Binding Object Properties – UpdateSourceTrigger

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Bindings that are TwoWay or OneWayToSource listen for changes in the target property and propagate them back to the source. This is known as updating the source. For example, you may edit the text of a TextBox to change the underlying source value. As described in the mode section, the direction of the data flow is determined by the value of the Mode property of the binding. In the following sample, TextBox1 has UpdateSourceTrigger property set to PropertyChanged, so as the user starts editing the text in this control, it automatically updates the source and so the textbox2 also gets updated.

<Window x:Class="Bindingtoclasses.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation&quot;
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml&quot;
        xmlns:src="clr-namespace:Bindingtoclasses"
        Title="MainWindow" Height="350" Width="525">
    <Grid DataContext="{Binding Path=EmployeeName}">
        <Grid.Resources>
                <src:Employee x:Key="myDataSource" EmployeeName="Kishore1021"/>
        </Grid.Resources>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
        </Grid.RowDefinitions>
        <TextBox Grid.Row="0" Text="{Binding Source={StaticResource myDataSource}, Path=EmployeeName, UpdateSourceTrigger=PropertyChanged}"/>
        <TextBox Grid.Row="1" Text="{Binding Source= {StaticResource myDataSource}, Path=EmployeeName}"></TextBox>
        <TextBox Grid.Row="2" Text="{Binding Path=.}"></TextBox>
        <TextBlock Grid.Row="3" Text="{Binding}"></TextBlock>
        </Grid>
</Window>

 

Does your source value get updated while you are editing the text or after you finish editing the text and point your mouse away from the TextBox? See the attached source code and run the exe. The UpdateSourceTrigger property of the binding determines what triggers the update of the source. The dots of the right arrows in the following figure illustrate the role of the UpdateSourceTrigger property:

If the UpdateSourceTrigger value is PropertyChanged, then the value pointed to by the right arrow of TwoWay or the OneWayToSource bindings gets updated as soon as the target property changes. However, if the UpdateSourceTrigger value is LostFocus, then that value only gets updated with the new value when the target property loses focus.

Similar to the Mode property, different dependency properties have different default UpdateSourceTrigger values. The default value for most dependency properties is PropertyChanged, while the Text property has a default value of LostFocus. This means that source updates usually happen whenever the target property changes, which is fine for CheckBoxes and other simple controls. However, for text fields, updating after every keystroke can diminish performance and it denies the user the usual opportunity to backspace and fix typing errors before committing to the new value. That is why the Text property has a default value of LostFocus instead of PropertyChanged.

 

UpdateSourceTrigger value	When the Source Value Gets Updated	Example Scenario for TextBox
LostFocus (default for TextBox.Text)	When the TextBox control loses focus	A TextBox that is associated with validation logic
PropertyChanged	As you type into the TextBox	TextBox controls in a chat room window
Explicit	When the application calls UpdateSource	TextBox controls in an editable form (updates the source values only when the user clicks the submit button)

Download the Source Code here.

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