Platform: implement {Glfw,Sdl2}Application::exitEvent().

SDL has to be special and behave weirdly, of course, as always.

doc/changelog.dox

@@ -80,13 +80,17 @@ See also:
     @ref Platform::AndroidApplication::ViewportEvent::dpiScaling(),
     @ref Platform::GlfwApplication::ViewportEvent::framebufferSize() and
     @ref Platform::GlfwApplication::ViewportEvent::dpiScaling() to ensure
-    feature parity with @ref Sdl2Application
+    feature parity with @ref Platform::Sdl2Application
 -   Clarified HiDPI behavior of Android apps and mentioning
     @ref platforms-android-apps-manifest-screen-compatibility-mode "the necessary steps"
     in the docs
 -   Implemented handling of @ref Platform::AndroidApplication::viewportEvent()
     and documenting how to get Android to call it instead of relaunching the
     app from scratch
+-   New @ref Platform::Sdl2Application::exitEvent() and
+    @ref Platform::GlfwApplication::exitEvent() events for responding to
+    application window close and possibility to cancel it (for example to
+    show an exit confirmation dialog)
 
 @subsection changelog-latest-changes Changes and improvements
 

src/Magnum/Platform/GlfwApplication.cpp

@@ -454,6 +454,11 @@ bool GlfwApplication::tryCreate(const Configuration& configuration, const GLConf
 
 void GlfwApplication::setupCallbacks() {
     glfwSetWindowUserPointer(_window, this);
+    glfwSetWindowCloseCallback(_window, [](GLFWwindow* const window){
+        ExitEvent e;
+        static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->exitEvent(e);
+        if(!e.isAccepted()) glfwSetWindowShouldClose(window, false);
+    });
     glfwSetWindowRefreshCallback(_window, [](GLFWwindow* const window){
         /* Properly redraw after the window is restored from minimized state */
         static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->drawEvent();
@@ -577,6 +582,10 @@ auto GlfwApplication::MouseScrollEvent::modifiers() -> Modifiers {
     return *_modifiers;
 }
 
+void GlfwApplication::exitEvent(ExitEvent& event) {
+    event.setAccepted();
+}
+
 void GlfwApplication::viewportEvent(ViewportEvent& event) {
     #ifdef MAGNUM_BUILD_DEPRECATED
     CORRADE_IGNORE_DEPRECATED_PUSH

src/Magnum/Platform/GlfwApplication.h

@@ -149,6 +149,7 @@ class GlfwApplication {
         #ifdef MAGNUM_TARGET_GL
         class GLConfiguration;
         #endif
+        class ExitEvent;
         class ViewportEvent;
         class InputEvent;
         class KeyEvent;
@@ -426,6 +427,17 @@ class GlfwApplication {
     #else
     private:
     #endif
+        /**
+         * @brief Exit event
+         *
+         * If implemented, it allows the application to react to an application
+         * exit (for example to save its internal state) and suppress it as
+         * well (for example to show a exit confirmation dialog). The default
+         * implementation calls @ref ExitEvent::setAccepted() on @p event,
+         * which tells the application that it's safe to exit.
+         */
+        virtual void exitEvent(ExitEvent& event);
+
         /**
          * @brief Viewport event
          *
@@ -1130,6 +1142,45 @@ class GlfwApplication::Configuration {
 
 CORRADE_ENUMSET_OPERATORS(GlfwApplication::Configuration::WindowFlags)
 
+/**
+@brief Exit event
+
+@see @ref exitEvent()
+*/
+class GlfwApplication::ExitEvent {
+    public:
+        /** @brief Copying is not allowed */
+        ExitEvent(const ExitEvent&) = delete;
+
+        /** @brief Moving is not allowed */
+        ExitEvent(ExitEvent&&) = delete;
+
+        /** @brief Copying is not allowed */
+        ExitEvent& operator=(const ExitEvent&) = delete;
+
+        /** @brief Moving is not allowed */
+        ExitEvent& operator=(ExitEvent&&) = delete;
+
+        /** @brief Whether the event is accepted */
+        bool isAccepted() const { return _accepted; }
+
+        /**
+         * @brief Set event as accepted
+         *
+         * If the event is ignored (i.e., not set as accepted) in
+         * @ref exitEvent(), the application won't exit. Default implementation
+         * of @ref exitEvent() accepts the event.
+         */
+        void setAccepted(bool accepted = true) { _accepted = accepted; }
+
+    private:
+        friend GlfwApplication;
+
+        explicit ExitEvent(): _accepted(false) {}
+
+        bool _accepted;
+};
+
 /**
 @brief Viewport event
 

