Updating Source on PropertyChanged on Silverlight

Download source code - 9.93 KB

Introduction

Don't you miss the "UpdateSourceTrigger=PropertyChanged" on a Binding expression on Silverlight? Well, me too, and since I don't like the idea of having a hidden control to change focus on every TextChange of my TextBoxes, I tried to make a more elegant solution for the problem.

Background

In both WPF and Silverlight, when we create a BindingExpression, you can define a UpdateSourceTrigger property of the Binding. But, there is a huge difference between them, Silverlight has only the Default mode (LostFocus) and Explicit, so we are missing PropertyChanged that in some cases makes our life way easier.

Using the Code

The code is fairly simple. It is only one AttachedProperty that will handle the TextChanged for you and will update the BindingSource if the BindingExpression of the TextProperty exists.

This was only implemented in the TextBox because there is no need to change the other input controls over the fact that LostFocus works perfect for them (like ComboBoxes, CheckBoxes, RadioButtons).

So, we define an AttachedProperty that shouldn't be a problem to any Silverlight developer:

public static bool GetUpdateOnPropertyChanged(DependencyObject obj)
{
    return (bool)obj.GetValue(UpdateOnPropertyChangedProperty);
}

public static void SetUpdateOnPropertyChanged(DependencyObject obj, bool value)
{
    obj.SetValue(UpdateOnPropertyChangedProperty, value);
}

public static readonly DependencyProperty UpdateOnPropertyChangedProperty =
    DependencyProperty.RegisterAttached(
        "UpdateOnPropertyChanged",
        typeof(bool),
        typeof(UpdateSourceManager),
        new PropertyMetadata(
            new PropertyChangedCallback(
                UpdateOnPropertyChangedPropertyCallback)));

Then we define the UpdateOnPropertyChangedPropertyCallback:

static void UpdateOnPropertyChangedPropertyCallback(
    DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    if (sender is TextBox)
    {
        TextBox txt = sender as TextBox;
        txt.TextChanged += new TextChangedEventHandler(txt_TextChanged);
    }
}

static void txt_TextChanged(object sender, TextChangedEventArgs e)
{
    TextBox txt = sender as TextBox;

    var bindingExpression = txt.GetBindingExpression(TextBox.TextProperty);
    if (bindingExpression != null)
    {
        bindingExpression.UpdateSource();
    }
}

Then, for using this, just add to your UserControl the proper XML namespace like:

<UserControl x:class="UpdateSourceTrigger.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:local="clr-namespace:UpdateSourceTrigger" 
    mc:ignorable="d">

And finally, add the Attached Property to the TextBox as follows:

<TextBox 
    Text="{Binding Context.FirstName, Mode=TwoWay}" 
    Grid.Column="1" 
    Grid.Row="0" 
    Margin="5" 
    local:UpdateSourceManager.UpdateOnPropertyChanged="True"
    />