Add API to restrict the windowing backend to load

In situations when the default backend would fail (for example
when compiled with X11 support but run without DISPLAY), or
when the application is using backend specific code, it makes
sense to let the application choose the backend explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=707869

clutter/clutter-backend-private.h

@@ -99,6 +99,8 @@ struct _ClutterBackendClass
   void (* settings_changed)   (ClutterBackend *backend);
 };
 
+ClutterBackend *        _clutter_create_backend                         (void);
+
 ClutterStageWindow *    _clutter_backend_create_stage                   (ClutterBackend         *backend,
                                                                          ClutterStage           *wrapper,
                                                                          GError                **error);

clutter/clutter-backend.c

@@ -83,6 +83,9 @@
 /* XXX - should probably warn, here */
 #include "tslib/clutter-event-tslib.h"
 #endif
+#ifdef CLUTTER_WINDOWING_EGL
+#include "egl/clutter-backend-eglnative.h"
+#endif
 #ifdef CLUTTER_INPUT_WAYLAND
 #include "wayland/clutter-device-manager-wayland.h"
 #endif
@@ -126,6 +129,7 @@ static guint backend_signals[LAST_SIGNAL] = { 0, };
 static struct wl_display *_wayland_compositor_display;
 #endif
 
+static const char *allowed_backend;
 
 static void
 clutter_backend_dispose (GObject *gobject)
@@ -463,6 +467,58 @@ clutter_backend_real_create_stage (ClutterBackend  *backend,
                        NULL);
 }
 
+ClutterBackend *
+_clutter_create_backend (void)
+{
+  const char *backend = allowed_backend;
+  ClutterBackend *retval = NULL;
+
+  if (backend == NULL)
+    {
+      const char *backend_env = g_getenv ("CLUTTER_BACKEND");
+
+      if (backend_env != NULL)
+	backend = g_intern_string (backend_env);
+    }
+
+#ifdef CLUTTER_WINDOWING_OSX
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_OSX))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_OSX, NULL);
+  else
+#endif
+#ifdef CLUTTER_WINDOWING_WIN32
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_WIN32))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_WIN32, NULL);
+  else
+#endif
+#ifdef CLUTTER_WINDOWING_X11
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_X11))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_X11, NULL);
+  else
+#endif
+#ifdef CLUTTER_WINDOWING_WAYLAND
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_WAYLAND))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_WAYLAND, NULL);
+  else
+#endif
+#ifdef CLUTTER_WINDOWING_EGL
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_EGL))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_EGL_NATIVE, NULL);
+  else
+#endif
+#ifdef CLUTTER_WINDOWING_GDK
+  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_GDK))
+    retval = g_object_new (CLUTTER_TYPE_BACKEND_GDK, NULL);
+  else
+#endif
+  if (backend == NULL)
+    g_error ("No default Clutter backend found.");
+  else
+    g_error ("Unsupported Clutter backend: '%s'", backend);
+
+  return retval;
+}
+
 static void
 clutter_backend_real_init_events (ClutterBackend *backend)
 {
@@ -506,8 +562,8 @@ clutter_backend_real_init_events (ClutterBackend *backend)
 #endif
 #ifdef CLUTTER_INPUT_EVDEV
   /* Evdev can be used regardless of the windowing system */
-  if (input_backend != NULL &&
+  if ((input_backend != NULL && strcmp (input_backend, CLUTTER_INPUT_EVDEV) == 0) ||
-      strcmp (input_backend, CLUTTER_INPUT_EVDEV) == 0)
+      clutter_check_windowing_backend (CLUTTER_WINDOWING_EGL))
     {
       _clutter_events_evdev_init (backend);
     }
@@ -1326,3 +1382,21 @@ clutter_wayland_set_compositor_display (void *display)
   _wayland_compositor_display = display;
 }
 #endif
+
+/**
+ * clutter_set_windowing_backend:
+ * @first_backend: the name of a clutter window backend
+ *
+ * Restricts clutter to only use the specified backend.
+ * This must be called before the first API call to clutter, including
+ * clutter_get_option_context()
+ *
+ * Since: 1.16
+ */
+void
+clutter_set_windowing_backend (const char *backend_type)
+{
+  g_return_if_fail (backend_type != NULL);
+
+  allowed_backend = g_intern_string (backend_type);
+}

clutter/clutter-backend.h

@@ -59,6 +59,9 @@ GType clutter_backend_get_type    (void) G_GNUC_CONST;
 
 ClutterBackend *clutter_get_default_backend (void);
 
+CLUTTER_AVAILABLE_IN_1_16
+void           clutter_set_windowing_backend (const char *backend_type);
+
 gdouble                     clutter_backend_get_resolution            (ClutterBackend             *backend);
 
 void                        clutter_backend_set_font_options          (ClutterBackend             *backend,

clutter/clutter-main.c

@@ -1390,53 +1390,6 @@ _clutter_context_is_initialized (void)
   return ClutterCntx->is_initialized;
 }
 
-static ClutterBackend *
-clutter_create_backend (void)
-{
-  const char *backend = g_getenv ("CLUTTER_BACKEND");
-  ClutterBackend *retval = NULL;
-
-  if (backend != NULL)
-    backend = g_intern_string (backend);
-
-#ifdef CLUTTER_WINDOWING_OSX
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_OSX))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_OSX, NULL);
-  else
-#endif
-#ifdef CLUTTER_WINDOWING_WIN32
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_WIN32))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_WIN32, NULL);
-  else
-#endif
-#ifdef CLUTTER_WINDOWING_X11
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_X11))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_X11, NULL);
-  else
-#endif
-#ifdef CLUTTER_WINDOWING_WAYLAND
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_WAYLAND))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_WAYLAND, NULL);
-  else
-#endif
-#ifdef CLUTTER_WINDOWING_EGL
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_EGL))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_EGL_NATIVE, NULL);
-  else
-#endif
-#ifdef CLUTTER_WINDOWING_GDK
-  if (backend == NULL || backend == I_(CLUTTER_WINDOWING_GDK))
-    retval = g_object_new (CLUTTER_TYPE_BACKEND_GDK, NULL);
-  else
-#endif
-  if (backend == NULL)
-    g_error ("No default Clutter backend found.");
-  else
-    g_error ("Unsupported Clutter backend: '%s'", backend);
-
-  return retval;
-}
-
 static ClutterMainContext *
 clutter_context_get_default_unlocked (void)
 {
@@ -1449,7 +1402,7 @@ clutter_context_get_default_unlocked (void)
       ctx->is_initialized = FALSE;
 
       /* create the windowing system backend */
-      ctx->backend = clutter_create_backend ();
+      ctx->backend = _clutter_create_backend ();
 
       /* create the default settings object, and store a back pointer to
        * the backend singleton

clutter/clutter.symbols

@@ -1193,6 +1193,7 @@ clutter_settings_get_type
 clutter_set_default_frame_rate
 clutter_set_font_flags
 clutter_set_motion_events_enabled
+clutter_set_windowing_backend
 clutter_shader_compile
 clutter_shader_effect_get_program
 clutter_shader_effect_get_shader