libcamera: message: Add user message types

Reserve identifiers for user-defined message types and add an operation
to the Message class to register the type identifiers.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/include/message.h

@@ -7,6 +7,8 @@
 #ifndef __LIBCAMERA_MESSAGE_H__
 #define __LIBCAMERA_MESSAGE_H__
 
+#include <atomic>
+
 namespace libcamera {
 
 class Object;
@@ -19,6 +21,7 @@ public:
 	enum Type {
 		None = 0,
 		SignalMessage = 1,
+		UserMessage = 1000,
 	};
 
 	Message(Type type);
@@ -27,11 +30,15 @@ public:
 	Type type() const { return type_; }
 	Object *receiver() const { return receiver_; }
 
+	static Type registerMessageType();
+
 private:
 	friend class Thread;
 
 	Type type_;
 	Object *receiver_;
+
+	static std::atomic_uint nextUserType_;
 };
 
 class SignalMessage : public Message

src/libcamera/message.cpp

@@ -31,6 +31,8 @@ namespace libcamera {
 
 LOG_DEFINE_CATEGORY(Message)
 
+std::atomic_uint Message::nextUserType_{ Message::UserMessage };
+
 /**
  * \class Message
  * \brief A message that can be posted to a Thread
@@ -43,6 +45,8 @@ LOG_DEFINE_CATEGORY(Message)
  * \brief Invalid message type
  * \var Message::SignalMessage
  * \brief Asynchronous signal delivery across threads
+ * \var Message::UserMessage
+ * \brief First value available for user-defined messages
  */
 
 /**
@@ -70,6 +74,38 @@ Message::~Message()
  * \return The message receiver
  */
 
+/**
+ * \brief Reserve and register a custom user-defined message type
+ *
+ * Custom message types use values starting at Message::UserMessage. Assigning
+ * custom types manually may lead to accidental duplicated types. To avoid this
+ * problem, this method reserves and returns the next available user-defined
+ * message type.
+ *
+ * The recommended way to use this method is to subclass Message and provide a
+ * static accessor for the custom message type.
+ *
+ * \code{.cpp}
+ * class MyCustomMessage : public Message
+ * {
+ * public:
+ *	MyCustomMessage() : Message(type()) { }
+ *
+ *	static Message::Type type()
+ *	{
+ *		static MessageType type = registerMessageType();
+ *		return type;
+ *	}
+ * };
+ * \endcode
+ *
+ * \return A new unique message type
+ */
+Message::Type Message::registerMessageType()
+{
+	return static_cast<Message::Type>(nextUserType_++);
+}
+
 /**
  * \class SignalMessage
  * \brief A message carrying a Signal across threads

test/message.cpp

@@ -52,6 +52,17 @@ class MessageTest : public Test
 protected:
 	int run()
 	{
+		Message::Type msgType[2] = {
+			Message::registerMessageType(),
+			Message::registerMessageType(),
+		};
+
+		if (msgType[0] != Message::UserMessage ||
+		    msgType[1] != Message::UserMessage + 1) {
+			cout << "Failed to register message types" << endl;
+			return TestFail;
+		}
+
 		MessageReceiver receiver;
 		receiver.moveToThread(&thread_);