libcamera: Add signal/slot communication mechanism

Introduce a Signal class that allows connecting event sources (signals)
to event listeners (slots) without adding any boilerplate code usually
associated with the observer or listener design patterns.

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

src/libcamera/meson.build

@@ -6,6 +6,7 @@ libcamera_sources = files([
     'media_device.cpp',
     'media_object.cpp',
     'pipeline_handler.cpp',
+    'signal.cpp',
 ])
 
 libcamera_headers = files([

src/libcamera/signal.cpp

@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * signal.cpp - Signal & slot implementation
+ */
+
+namespace libcamera {
+
+/**
+ * \class Signal
+ * \brief Generic signal and slot communication mechanism
+ *
+ * Signals and slots are a language construct aimed at communication between
+ * objects through the observer pattern without the need for boilerplate code.
+ * See http://doc.qt.io/qt-5/signalsandslots.html for more information.
+ *
+ * Signals model events that can be observed from objects unrelated to the event
+ * source. Slots are functions that are called in response to a signal. Signals
+ * can be connected to and disconnected from slots dynamically at runtime. When
+ * a signal is emitted, all connected slots are called sequentially in the order
+ * they have been connected.
+ *
+ * Signals are defined with zero, one or more typed parameters. They are emitted
+ * with a value for each of the parameters, and those values are passed to the
+ * connected slots.
+ *
+ * Slots are normal static or class member functions. In order to be connected
+ * to a signal, their signature must match the signal type (taking the same
+ * arguments as the signal and returning void).
+ *
+ * Connecting a signal to a slot results in the slot being called with the
+ * arguments passed to the emit() function when the signal is emitted. Multiple
+ * slots can be connected to the same signal, and multiple signals can connected
+ * to the same slot. Duplicate connections between a signal and a slot are
+ * allowed and result in the slot being called multiple times for the same
+ * signal emission.
+ */
+
+/**
+ * \fn Signal::connect(T *object, void(T::*func)(Args...))
+ * \brief Connect the signal to a member function slot
+ * \param object The slot object pointer
+ * \param func The slot member function
+ */
+
+/**
+ * \fn Signal::connect(void(*func)(Args...))
+ * \brief Connect the signal to a static function slot
+ * \param func The slot static function
+ */
+
+/**
+ * \fn Signal::disconnect()
+ * \brief Disconnect the signal from all slots
+ */
+
+/**
+ * \fn Signal::disconnect(T *object)
+ * \brief Disconnect the signal from all slots of the \a object
+ */
+
+/**
+ * \fn Signal::disconnect(T *object, void(T::*func)(Args...))
+ * \brief Disconnect the signal from the \a object slot member function \a func
+ */
+
+/**
+ * \fn Signal::disconnect(void(*func)(Args...))
+ * \brief Disconnect the signal from the slot static function \a func
+ */
+
+/**
+ * \fn Signal::emit(Args... args)
+ * \brief Emit the signal and call all connected slots
+ *
+ * Emitting a signal calls all connected slots synchronously and sequentially in
+ * the order the slots have been connected. The arguments passed to the emit()
+ * function are passed to the slot functions unchanged. If a slot modifies one
+ * of the arguments (when passed by pointer or reference), the modification is
+ * thus visible to all subsequently called slots.
+ */
+
+} /* namespace libcamera */