libcamera: device_enumerator: Emit a signal when new devices are added

Emit a signal whenever new MediaDevices are added to the
DeviceEnumerator. This will allow CameraManager to be notified
about the new devices and it can re-emumerate all the devices
currently present on the system.

Device enumeration by the CameraManger is an expensive operation hence,
we want one signal emission per 'x' milliseconds to notify multiple
devices additions as a single batch, by the DeviceEnumerator.
Add a \todo to investigate the support for that.

Signed-off-by: Umang Jain <email@uajain.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/device_enumerator.h

@@ -13,6 +13,8 @@
 
 #include <linux/media.h>
 
+#include <libcamera/signal.h>
+
 namespace libcamera {
 
 class MediaDevice;
@@ -43,6 +45,8 @@ public:
 
 	std::shared_ptr<MediaDevice> search(const DeviceMatch &dm);
 
+	Signal<> devicesAdded;
+
 protected:
 	std::unique_ptr<MediaDevice> createDevice(const std::string &deviceNode);
 	void addDevice(std::unique_ptr<MediaDevice> &&media);

src/libcamera/camera_manager.cpp

@@ -155,12 +155,12 @@ void CameraManager::Private::createPipelineHandlers()
 		}
 	}
 
-	/* \todo Register hot-plug callback here */
+	enumerator_->devicesAdded.connect(this, &Private::createPipelineHandlers);
 }
 
 void CameraManager::Private::cleanup()
 {
-	/* \todo Unregister hot-plug callback here */
+	enumerator_->devicesAdded.disconnect(this, &Private::createPipelineHandlers);
 
 	/*
 	 * Release all references to cameras to ensure they all get destroyed

src/libcamera/device_enumerator.cpp

@@ -227,6 +227,16 @@ std::unique_ptr<MediaDevice> DeviceEnumerator::createDevice(const std::string &d
 	return media;
 }
 
+/**
+* \var DeviceEnumerator::devicesAdded
+* \brief Notify of new media devices being found
+*
+* This signal is emitted when the device enumerator finds new media devices in
+* the system. It may be emitted for every newly detected device, or once for
+* multiple devices, at the discretion of the device enumerator. Not all device
+* enumerator types may support dynamic detection of new devices.
+*/
+
 /**
  * \brief Add a media device to the enumerator
  * \param[in] media media device instance to add
@@ -242,6 +252,9 @@ void DeviceEnumerator::addDevice(std::unique_ptr<MediaDevice> &&media)
 		<< "Added device " << media->deviceNode() << ": " << media->driver();
 
 	devices_.push_back(std::move(media));
+
+	/* \todo To batch multiple additions, emit with a small delay here. */
+	devicesAdded.emit();
 }
 
 /**