src/Magnum/Platform/Sdl2Application.cpp

@@ -91,6 +91,12 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
     args.parse(arguments.argc, arguments.argv);
     #endif
 
+    /* Available since 2.0.4, disables interception of SIGINT and SIGTERM so
+       it's possible to Ctrl-C the application even if exitEvent() doesn't set
+       event.setAccepted(). */
+    #ifdef SDL_HINT_NO_SIGNAL_HANDLERS
+    SDL_SetHint(SDL_HINT_NO_SIGNAL_HANDLERS, "1");
+    #endif
     /* Available since 2.0.8, disables compositor bypass on X11, which causes
        flickering on KWin as the compositor gets shut down on every startup */
     #ifdef SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR
@@ -713,13 +719,18 @@ void Sdl2Application::mainLoopIteration() {
                 textEditingEvent(e);
             } break;
 
-            case SDL_QUIT:
-                #ifndef CORRADE_TARGET_EMSCRIPTEN
-                _flags |= Flag::Exit;
-                #else
-                emscripten_cancel_main_loop();
-                #endif
-                return;
+            case SDL_QUIT: {
+                ExitEvent e;
+                exitEvent(e);
+                if(e.isAccepted()) {
+                    #ifndef CORRADE_TARGET_EMSCRIPTEN
+                    _flags |= Flag::Exit;
+                    #else
+                    emscripten_cancel_main_loop();
+                    #endif
+                    return;
+                }
+            } break;
         }
     }
 
@@ -795,6 +806,10 @@ void Sdl2Application::setTextInputRect(const Range2Di& rect) {
     SDL_SetTextInputRect(&r);
 }
 
+void Sdl2Application::exitEvent(ExitEvent& event) {
+    event.setAccepted();
+}
+
 void Sdl2Application::tickEvent() {
     /* If this got called, the tick event is not implemented by user and thus
        we don't need to call it ever again */

src/Magnum/Platform/Sdl2Application.h

@@ -239,13 +239,35 @@ If no other application header is included, this class is also aliased to
 @cpp Platform::Application @ce and the macro is aliased to @cpp MAGNUM_APPLICATION_MAIN() @ce
 to simplify porting.
 
+@subsection Platform-Sdl2Application-usage-posix POSIX specifics
+
+On POSIX systems, SDL by default intercepts the `SIGTERM` signal and generates
+an exit event for it, instead of doing the usual application exit. This would
+mean that if the application fails to set @ref ExitEvent::setAccepted() in an
+@ref exitEvent() override for some reason, pressing
+@m_class{m-label m-warning} **Ctrl** @m_class{m-label m-default} **C** would
+not terminate it either and you'd have to forcibly kill it instead. When using
+SDL >= 2.0.4, @ref Sdl2Application turns this behavior off, making
+@ref exitEvent() behave consistently with other application implementations
+such as @ref GlfwApplication. You can turn this behavior back on by enabling
+the [corresponding SDL hint](https://wiki.libsdl.org/SDL_HINT_NO_SIGNAL_HANDLERS)
+through an environment variable:
+
+@code{.sh}
+SDL_NO_SIGNAL_HANDLERS=1 ./your-app
+@endcode
+
+See also the [SDL Wiki](https://wiki.libsdl.org/SDL_EventType#SDL_QUIT) for
+details.
+
 @subsection Platform-Sdl2Application-usage-linux Linux specifics
 
