libcamera: device_enumerator: add factory for DeviceEnumerators

Provide a factory for DeviceEnumerator objects. Depending on which
libraries are available there will be different ways to enumerate
information in the system. This factory hides this from the rest of the
library.

Currently udev enumeration is the only supported implementation, a sysfs
implementation is another method that surely will be added in the
future.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/device_enumerator.cpp

@@ -121,6 +121,25 @@ bool DeviceMatch::match(const DeviceInfo *info) const
  * Enumerator Base
  */
 
+DeviceEnumerator *DeviceEnumerator::create()
+{
+	DeviceEnumerator *enumerator;
+
+	/* TODO: add compile time checks to only try udev enumerator if libudev is available */
+	enumerator = new DeviceEnumeratorUdev();
+	if (!enumerator->init())
+		return enumerator;
+
+	/*
+	 * NOTE: Either udev is not available or initialization of it
+	 * failed, use/fallback on sysfs enumerator
+	 */
+
+	/* TODO: add a sysfs based enumerator */
+
+	return nullptr;
+}
+
 DeviceEnumerator::~DeviceEnumerator()
 {
 	for (DeviceInfo *dev : devices_) {

src/libcamera/include/device_enumerator.h

@@ -56,6 +56,8 @@ private:
 class DeviceEnumerator
 {
 public:
+	static DeviceEnumerator *create();
+
 	virtual ~DeviceEnumerator();
 
 	virtual int init() = 0;