cam: Use libevent to implement event loop

To prepare for removal of the EventDispatcher from the libcamera public
API, switch to libevent to handle the event loop.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

README.rst

@@ -78,6 +78,9 @@ for documentation: [optional]
 for gstreamer: [optional]
 	libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev
 
+for cam: [optional]
+        libevent-dev
+
 for qcam: [optional]
 	qtbase5-dev libqt5core5a libqt5gui5 libqt5widgets5 qttools5-dev-tools libtiff-dev
 

src/cam/event_loop.cpp

@@ -5,19 +5,34 @@
  * event_loop.cpp - cam - Event loop
  */
 
-#include <libcamera/event_dispatcher.h>
-
 #include "event_loop.h"
 
-using namespace libcamera;
+#include <assert.h>
+#include <event2/event.h>
+#include <event2/thread.h>
 
-EventLoop::EventLoop(EventDispatcher *dispatcher)
-	: dispatcher_(dispatcher)
+EventLoop *EventLoop::instance_ = nullptr;
+
+EventLoop::EventLoop()
 {
+	assert(!instance_);
+
+	evthread_use_pthreads();
+	event_ = event_base_new();
+	instance_ = this;
 }
 
 EventLoop::~EventLoop()
 {
+	instance_ = nullptr;
+
+	event_base_free(event_);
+	libevent_global_shutdown();
+}
+
+EventLoop *EventLoop::instance()
+{
+	return instance_;
 }
 
 int EventLoop::exec()
@@ -26,7 +41,7 @@ int EventLoop::exec()
 	exit_.store(false, std::memory_order_release);
 
 	while (!exit_.load(std::memory_order_acquire))
-		dispatcher_->processEvents();
+		event_base_loop(event_, EVLOOP_NO_EXIT_ON_EMPTY);
 
 	return exitCode_;
 }
@@ -35,5 +50,10 @@ void EventLoop::exit(int code)
 {
 	exitCode_ = code;
 	exit_.store(true, std::memory_order_release);
-	dispatcher_->interrupt();
+	interrupt();
+}
+
+void EventLoop::interrupt()
+{
+	event_base_loopbreak(event_);
 }

src/cam/event_loop.h

@@ -9,26 +9,27 @@
 
 #include <atomic>
 
-#include <libcamera/event_notifier.h>
-
-namespace libcamera {
-class EventDispatcher;
-}
+struct event_base;
 
 class EventLoop
 {
 public:
-	EventLoop(libcamera::EventDispatcher *dispatcher);
+	EventLoop();
 	~EventLoop();
 
+	static EventLoop *instance();
+
 	int exec();
 	void exit(int code = 0);
 
 private:
-	libcamera::EventDispatcher *dispatcher_;
+	static EventLoop *instance_;
 
+	struct event_base *event_;
 	std::atomic<bool> exit_;
 	int exitCode_;
+
+	void interrupt();
 };
 
 #endif /* __CAM_EVENT_LOOP_H__ */

src/cam/main.cpp

@@ -52,7 +52,7 @@ private:
 	CameraManager *cm_;
 	std::shared_ptr<Camera> camera_;
 	std::unique_ptr<libcamera::CameraConfiguration> config_;
-	EventLoop *loop_;
+	EventLoop loop_;
 
 	bool strictFormats_;
 };
@@ -60,7 +60,7 @@ private:
 CamApp *CamApp::app_ = nullptr;
 
 CamApp::CamApp()
-	: cm_(nullptr), camera_(nullptr), config_(nullptr), loop_(nullptr),
+	: cm_(nullptr), camera_(nullptr), config_(nullptr),
 	  strictFormats_(false)
 {
 	CamApp::app_ = this;
@@ -134,16 +134,11 @@ int CamApp::init(int argc, char **argv)
 		std::cout << "Monitoring new hotplug and unplug events" << std::endl;
 	}
 
-	loop_ = new EventLoop(cm_->eventDispatcher());
-
 	return 0;
 }
 
 void CamApp::cleanup()
 {
-	delete loop_;
-	loop_ = nullptr;
-
 	if (camera_) {
 		camera_->release();
 		camera_.reset();
@@ -166,8 +161,7 @@ int CamApp::exec()
 
 void CamApp::quit()
 {
-	if (loop_)
-		loop_->exit();
+	loop_.exit();
 }
 
 int CamApp::parseOptions(int argc, char *argv[])
@@ -366,13 +360,13 @@ int CamApp::run()
 	}
 
 	if (options_.isSet(OptCapture)) {
-		Capture capture(camera_, config_.get(), loop_);
+		Capture capture(camera_, config_.get(), &loop_);
 		return capture.run(options_);
 	}
 
 	if (options_.isSet(OptMonitor)) {
 		std::cout << "Press Ctrl-C to interrupt" << std::endl;
-		ret = loop_->exec();
+		ret = loop_.exec();
 		if (ret)
 			std::cout << "Failed to run monitor loop" << std::endl;
 	}

src/cam/meson.build

@@ -1,5 +1,12 @@
 # SPDX-License-Identifier: CC0-1.0
 
+libevent = dependency('libevent_pthreads', required : false)
+
+if not libevent.found()
+    warning('libevent_pthreads not found, \'cam\' application will not be compiled')
+    subdir_done()
+endif
+
 cam_sources = files([
     'buffer_writer.cpp',
     'capture.cpp',
@@ -10,5 +17,9 @@ cam_sources = files([
 ])
 
 cam  = executable('cam', cam_sources,
-                  dependencies : [ libatomic, libcamera_dep ],
+                  dependencies : [
+                      libatomic,
+                      libcamera_dep,
+                      libevent,
+                  ],
                   install : true)