csharp
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gui-cs.Terminal.Gui
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<h1 id="event-processing-and-the-application-main-loop">Event Processing and the Application Main Loop</h1>

<p>The method <code>Application.Run</code> that we covered before will wait for
events from either the keyboard or mouse and route those events to the
proper view.</p>
<p>The job of waiting for events and dispatching them in the
<code>Application</code> is implemented by an instance of the
<a href=""><code>MainLoop</code></a>
class.</p>
<p>Mainloops are a common idiom in many user interface toolkits so many
of the concepts will be familiar to you if you have used other
toolkits before.</p>
<p>This class provides the following capabilities:</p>
<ul>
<li>Keyboard and mouse processing</li>
<li>.NET Async support</li>
<li>Timers processing</li>
<li>Invoking of UI code from a background thread</li>
<li>Idle processing handlers</li>
<li>Possibility of integration with other mainloops.</li>
<li>On Unix systems, it can monitor file descriptors for readability or writability.</li>
</ul>
<p>The <code>MainLoop</code> property in the the
<a href="../api/Terminal.Gui/Terminal.Gui.Application.html"><code>Application</code></a>
provides access to these functions.</p>
<p>When your code invokes <code>Application.Run (Toplevel)</code>, the application
will prepare the current
<a href="../api/Terminal.Gui/Terminal.Gui.Toplevel.html"><code>Toplevel</code></a> instance by
redrawing the screen appropriately and then calling the mainloop to
run.    </p>
<p>You can configure the Mainloop before calling Application.Run, or you
can configure the MainLoop in response to events during the execution.</p>
<p>The keyboard inputs is dispatched by the application class to the
current TopLevel window this is covered in more detail in the
<a href="keyboard.html">Keyboard Event Processing</a> document.</p>
<h2 id="async-execution">Async Execution</h2>
<p>On startup, the <code>Application</code> class configured the .NET Asynchronous
machinery to allow you to use the <code>await</code> keyword to run tasks in the
background and have the execution of those tasks resume on the context
of the main thread running the main loop.</p>
<p>Once you invoke <code>Application.Main</code> the async machinery will be ready
to use, and you can merely call methods using <code>await</code> from your main
thread, and the awaited code will resume execution on the main
thread. </p>
<h2 id="timers-processing">Timers Processing</h2>
<p>You can register timers to be executed at specified intervals by
calling the <a href=""><code>AddTimeout</code></a> method, like this:</p>
<pre><code class="lang-csharp">void UpdateTimer ()
{
    time.Text = DateTime.Now.ToString ();
}

var token = Application.MainLoop.AddTimeout (TimeSpan.FromSeconds (20), UpdateTimer);
</code></pre><p>The return value from AddTimeout is a token value that you can use if
you desire to cancel the timer before it runs:</p>
<pre><code class="lang-csharup">Application.MainLoop.RemoveTimeout (token);
</code></pre><h2 id="idle-handlers">Idle Handlers</h2>
<p>You can register code to be executed when the application is idling
and there are no events to process by calling the
<a href=""><code>AddIdle</code></a>
method.  This method takes as a parameter a function that will be
invoked when the application is idling.  </p>
<p>Idle functions should return <code>true</code> if they should be invoked again,
and <code>false</code> if the idle invocations should stop.</p>
<p>Like the timer APIs, the return value is a token that can be used to
cancel the scheduled idle function from being executed.</p>
<h2 id="threading">Threading</h2>
<p>Like other UI toolkits, Terminal.Gui is generally not thread safe.
You should avoid calling methods in the UI classes from a background
thread as there is no guarantee that they will not corrupt the state
of the UI application.  </p>
<p>Generally, as there is not much state, you will get lucky, but the
application will not behave properly.</p>
<p>You will be served better off by using C# async machinery and the
various APIs in the <code>System.Threading.Tasks.Task</code> APIs.   But if you
absolutely must work with threads on your own you should only invoke
APIs in Terminal.Gui from the main thread.</p>
<p>To make this simple, you can use the <code>Application.MainLoop.Invoke</code>
method and pass an <code>Action</code>.  This action will be queued for execution
on the main thread at an appropriate time and will run your code
there.</p>
<p>For example, the following shows how to properly update a label from a
background thread:</p>
<pre><code>void BackgroundThreadUpdateProgress ()
{
    Application.MainLoop.Invoke (() =&gt; {
        progress.Text = $&quot;Progress: {bytesDownloaded/totalBytes}&quot;;
        });
}
</code></pre><h2 id="integration-with-other-main-loop-drivers">Integration With Other Main Loop Drivers</h2>
<p>It is possible to run the main loop in a way that it does not take
over control of your application, but rather in a cooperative way.</p>
<p>To do this, you must use the lower-level APIs in <code>Application</code>: the
<code>Begin</code> method to prepare a toplevel for execution, followed by calls
to <code>MainLoop.EventsPending</code> to determine whether the events must be
processed, and in that case, calling <code>RunLoop</code> method and finally
completing the process  by calling <code>End</code>.</p>
<p>The method <code>Run</code> is implemented like this:</p>
<pre><code>void Run (Toplevel top)
{
    var runToken = Begin (view);
    RunLoop (runToken);
    End (runToken);
}
</code></pre><h2 id="unix-file-descriptor-monitoring">Unix File Descriptor Monitoring</h2>
<p>On Unix, it is possible to monitor file descriptors for input being
available, or for the file descriptor being available for data to be
written without blocking the application.</p>
<p>To do this, you on Unix, you can cast the <code>MainLoop</code> instance to a
<a href=""><code>UnixMainLoop</code></a>
and use the <code>AddWatch</code> method to register an interest on a particular
condition.</p>
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