if ( value == null )
        {
            ...
        } else if ( value == string.Empty )
        {
            ...
        } else
        {
        }
    }

    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
        "Value", typeof(string), typeof(MyControl),
        new FrameworkPropertyMetadata(default(string),
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Journal, OnValueChanged));

    public string Value
    {
        get { return (string)GetValue(MyControl.ValueProperty); }
        set { SetValue(MyControl.ValueProperty, value); }
    }

    static MyControl()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyControl), new FrameworkPropertyMetadata(typeof(MyControl)));
    }

}