UI: Changes to Graph Editor selection and transform

When introducing "drag-all-selected" support all over Blender, we
figured this wouldn't work well with the Graph Editor
selection/transform behavior.
Hence, William and I worked on the following changes, although we used
this chance to improve the behavior in general too.
For more info see T70634.

* Handles now always move with the key, regardless if they are selected
  or not.
* Selecting the key doesn't select the handles anymore, their selection
  is separate.
* Multiple keys and handles can now be dragged.
* Dragging a handle moves all selected handles **on the same side**.
* Tweak-dragging any handle can never affect any keyframe location,
  only handles.
* G/R/S should behave as before.
* Changing the handle type with a key selected always applies the change
  to both handles.
* Box selection with Ctrl+Drag now allows deselecting handles (used to
  act on entire triple only).
* Box selection //Include Handles// option now only acts on visible
  handles, wasn't the case with Only Selected Keyframes Handles enabled.
* Box selection //Include Handles// is now enabled by default in all
  bundled keymaps.

The changes have been tested for some days by the animators here in the
Blender Animation Studio. Some changes are based on their feedback.

Also, this improves/adds comments for related code.

Differential Revision: https://developer.blender.org/D6235

Reviewed by: Sybren Stüvel, William Reynish

1 files changed, 30 insertions, 0 deletions

diff --git a/source/blender/windowmanager/intern/wm_operator_props.c b/source/blender/windowmanager/intern/wm_operator_props.c
index 21636153904..72646718e41 100644
--- a/source/blender/windowmanager/intern/wm_operator_props.c
+++ b/source/blender/windowmanager/intern/wm_operator_props.c
@@ -396,8 +396,38 @@ void WM_operator_properties_select_operation_simple(wmOperatorType *ot)
   RNA_def_property_flag(prop, PROP_SKIP_SAVE);
 }
 
+/**
+ * Selecting and tweaking items are overlapping operations. Getting both to work without conflicts
+ * requires special care. See
+ * https://wiki.blender.org/wiki/Human_Interface_Guidelines/Selection#Select-tweaking for the
+ * desired behavior.
+ *
+ * For default click selection (with no modifier keys held), the select operators can do the
+ * following:
+ * - On a mouse press on an unselected item, change selection and finish immidiately after.
+ *   This sends an undo push and allows transform to take over should a tweak event be caught now.
+ * - On a mouse press on a selected item, don't change selection state, but start modal execution
+ *   of the operator. Idea is that we wait with deselecting other items until we know that the
+ *   intention wasn't to tweak (mouse press+drag) all selected items.
+ * - If a tweak is recognized before the release event happens, cancel the operator, so that
+ *   transform can take over and no undo-push is sent.
+ * - If the release event occurs rather than a tweak one, deselect all items but the one under the
+ *   cursor, and finish the modal operator.
+ *
+ * This utility, together with #WM_generic_select_invoke() and #WM_generic_select_modal() should
+ * help getting the wanted behavior to work. Most generic logic should be handled in these, so that
+ * the select operators only have to care for the case dependent handling.
+ *
+ * Every select operator has slightly diferent requirements, e.g. VSE strip selection also needs to
+ * account for handle selection. This should be the baseline behavior though.
+ */
 void WM_operator_properties_generic_select(wmOperatorType *ot)
 {
+  /* On the initial mouse press, this is set by #WM_generic_select_modal() to let the select
+   * operator exec callback know that it should not __yet__ deselect other items when clicking on
+   * an already selected one. Instead should make sure the operator executes modal then (see
+   * #WM_generic_select_modal()), so that the exec callback can be called a second time on the
+   * mouse release event to do this part. */
   PropertyRNA *prop = RNA_def_boolean(
       ot->srna, "wait_to_deselect_others", false, "Wait to Deselect Others", "");
   RNA_def_property_flag(prop, PROP_HIDDEN | PROP_SKIP_SAVE);