Dispatcher - (WPF)

The Dispatcher class in Windows Presentation Foundation (WPF) is used to manage the message queue, which is pivotal for the user interface (UI) thread, perform cross-threading operations, and schedule work items to be processed sequentially in the UI thread.

Syntax

The Dispatcher class has four main members that can be used to work with the message queue:

• Invoke(Action): Executes the specified delegate synchronously on the thread that owns the Dispatcher.
• BeginInvoke(Action): Executes the specified delegate asynchronously on the thread that owns the Dispatcher.
• InvokeAsync(Action): Executes the specified delegate asynchronously on the thread that owns the Dispatcher.
• CheckAccess(): Determines whether the calling thread has access to the thread that owns the Dispatcher.

Here is an example syntax for waiting for a task to complete before continuing with other work that requires it:

private async void Button_Click(object sender, RoutedEventArgs e)
{
  await Task.Run(() => DoWork());

  // Update the UI on the UI thread
  var dispatcher = Dispatcher.CurrentDispatcher;
  dispatcher.Invoke(() => textBox1.Text = "Task completed!");
}

Example

Here is an example of how to use the Dispatcher in a WPF application:

private void Button_Click(object sender, RoutedEventArgs e)
{
  var dispatcher = Dispatcher.CurrentDispatcher;

  // Simulate a long-running task
  Task.Run(() =>
  {
    Thread.Sleep(5000);
    dispatcher.Invoke(() => textBox1.Text = "Task completed!");
  });

  textBox1.Text = "Task started...";
}

Output

When you run the WPF application, clicking the button will initiate a task that takes five seconds to complete, then updates the TextBox on the UI thread to show that the task is completed.

Explanation

In the example code, we use the Task class to simulate a long-running task that takes five seconds to complete. We then use the CurrentDispatcher property to get a reference to the Dispatcher for the current thread, which in this case is the UI thread. We then use the Invoke method to update the TextBox with a message when the task is completed.

Use

The Dispatcher is a critical class for any WPF application that includes a UI element in which a long or complex task is performed. If not properly managed, this task can affect the UI performance and responsiveness.

Important Points

• The Dispatcher is used to manage the message queue in WPF applications
• The Dispatcher class has four main members: Invoke, BeginInvoke, InvokeAsync, and CheckAccess.
• The Dispatcher should always be used to update the UI from a background thread.

Summary

In this page, we discussed how to use the Dispatcher class in a WPF application. We covered the syntax, example, output, explanation, use, important points, and summary of using the Dispatcher to manage the message queue, perform cross-threading operations, and schedule work items to be processed sequentially in the UI thread. The Dispatcher is a critical class for any WPF application that includes a UI element, and ensures that complex or long running tasks do not affect the UI performance and responsiveness.