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Documentation: Update the "Start an event loop" section

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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>
This commit is contained in:
Daniel Semkowicz 2022-06-17 16:35:49 +02:00 • committed by Laurent Pinchart
parent d85a91cf56
commit d2c097c482
1 changed files with 17 additions and 23 deletions
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40
Documentation/guides/application-developer.rst
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@ -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
generated by the video devices part of the capture pipeline. libcamera provides
its own ``EventDispatcher`` class (inspired by the `Qt event system`_) to
process and deliver events generated by ``EventNotifiers``.
libcamera creates an internal execution thread at `CameraManager::start()`_
time to decouple its own event processing from the application's main thread.
Applications are thus free to manage their own execution opportunely, and only
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
``EventNotifier`` class, which models a file descriptor event source registered
to an ``EventDispatcher``. Whenever the ``EventDispatcher`` detects an event on
a notifier it is monitoring, it emits the notifier's
``EventNotifier::activated`` signal. The libcamera components connect to the
notifiers' signals and emit application visible events, such as the
``Camera::bufferReady`` and ``Camera::requestCompleted`` signals.
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.
Real-world applications will likely either integrate with the event loop of the
framework they use, or create their own event loop to respond to user events.
For the simple application presented in this example, it is enough to prevent
immediate termination by pausing for 3 seconds. During that time, the libcamera
thread will generate request completion events that the application will handle
in the ``requestComplete()`` slot connected to the ``Camera::requestCompleted``
signal.
.. code:: cpp
EventDispatcher *dispatcher = cm->eventDispatcher();
Timer timer;
timer.start(3000);
while (timer.isRunning())
dispatcher->processEvents();
std::this_thread::sleep_for(3000ms);
Clean up and stop the application
---------------------------------