 SDL by default attempts to disable compositing, which may cause ugly flickering
 for non-fullscreen apps (KWin, among others, is known to respect this setting).
 When using SDL >= 2.0.8, @ref Sdl2Application turns this behavior off, keeping
 the compositor running to avoid the flicker. You can turn this behavior back on
-by enabling the [corresponding SDL hint](https://wiki.libsdl.org/CategoryHints) through an environment variable:
+by enabling the [corresponding SDL hint](https://wiki.libsdl.org/CategoryHints)
+through an environment variable:
 
 @code{.sh}
 SDL_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR=1 ./your-app
@@ -381,6 +403,7 @@ class Sdl2Application {
         #ifdef MAGNUM_TARGET_GL
         class GLConfiguration;
         #endif
+        class ExitEvent;
         class ViewportEvent;
         class InputEvent;
         class KeyEvent;
@@ -715,6 +738,21 @@ class Sdl2Application {
     #else
     private:
     #endif
+        /**
+         * @brief Exit event
+         *
+         * If implemented, it allows the application to react to an application
+         * exit (for example to save its internal state) and suppress it as
+         * well (for example to show a exit confirmation dialog). The default
+         * implementation calls @ref ExitEvent::setAccepted() on @p event,
+         * which tells the application that it's safe to exit.
+         *
+         * SDL has special behavior on POSIX systems regarding `SIGINT` and
+         * `SIGTERM` handling, see @ref Platform-Sdl2Application-usage-posix
+         * for more information.
+         */
+        virtual void exitEvent(ExitEvent& event);
+
         /**
          * @brief Tick event
          *
@@ -1606,6 +1644,45 @@ class Sdl2Application::Configuration {
         #endif
 };
 
+/**
+@brief Exit event
+
+@see @ref exitEvent()
+*/
+class Sdl2Application::ExitEvent {
+    public:
+        /** @brief Copying is not allowed */
+        ExitEvent(const ExitEvent&) = delete;
+
+        /** @brief Moving is not allowed */
+        ExitEvent(ExitEvent&&) = delete;
+
+        /** @brief Copying is not allowed */
+        ExitEvent& operator=(const ExitEvent&) = delete;
+
+        /** @brief Moving is not allowed */
+        ExitEvent& operator=(ExitEvent&&) = delete;
+
+        /** @brief Whether the event is accepted */
+        bool isAccepted() const { return _accepted; }
+
+        /**
+         * @brief Set event as accepted
+         *
+         * If the event is ignored (i.e., not set as accepted) in
+         * @ref exitEvent(), the application won't exit. Default implementation
+         * of @ref exitEvent() accepts the event.
+         */
+        void setAccepted(bool accepted = true) { _accepted = accepted; }
+
+    private:
+        friend Sdl2Application;
+
+        explicit ExitEvent(): _accepted(false) {}
+
+        bool _accepted;
+};
+
 /**
 @brief Viewport event
 

src/Magnum/Platform/Test/GlfwApplicationTest.cpp

@@ -29,6 +29,12 @@ namespace Magnum { namespace Platform { namespace Test {
 
 struct GlfwApplicationTest: Platform::Application {
     explicit GlfwApplicationTest(const Arguments& arguments): Platform::Application{arguments} {}
+
+    void exitEvent(ExitEvent& event) override {
+        Debug{} << "application exiting";
+        event.setAccepted(); /* Comment-out to test app exit suppression */
+    }
+
     void drawEvent() override {}
 };
 

src/Magnum/Platform/Test/Sdl2ApplicationTest.cpp

@@ -31,6 +31,11 @@ struct Sdl2ApplicationTest: Platform::Application {
     /* For testing resize events */
     explicit Sdl2ApplicationTest(const Arguments& arguments): Platform::Application{arguments, Configuration{}.setWindowFlags(Configuration::WindowFlag::Resizable)} {}
 
+    void exitEvent(ExitEvent& event) override {
+        Debug{} << "application exiting";
+        event.setAccepted(); /* Comment-out to test app exit suppression */
+    }
+
     void drawEvent() override {}
 
     /* For testing HiDPI resize events */