Why am i using UpdateSourceTrigger=PropertyChanged ,TwoWay is not enough?

Question

hi; there are Source and target textbox txttarget has a binding to txtsource. when writing something in txtsource, txttarget is changed.Everything is good. But writing on txttarget, i dont see any changes at txttarget? there is TwoWay mode. Twoway mode is not enough? can i write without using "UpdateSourceTrigger=PropertyChanged"?

   <Grid>
        <TextBox Height="23" HorizontalAlignment="Left" Margin="155,62,0,0" Name="txtSource" VerticalAlignment="Top" Width="120" />
        <TextBox Height="23" HorizontalAlignment="Left"
                 Text="{Binding ElementName=txtSource,Path=Text,Mode=TwoWay}" 
                 Margin="155,113,0,0" Name="txtTarget" VerticalAlignment="Top" Width="120" />
    </Grid>

Answer 1

txtTarget.Text is updated whenever the bound source (txtSource.Text) changes.

The binding mode is TwoWay which means that changes to txtTarget.Text will be reflected to the bound source. When? It depends on the Binding.UpdataSourceTrigger property.

If you want your target binding to update your source binding when changing you must use Binding.UpdataSourceTrigger = OnPropertyChanged, otherwise you will update the binding source when txtTarget losts focus (default behavior).

Answer 2

The default UpdateSourceTrigger for a TextBox is LostFocus (see Binding.UpdateSourceTrigger). If you do not specify PropertyChanged as the UpdateSourceTrigger, what you type into txtTarget will not be written to txtSource until txtTarget loses focus (that is you tab off of it).