Why does KeyDown event not have access to the current value of bound variable?

Question

In the example below:

• I start program, type text, click button, see text above. Press ENTER see text again.

BUT:

• I start program, type text, press ENTER, see no text.

It seems that the KeyDown event doesn't get access to the current value of the bound variable, as if it is always "one behind".

What do I have to change so that when I press ENTER I have access to the value that is in the textbox so I can add it to the chat window?

XAML:

<Window x:Class="TestScroll.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="290" Width="300" Background="#eee">
    <StackPanel Margin="10">

        <ScrollViewer Height="200" Width="260" Margin="0 0 0 10"
                      VerticalScrollBarVisibility="Auto"
                      HorizontalScrollBarVisibility="Auto">
            <TextBlock Text="{Binding TextContent}"
                       Background="#fff"/>
        </ScrollViewer>

        <StackPanel HorizontalAlignment="Left" Orientation="Horizontal">
            <TextBox x:Name="TheLineTextBox" 
                     Text="{Binding TheLine}" 
                     Width="205" 
                     Margin="0 0 5 0"
                     KeyDown="TheLineTextBox_KeyDown"/>
            <Button Content="Enter" Click="Button_Click"/>
        </StackPanel>

    </StackPanel>
</Window>

Code-Behind:

using System;
using System.Windows;
using System.Windows.Input;
using System.ComponentModel;

namespace TestScroll
{
    public partial class Window1 : Window, INotifyPropertyChanged
    {

        #region ViewModelProperty: TextContent
        private string _textContent;
        public string TextContent
        {
            get
            {
                return _textContent;
            }

            set
            {
                _textContent = value;
                OnPropertyChanged("TextContent");
            }
        }
        #endregion

        #region ViewModelProperty: TheLine
        private string _theLine;
        public string TheLine
        {
            get
            {
                return _theLine;
            }

            set
            {
                _theLine = value;
                OnPropertyChanged("TheLine");
            }
        }
        #endregion

        public Window1()
        {
            InitializeComponent();
            DataContext = this;
            TheLineTextBox.Focus();

        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            AddLine();
        }

        void AddLine()
        {
            TextContent += TheLine + Environment.NewLine;
        }

        private void TheLineTextBox_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Return)
            {
                AddLine();
            }
        }

        #region INotifiedProperty Block
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;

            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion
    }
}

Answer 1

Does it work if you change AddLine to use TheLineTextBox.Text?

    void AddLine() 
    { 
        TextContent += TheLineTextBox.Text + Environment.NewLine; 
    } 

Answer 2

Your textbox->property binding is only happening after the textbox loses focus. When you type in text and press enter, you have not set the focus anywhere else on the form. You can demonstrate this by typing in your text, clicking on the scroll viewer, then clicking back on the textbox and hitting enter. You will then see your text change in the viewer.

To get around that, update your Text property of the textbox to this.

Text="{Binding TheLine, UpdateSourceTrigger=PropertyChanged}"

http://blogs.msdn.com/wpfsdk/archive/2006/10/19/wpf-basic-data-binding-faq.aspx

See about a quarter of the way down the page: How do I make my data-bound TextBox update the source value as I type?