UI: List Panel System

This implements a general system to implement drag and drop, subpanels,
and UI animation for the stack UIs in Blender. There are NO functional
changes in this patch, but it makes it relatively trivial to implement
these features for stacks.

The biggest complication to using panels to implement the UI for lists
is that there can be multiple modifiers of the same type. Currently there
is an assumed 1 to 1 relationship between every panel and its type, but
there can be multiple list items of the same type, so we have to break
this relationship. The mapping between panels and their data is stored
with an index in the panel's runtime struct.

To make use the system for a list like modifiers, four components
must be added:
1. A panel type defined and registered for each list data type, with a
    known mapping between list data types and panel idnames.
1. A function called by interface code to build the add the panel
    layouts with the provided helper functions.
    - UI_panel_list_matches_data will check if the panel list needs to
      be rebuilt.
    - UI_panels_free_instanced will remove the existing list panels
    - UI_panel_add_instanced adds a list panel of a given type.
3. An expand flag for the list data and implementations of
    get_list_data_expand_flag and set_list_data_expand_flag.
4. For reordering, the panel type's reorder callback. This is called
   when the instanced panels are drag-dropped. This requires
   implementing a "move to index" operator for the list data.

Reviewed By: Severin, brecht

Differential Revision: https://developer.blender.org/D7490

1 files changed, 19 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_screen.h b/source/blender/blenkernel/BKE_screen.h
index 0d8f5ed550c..8794558b81f 100644
--- a/source/blender/blenkernel/BKE_screen.h
+++ b/source/blender/blenkernel/BKE_screen.h
@@ -229,6 +229,25 @@ typedef struct PanelType {
   /* draw entirely, view changes should be handled here */
   void (*draw)(const struct bContext *C, struct Panel *panel);
 
+  /* For instanced panels corresponding to a list: */
+
+  /** Reorder function, called when drag and drop finishes. */
+  void (*reorder)(struct bContext *C, struct Panel *pa, int new_index);
+  /**
+   * Get the panel and subpanel's expansion state from the expansion flag in the corresponding data
+   * item. Called on draw updates.
+   * \note Subpanels are indexed in depth first order, the visualorder you would see if all panels
+   * were expanded.
+   */
+  short (*get_list_data_expand_flag)(const struct bContext *C, struct Panel *pa);
+  /**
+   * Set the expansion bitfield from the closed / open state of this panel and its subpanels.
+   * Called when the expansion state of the panel changes with user input.
+   * \note Subpanels are indexed in depth first order, the visual order you would see if all panels
+   * were expanded.
+   */
+  void (*set_list_data_expand_flag)(const struct bContext *C, struct Panel *pa, short expand_flag);
+
   /* sub panels */
   struct PanelType *parent;
   ListBase children;