libcamera: object: Notify objects of thread move

Send a synchronous message to objects just before they get moved to a
new thread. This allows the object to perform any required processing.
EventNotifier and Timer objects will use this mechanism to move
themselves to the new thread's event disaptcher.

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

src/libcamera/include/message.h

@@ -23,6 +23,7 @@ public:
 	enum Type {
 		None = 0,
 		InvokeMessage = 1,
+		ThreadMoveMessage = 2,
 		UserMessage = 1000,
 	};
 

src/libcamera/message.cpp

@@ -47,6 +47,8 @@ std::atomic_uint Message::nextUserType_{ Message::UserMessage };
  * \brief Invalid message type
  * \var Message::InvokeMessage
  * \brief Asynchronous method invocation across threads
+ * \var Message::ThreadMoveMessage
+ * \brief Object is being moved to a different thread
  * \var Message::UserMessage
  * \brief First value available for user-defined messages
  */

src/libcamera/object.cpp

@@ -135,6 +135,10 @@ void Object::invokeMethod(BoundMethodBase *method, void *args)
  * This method moves the object from the current thread to the new \a thread.
  * It shall be called from the thread in which the object currently lives,
  * otherwise the behaviour is undefined.
+ *
+ * Before the object is moved, a Message::ThreadMoveMessage message is sent to
+ * it. The message() method can be reimplement in derived classes to be notified
+ * of the upcoming thread move and perform any required processing.
  */
 void Object::moveToThread(Thread *thread)
 {
@@ -143,9 +147,17 @@ void Object::moveToThread(Thread *thread)
 	if (thread_ == thread)
 		return;
 
+	notifyThreadMove();
+
 	thread->moveObject(this);
 }
 
+void Object::notifyThreadMove()
+{
+	Message msg(Message::ThreadMoveMessage);
+	message(&msg);
+}
+
 void Object::connect(SignalBase *signal)
 {
 	signals_.push_back(signal);