TextBox TextChanged event on programmatic versus user change of text contents

Question

I would like to differentiate between changing the text programmatically (for example in a button click handler event) and user input (typing, cutting and pasting text).
Is it possible?

Answer 1

User input in a TextBox can be identified with

• Typing : PreviewTextInput event
• Backspace, Delete, Enter : PreviewKeyDown event
• Pasting : DataObject.PastingEvent

Combining these three events with a bool flag to indicate if any of the above occured before the TextChanged event and you'll know the reason for the update.

Typing and Pasting are easy, but Backspace doesn't always trigger TextChanged (if no text is selected and the cursor is at position 0 for example). So some logic is needed in PreviewTextInput.

Here is an Attached Behavior that implements the logic above and executes a command with a bool flag when TextChanged is raised.

<TextBox ex:TextChangedBehavior.TextChangedCommand="{Binding TextChangedCommand}" />

And in code you can find out the source for the update like

private void TextChanged_Executed(object parameter)
{
    object[] parameters = parameter as object[];
    object sender = parameters[0];
    TextChangedEventArgs e = (TextChangedEventArgs)parameters[1];
    bool userInput = (bool)parameters[2];

    if (userInput == true)
    {
        // User input update..
    }
    else
    {
        // Binding, Programatic update..
    }
}

Here is a small sample project demonstrating the effect: SourceOfTextChanged.zip

TextChangedBehavior

public class TextChangedBehavior
{
    public static DependencyProperty TextChangedCommandProperty =
        DependencyProperty.RegisterAttached("TextChangedCommand",
                                            typeof(ICommand),
                                            typeof(TextChangedBehavior),
                                            new UIPropertyMetadata(TextChangedCommandChanged));

    public static void SetTextChangedCommand(DependencyObject target, ICommand value)
    {
        target.SetValue(TextChangedCommandProperty, value);
    }

    // Subscribe to the events if we have a valid command
    private static void TextChangedCommandChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
    {
        TextBox textBox = target as TextBox;
        if (textBox != null)
        {
            if ((e.NewValue != null) && (e.OldValue == null))
            {
                textBox.PreviewKeyDown += textBox_PreviewKeyDown;
                textBox.PreviewTextInput += textBox_PreviewTextInput;
                DataObject.AddPastingHandler(textBox, textBox_TextPasted);
                textBox.TextChanged += textBox_TextChanged;
            }
            else if ((e.NewValue == null) && (e.OldValue != null))
            {
                textBox.PreviewKeyDown -= textBox_PreviewKeyDown;
                textBox.PreviewTextInput -= textBox_PreviewTextInput;
                DataObject.RemovePastingHandler(textBox, textBox_TextPasted);
                textBox.TextChanged -= textBox_TextChanged;
            }
        }
    }

    // Catches User input
    private static void textBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        TextBox textBox = sender as TextBox;
        SetUserInput(textBox, true);
    }
    // Catches Backspace, Delete, Enter
    private static void textBox_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        TextBox textBox = sender as TextBox;
        if (e.Key == Key.Return)
        {
            if (textBox.AcceptsReturn == true)
            {
                SetUserInput(textBox, true);
            }
        }
        else if (e.Key == Key.Delete)
        {
            if (textBox.SelectionLength > 0 || textBox.SelectionStart < textBox.Text.Length)
            {
                SetUserInput(textBox, true);
            }
        }
        else if (e.Key == Key.Back)
        {
            if (textBox.SelectionLength > 0 || textBox.SelectionStart > 0)
            {
                SetUserInput(textBox, true);
            }
        }
    }
    // Catches pasting
    private static void textBox_TextPasted(object sender, DataObjectPastingEventArgs e)
    {
        TextBox textBox = sender as TextBox;
        if (e.SourceDataObject.GetDataPresent(DataFormats.Text, true) == false)
        {
            return;
        }
        SetUserInput(textBox, true);
    }
    private static void textBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        TextBox textBox = sender as TextBox;
        TextChangedFired(textBox, e);
        SetUserInput(textBox, false);
    }

    private static void TextChangedFired(TextBox sender, TextChangedEventArgs e)
    {
        ICommand command = (ICommand)sender.GetValue(TextChangedCommandProperty);
        object[] arguments = new object[] { sender, e, GetUserInput(sender) };
        command.Execute(arguments);
    }

    #region UserInput

    private static DependencyProperty UserInputProperty =
        DependencyProperty.RegisterAttached("UserInput",
                                            typeof(bool),
                                            typeof(TextChangedBehavior));
    private static void SetUserInput(DependencyObject target, bool value)
    {
        target.SetValue(UserInputProperty, value);
    }
    private static bool GetUserInput(DependencyObject target)
    {
        return (bool)target.GetValue(UserInputProperty);
    }

    #endregion // UserInput
}