Enforcing a delay on textbox_textchanged

Question

I have a barcode scanner that emulates keyboard entry. I'm using it to enter ISBN numbers into a textbox, which then performs a search for that title. I need the textbox method to wait for wither a 10 or 13 character entry before doing anything, however I'm not sure how to go about doing it.

So far, I have the following:

private void scanBox_TextChanged(object sender, EventArgs e)
        {

            if (scanBox.Text.Length == 10)
            {
                getRecord10();
            }
            else if (scanBox.Text.Length == 13)
            {
                getRecord13();
            }
            else
            {
                MessageBox.Show("Not in directory", "Error");
            }
        }

I'm considering some sort of timer implementation to hold off on that last condition, but what I really need is for the method to wait for either 10 or 13 digits. The barcode scanner emulates individual keys being pressed, which is why it is currently failing.

Answer 1

You can use Timer (or DispatcherTimer in WPF). This sample app updates window's title 300ms after the last keystroke.

    System.Windows.Forms.Timer _typingTimer; // WinForms
    // System.Windows.Threading.DispatcherTimer _typingTimer; // WPF

    public MainWindow()
    {
        InitializeComponent();
    }

    private void scanBox_TextChanged(object sender, EventArgs e)
    {
        if (_typingTimer == null)
        {
            /* WinForms: */
            _typingTimer = new Timer();
            _typingTimer.Interval = 300;
            /* WPF: 
            _typingTimer = new DispatcherTimer();
            _typingTimer.Interval = TimeSpan.FromMilliseconds(300);
            */

            _typingTimer.Tick += new EventHandler(this.handleTypingTimerTimeout);
        }
        _typingTimer.Stop(); // Resets the timer
        _typingTimer.Tag = (sender as TextBox).Text; // This should be done with EventArgs
        _typingTimer.Start(); 
    }

    private void handleTypingTimerTimeout(object sender, EventArgs e)
    {
        var timer = sender as Timer; // WinForms
        // var timer = sender as DispatcherTimer; // WPF
        if (timer == null)
        {
            return;
        }

        // Testing - updates window title
        var isbn = timer.Tag.ToString();
        windowFrame.Text = isbn; // WinForms
        // windowFrame.Title = isbn; // WPF

        // The timer must be stopped! We want to act only once per keystroke.
        timer.Stop();
    }

Parts of code are taken from the Roslyn syntax visualizer

Answer 2

I propose a solution using Microsoft Reactive Extensions which are available as a nuget package.

Reactive Extensions is a library to compose asynchronous and event-based programs using observable collections and LINQ-style query operators.

If you use the RX extensions your problem can be solved with just two lines of code:

Sign up for an event: here with count == 10

    IObservable<string> textChangedObservable =
    Observable.FromEventPattern(textBox1, "TextChanged")
    .Select(evt => ((TextBox)evt.Sender).Text).Where(x => x.Length == 10);

Subscribe to the Event:

    textChangedObservable.Subscribe(e => MessageBox.Show(e));