Execute Silverlight 4 Command on Load

Question

How do you implement a Silverlight 4 command to execute when the user control loads instead of being mapped to an explicit button click?

Answer 1

Are you looking for the FrameworkElement.Loaded event?

According to MSDN, this event occurs "when a FrameworkElement has been constructed and added to the object tree."

So, you could use it to do something like this:

public MyCustomUserControl()
{
   InitializeComponent();

   this.Loaded += new RoutedEventHandler(MyCustomUserControl_Loaded);
}

private void MyCustomUserControl_Loaded(object sender, RoutedEventArgs e)
{
   // Do whatever you want here; it'll get executed when the control is loaded
}

Note that you can also declaratively specify the event handler for the Loaded event in XAML:

<UserControl Loaded="MyCustomUserControl_Loaded" (...)

This has the same effect as the first example; it's really a matter of preference.

Hope this helps!

Answer 2

Create a DependencyProperty of type ICommand:-

    #region public ICommand LoadedCommand

    public ICommand LoadedCommand
    {
        get { return GetValue(LoadedCommandProperty) as ICommand; }
        set { SetValue(LoadedCommandProperty, value); }
    }

    public static readonly DependencyProperty LoadedCommandProperty =
            DependencyProperty.Register(
                    "LoadedCommand",
                    typeof(ICommand),
                    typeof(MainPage),
                    new PropertyMetadata(null));

    #endregion public ICommand LoadedCommand

Also add a something to act as the command parameter:-

    #region public object LoadedCommandParameter

    public object LoadedCommandParameter
    {
        get { return GetValue(LoadedCommandParameterProperty) as object; }
        set { SetValue(LoadedCommandParameterProperty, value); }
    }

    public static readonly DependencyProperty LoadedCommandParameterProperty =
            DependencyProperty.Register(
                    "LoadedCommandParameter",
                    typeof(object),
                    typeof(MainPage),
                    new PropertyMetadata(null));

    #endregion public object LoadedCommandParameter

Now set up its execution like this:-

    public UserControl1()
    {
        InitializeComponent();
        Loaded += UserControl1_Loaded;
    }

    void UserControl1_Loaded(object sender, RoutedEventArgs e)
    {
        if (LoadedCommand != null && LoadedCommand.CanExecute(LoadedCommandParameter))
        {
            LoadedCommand.Execute(LoadedCommandParameter);
        }
    }

Now if your ViewModel (has a command called StartStuff) then:-

  <UserControl1 LoadedCommand="{Binding StartStuff}" .... >

Answer 3

Or simply add a trigger in xaml for your UserControl:

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Loaded">
        <si:InvokeDataCommand Command="{Binding MyCommand}"/>
    </i:EventTrigger>
</i:Interaction.Triggers>