[gimp] app: in GimpHistogramEditor, don't block when creating bg histogram

[Ell <ell src gnome org>]

commit 471efee769555a04782f7f9fe544b256b229753a
Author: Ell <ell_se yahoo com>
Date:   Fri Jan 4 15:13:43 2019 -0500

    app: in GimpHistogramEditor, don't block when creating bg histogram
    
    In GimpHistogramEditor, when the drawable preview is frozen, don't
    duplicate the main histogram as the bg histogram if calculation is
    still ongoing, since this will block until histogram calculation is
    complete.  In particular, this creates a noticeable stall when
    beginning a paint stroke while the histogram is being calculated.
    
    Instead, defer the creation of the bg histogram to the completion
    of the calculation of the main histogram.

 app/widgets/gimphistogrameditor.c | 69 ++++++++++++++++++++++++++++++++++-----
 app/widgets/gimphistogrameditor.h |  5 ++-
 2 files changed, 65 insertions(+), 9 deletions(-)
---
diff --git a/app/widgets/gimphistogrameditor.c b/app/widgets/gimphistogrameditor.c
index 3212d3f625..2552762df4 100644
--- a/app/widgets/gimphistogrameditor.c
+++ b/app/widgets/gimphistogrameditor.c
@@ -336,6 +336,8 @@ gimp_histogram_editor_set_image (GimpImageEditor *image_editor,
           editor->idle_id = 0;
         }
 
+      editor->update_pending = FALSE;
+
       g_signal_handlers_disconnect_by_func (image_editor->image,
                                             gimp_histogram_editor_update,
                                             editor);
@@ -462,8 +464,24 @@ gimp_histogram_editor_calculate_async_callback (GimpAsync           *async,
 {
   editor->calculate_async = NULL;
 
-  if (gimp_async_is_finished (async))
-    gimp_histogram_editor_info_update (editor);
+  if (gimp_async_is_finished (async) && editor->histogram)
+    {
+      if (editor->bg_pending)
+        {
+          GimpHistogramView *view = GIMP_HISTOGRAM_BOX (editor->box)->view;
+
+          editor->bg_histogram = gimp_histogram_duplicate (editor->histogram);
+
+          gimp_histogram_view_set_background (view, editor->bg_histogram);
+        }
+
+      gimp_histogram_editor_info_update (editor);
+    }
+
+  editor->bg_pending = FALSE;
+
+  if (update_pending)
+    gimp_histogram_editor_update (editor);
 }
 
 static gboolean
@@ -543,16 +561,42 @@ gimp_histogram_editor_frozen_update (GimpHistogramEditor *editor,
       if (! editor->bg_histogram &&
           gtk_widget_is_drawable (GTK_WIDGET (editor)))
         {
-          if (gimp_histogram_editor_validate (editor))
-            editor->bg_histogram = gimp_histogram_duplicate (editor->histogram);
+          if (editor->idle_id)
+            {
+              g_source_remove (editor->idle_id);
 
-          gimp_histogram_view_set_background (view, editor->bg_histogram);
+              gimp_histogram_editor_idle_update (editor);
+            }
+
+          if (gimp_histogram_editor_validate (editor))
+            {
+              if (editor->calculate_async)
+                {
+                  editor->bg_pending = TRUE;
+                }
+              else
+                {
+                  editor->bg_histogram = gimp_histogram_duplicate (
+                    editor->histogram);
+
+                  gimp_histogram_view_set_background (view,
+                                                      editor->bg_histogram);
+                }
+            }
         }
     }
-  else if (editor->bg_histogram)
+  else
     {
-      g_clear_object (&editor->bg_histogram);
-      gimp_histogram_view_set_background (view, NULL);
+      if (editor->bg_histogram)
+        {
+          g_clear_object (&editor->bg_histogram);
+          gimp_histogram_view_set_background (view, NULL);
+        }
+
+      editor->bg_pending = FALSE;
+
+      if (editor->update_pending)
+        gimp_async_cancel_and_wait (editor->calculate_async);
     }
 }
 
@@ -568,6 +612,15 @@ gimp_histogram_editor_buffer_update (GimpHistogramEditor *editor,
 static void
 gimp_histogram_editor_update (GimpHistogramEditor *editor)
 {
+  if (editor->bg_pending)
+    {
+      editor->update_pending = TRUE;
+
+      return;
+    }
+
+  editor->update_pending = FALSE;
+
   if (editor->calculate_async)
     gimp_async_cancel_and_wait (editor->calculate_async);
 
diff --git a/app/widgets/gimphistogrameditor.h b/app/widgets/gimphistogrameditor.h
index b1dbfb56dd..bed27cfd85 100644
--- a/app/widgets/gimphistogrameditor.h
+++ b/app/widgets/gimphistogrameditor.h
@@ -41,11 +41,14 @@ struct _GimpHistogramEditor
   GimpDrawable         *drawable;
   GimpHistogram        *histogram;
   GimpHistogram        *bg_histogram;
-  GimpAsync            *calculate_async;
 
   guint                 idle_id;
   gboolean              recompute;
 
+  GimpAsync            *calculate_async;
+  gboolean              bg_pending;
+  gboolean              update_pending;
+
   GtkWidget            *menu;
   GtkWidget            *box;
   GtkWidget            *labels[6];