[gtk/wip/otte/listview: 197/309] listview: Try to keep the list items in order when scrolling

[Benjamin Otte <otte src gnome org>]

commit c2334a52ce3dfc90c5d3dbaa085e3fb32c22dca5
Author: Benjamin Otte <otte redhat com>
Date:   Fri Sep 28 03:33:16 2018 +0200

    listview: Try to keep the list items in order when scrolling
    
    Instead of just destroying all items and then recreating them (or even
    hide()ing and then show()ing them again (or even even repositioning
    them in the widget tree)), just try to reust them in the order they are.
    
    This works surprisingly well when scrolling and most/all widgets
    just moved.

 gtk/gtklistitemmanager.c        | 29 +++++++++++++++++++++++++++++
 gtk/gtklistitemmanagerprivate.h |  4 ++++
 gtk/gtklistview.c               | 34 +++++++++++++++++++++++++---------
 3 files changed, 58 insertions(+), 9 deletions(-)
---
diff --git a/gtk/gtklistitemmanager.c b/gtk/gtklistitemmanager.c
index 32687440b2..62888f366a 100644
--- a/gtk/gtklistitemmanager.c
+++ b/gtk/gtklistitemmanager.c
@@ -21,6 +21,8 @@
 
 #include "gtklistitemmanagerprivate.h"
 
+#include "gtkwidgetprivate.h"
+
 struct _GtkListItemManager
 {
   GObject parent_instance;
@@ -312,6 +314,33 @@ gtk_list_item_manager_try_reacquire_list_item (GtkListItemManager       *self,
   return GTK_WIDGET (result);
 }
 
+/**
+ * gtk_list_item_manager_move_list_item:
+ * @self: a #GtkListItemManager
+ * @list_item: an acquired #GtkListItem that should be moved to represent
+ *     a different row
+ * @position: the new position of that list item
+ * @prev_sibling: the new previous sibling
+ *
+ * Moves the widget to represent a new position in the listmodel without
+ * releasing the item.
+ *
+ * This is most useful when scrolling.
+ **/
+void
+gtk_list_item_manager_move_list_item (GtkListItemManager     *self,
+                                      GtkWidget              *list_item,
+                                      guint                   position,
+                                      GtkWidget              *prev_sibling)
+{
+  gpointer item;
+
+  item = g_list_model_get_item (self->model, position);
+  gtk_list_item_factory_bind (self->factory, GTK_LIST_ITEM (list_item), position, item);
+  gtk_widget_insert_after (list_item, _gtk_widget_get_parent (list_item), prev_sibling);
+  g_object_unref (item);
+}
+
 /**
  * gtk_list_item_manager_update_list_item:
  * @self: a #GtkListItemManager
diff --git a/gtk/gtklistitemmanagerprivate.h b/gtk/gtklistitemmanagerprivate.h
index 0f1d1bddd1..d34e346a21 100644
--- a/gtk/gtklistitemmanagerprivate.h
+++ b/gtk/gtklistitemmanagerprivate.h
@@ -69,6 +69,10 @@ GtkWidget *             gtk_list_item_manager_try_reacquire_list_item
 void                    gtk_list_item_manager_update_list_item  (GtkListItemManager     *self,
                                                                  GtkWidget              *item,
                                                                  guint                   position);
+void                    gtk_list_item_manager_move_list_item    (GtkListItemManager     *self,
+                                                                 GtkWidget              *list_item,
+                                                                 guint                   position,
+                                                                 GtkWidget              *prev_sibling);
 void                    gtk_list_item_manager_release_list_item (GtkListItemManager     *self,
                                                                  GtkListItemManagerChange *change,
                                                                  GtkWidget              *widget);
diff --git a/gtk/gtklistview.c b/gtk/gtklistview.c
index c9cfc7c840..35cf3b6da3 100644
--- a/gtk/gtklistview.c
+++ b/gtk/gtklistview.c
@@ -336,7 +336,8 @@ gtk_list_view_merge_list_rows (GtkListView *self,
 }
 
 static void
-gtk_list_view_release_rows (GtkListView *self)
+gtk_list_view_release_rows (GtkListView *self,
+                            GQueue      *released)
 {
   ListRow *row, *prev, *next;
   guint i;
@@ -347,7 +348,7 @@ gtk_list_view_release_rows (GtkListView *self)
     {
       if (row->widget)
         {
-          gtk_list_item_manager_release_list_item (self->item_manager, NULL, row->widget);
+          g_queue_push_tail (released, row->widget);
           row->widget = NULL;
           i++;
           prev = gtk_rb_tree_node_get_previous (row);
@@ -379,7 +380,7 @@ gtk_list_view_release_rows (GtkListView *self)
   
   if (row->widget)
     {
-      gtk_list_item_manager_release_list_item (self->item_manager, NULL, row->widget);
+      g_queue_push_tail (released, row->widget);
       row->widget = NULL;
       prev = gtk_rb_tree_node_get_previous (row);
       if (prev && gtk_list_view_merge_list_rows (self, prev, row))
@@ -390,7 +391,7 @@ gtk_list_view_release_rows (GtkListView *self)
     {
       if (next->widget)
         {
-          gtk_list_item_manager_release_list_item (self->item_manager, NULL, next->widget);
+          g_queue_push_tail (released, next->widget);
           next->widget = NULL;
         }
       gtk_list_view_merge_list_rows (self, row, next);
@@ -404,9 +405,10 @@ gtk_list_view_ensure_rows (GtkListView              *self,
 {
   ListRow *row, *new_row;
   guint i, offset;
-  GtkWidget *insert_after;
+  GtkWidget *widget, *insert_after;
+  GQueue released = G_QUEUE_INIT;
 
-  gtk_list_view_release_rows (self);
+  gtk_list_view_release_rows (self, &released);
 
   row = gtk_list_view_get_row (self, self->anchor_start, &offset);
   if (offset > 0)
@@ -444,9 +446,20 @@ gtk_list_view_ensure_rows (GtkListView              *self,
             }
           if (new_row->widget == NULL)
             {
-              new_row->widget = gtk_list_item_manager_acquire_list_item (self->item_manager,
-                                                                         i,
-                                                                         insert_after);
+              new_row->widget = g_queue_pop_head (&released);
+              if (new_row->widget)
+                {
+                  gtk_list_item_manager_move_list_item (self->item_manager,
+                                                        new_row->widget,
+                                                        i,
+                                                        insert_after);
+                }
+              else
+                {
+                  new_row->widget = gtk_list_item_manager_acquire_list_item (self->item_manager,
+                                                                             i,
+                                                                             insert_after);
+                }
             }
         }
       else
@@ -456,6 +469,9 @@ gtk_list_view_ensure_rows (GtkListView              *self,
         }
       insert_after = new_row->widget;
     }
+
+  while ((widget = g_queue_pop_head (&released)))
+    gtk_list_item_manager_release_list_item (self->item_manager, NULL, widget);
 }
 
 static void