app: add GimpCancelable interface

... which provides a method for canceling an operation managed by
an object.

app/core/Makefile.am

@@ -117,6 +117,8 @@ libappcore_a_sources = \
 	gimpbrushpipe-load.h			\
 	gimpbuffer.c				\
 	gimpbuffer.h				\
+	gimpcancelable.c			\
+	gimpcancelable.h			\
 	gimpchannel.c				\
 	gimpchannel.h				\
 	gimpchannel-combine.c			\

app/core/core-types.h

@@ -198,6 +198,7 @@ typedef struct _GimpTreeHandler     GimpTreeHandler;
 
 /*  interfaces  */
 
+typedef struct _GimpCancelable      GimpCancelable;  /* dummy typedef */
 typedef struct _GimpPickable        GimpPickable;    /* dummy typedef */
 typedef struct _GimpProgress        GimpProgress;    /* dummy typedef */
 typedef struct _GimpProjectable     GimpProjectable; /* dummy typedef */

app/core/gimpcancelable.c

@@ -0,0 +1,72 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * gimpcancelable.c
+ * Copyright (C) 2018 Ell
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+#include "config.h"
+
+#include <gdk-pixbuf/gdk-pixbuf.h>
+#include <gegl.h>
+
+#include "core-types.h"
+
+#include "gimpcancelable.h"
+#include "gimpmarshal.h"
+
+
+enum
+{
+  CANCEL,
+  LAST_SIGNAL
+};
+
+
+G_DEFINE_INTERFACE (GimpCancelable, gimp_cancelable, G_TYPE_OBJECT)
+
+
+static guint cancelable_signals[LAST_SIGNAL] = { 0 };
+
+
+/*  private functions  */
+
+
+static void
+gimp_cancelable_default_init (GimpCancelableInterface *iface)
+{
+  cancelable_signals[CANCEL] =
+    g_signal_new ("cancel",
+                  G_TYPE_FROM_CLASS (iface),
+                  G_SIGNAL_RUN_FIRST,
+                  G_STRUCT_OFFSET (GimpCancelableInterface, cancel),
+                  NULL, NULL,
+                  gimp_marshal_VOID__VOID,
+                  G_TYPE_NONE, 0);
+}
+
+
+/*  public functions  */
+
+
+void
+gimp_cancelable_cancel (GimpCancelable *cancelable)
+{
+  g_return_if_fail (GIMP_IS_CANCELABLE (cancelable));
+
+  g_signal_emit (cancelable, cancelable_signals[CANCEL], 0);
+}

app/core/gimpcancelable.h

@@ -0,0 +1,47 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995-1997 Spencer Kimball and Peter Mattis
+ *
+ * gimpcancelable.h
+ * Copyright (C) 2018 Ell
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_CANCELABLE_H__
+#define __GIMP_CANCELABLE_H__
+
+
+#define GIMP_TYPE_CANCELABLE               (gimp_cancelable_get_type ())
+#define GIMP_IS_CANCELABLE(obj)            (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GIMP_TYPE_CANCELABLE))
+#define GIMP_CANCELABLE(obj)               (G_TYPE_CHECK_INSTANCE_CAST ((obj), GIMP_TYPE_CANCELABLE, GimpCancelable))
+#define GIMP_CANCELABLE_GET_INTERFACE(obj) (G_TYPE_INSTANCE_GET_INTERFACE ((obj), GIMP_TYPE_CANCELABLE, GimpCancelableInterface))
+
+
+typedef struct _GimpCancelableInterface GimpCancelableInterface;
+
+struct _GimpCancelableInterface
+{
+  GTypeInterface base_iface;
+
+  /*  signals  */
+  void   (* cancel) (GimpCancelable *cancelable);
+};
+
+
+GType   gimp_cancelable_get_type (void) G_GNUC_CONST;
+
+void    gimp_cancelable_cancel   (GimpCancelable *cancelable);
+
+
+#endif  /* __GIMP_CANCELABLE_H__ */