Documentation: Update the "Start an event loop" section

Event loop was moved to be a part of CameraManager, so it is no longer
a user responsibility to control the event dispatching.

Signed-off-by: Daniel Semkowicz <dse@thaumatec.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/guides/application-developer.rst

@@ -27,10 +27,12 @@ defined names and types without the need of prefixing them.
    #include <iomanip>
    #include <iostream>
    #include <memory>
+   #include <thread>
 
    #include <libcamera/libcamera.h>
 
    using namespace libcamera;
+   using namespace std::chrono_literals;
 
    int main()
    {
@@ -506,35 +508,27 @@ and queue all the previously created requests.
    for (std::unique_ptr<Request> &request : requests)
       camera->queueRequest(request.get());
 
-Start an event loop
+Event processing
-~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~
 
-The libcamera library needs an event loop to monitor and dispatch events
+libcamera creates an internal execution thread at `CameraManager::start()`_
-generated by the video devices part of the capture pipeline. libcamera provides
+time to decouple its own event processing from the application's main thread.
-its own ``EventDispatcher`` class (inspired by the `Qt event system`_) to
+Applications are thus free to manage their own execution opportunely, and only
-process and deliver events generated by ``EventNotifiers``.
+need to respond to events generated by libcamera emitted through signals.
 
-.. _Qt event system: https://doc.qt.io/qt-5/eventsandfilters.html
+.. _CameraManager::start(): https://libcamera.org/api-html/classlibcamera_1_1CameraManager.html#a49e322880a2a26013bb0076788b298c5
 
-The libcamera library implements this by creating instances of the
+Real-world applications will likely either integrate with the event loop of the
-``EventNotifier`` class, which models a file descriptor event source registered
+framework they use, or create their own event loop to respond to user events.
-to an ``EventDispatcher``. Whenever the ``EventDispatcher`` detects an event on
+For the simple application presented in this example, it is enough to prevent
-a notifier it is monitoring, it emits the notifier's
+immediate termination by pausing for 3 seconds. During that time, the libcamera
-``EventNotifier::activated`` signal. The libcamera components connect to the
+thread will generate request completion events that the application will handle
-notifiers' signals and emit application visible events, such as the
+in the ``requestComplete()`` slot connected to the ``Camera::requestCompleted``
-``Camera::bufferReady`` and ``Camera::requestCompleted`` signals.
+signal.
-
-The code below retrieves a reference to the system-wide event dispatcher and for
-the a fixed duration of 3 seconds, processes all the events detected in the
-system.
 
 .. code:: cpp
 
-   EventDispatcher *dispatcher = cm->eventDispatcher();
+   std::this_thread::sleep_for(3000ms);
-   Timer timer;
-   timer.start(3000);
-   while (timer.isRunning())
-       dispatcher->processEvents();
 
 Clean up and stop the application
 ---------------------------------