Set property of UserControl from code behind of parent during page initialization - user-controls

Question

I have a page with a user control on it, and the user control has a dependency property. The logic for setting the value of the property is slightly complicated so I want to do from the code-behind for the page.
The intended flow is:
MainPage:
- In Loaded event, set property on control
ChildControl
- In Loaded event, push property to XAML
I've tried this in WPF and WinRT, and followed it with breakpoints in the debugger. It works in WPF as intended, but in WinRT the Loaded event for the child control is called before the event for the MainPage, so the sequence fails.
ChildControl.xaml
<UserControl x:Class="UserControlFromWinRT.ChildControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300"
Loaded="UserControl_Loaded">
<Grid>
<TextBlock x:Name="greetingTextBlock"/>
</Grid>
ChildControl.xaml.cs
public partial class ChildControl : UserControl
{
public ChildControl() {
InitializeComponent();
}
public string Greeting {
get { return (string)GetValue(GreetingProperty); }
set { SetValue(GreetingProperty, value); }
}
public static readonly DependencyProperty GreetingProperty =
DependencyProperty.Register("Greeting", typeof(string), typeof(ChildControl), new PropertyMetadata(""));
private void UserControl_Loaded(object sender, RoutedEventArgs e) {
greetingTextBlock.Text = Greeting;
}
}
MainPage.xaml
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<local:ChildControl x:Name="childControl" Margin="20,30,0,0" FontSize="30"/>
</Grid>
MainPage.xaml.cs
private void Page_Loaded(object sender, RoutedEventArgs e) {
childControl.Greeting = "Hello";
}

Answer 1

You need to add a PropertyChangedCallback to your PropertyMetadata Constructor
Like:
public static readonly DependencyProperty GreetingProperty = DependencyProperty.Register("Greeting", typeof(string), typeof(ChildControl), new PropertyMetadata("", OnGreetingChanged));
private static void OnGreetingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
// do something e.NewValue
}