Get UI ready for using drag & drop to nest a collection into another one

Nothing happen yet when it's supposed to insert the collection into
another one, that part will be handled by @dfelinto.

See gif for demo of how it works UI wise: {F500337}

Also fixed off-by-one error in utility function.

7 files changed, 81 insertions, 24 deletions

diff --git a/source/blender/blenkernel/BKE_layer.h b/source/blender/blenkernel/BKE_layer.h
index 863f33dea8d..aa1d537a28e 100644
--- a/source/blender/blenkernel/BKE_layer.h
+++ b/source/blender/blenkernel/BKE_layer.h
@@ -86,6 +86,7 @@ int BKE_layer_collection_count(struct SceneLayer *sl);
 int BKE_layer_collection_findindex(struct SceneLayer *sl, struct LayerCollection *lc);
 void BKE_layer_collection_reinsert_after(const struct Scene *scene, struct SceneLayer *sl,
                                          struct LayerCollection *lc_reinsert, struct LayerCollection *lc_after);
+void BKE_layer_collection_reinsert_into(struct LayerCollection *lc_reinsert, struct LayerCollection *lc_into);
 
 struct LayerCollection *BKE_collection_link(struct SceneLayer *sl, struct SceneCollection *sc);
 
diff --git a/source/blender/blenkernel/intern/layer.c b/source/blender/blenkernel/intern/layer.c
index 605705c483a..77501c4acce 100644
--- a/source/blender/blenkernel/intern/layer.c
+++ b/source/blender/blenkernel/intern/layer.c
@@ -536,6 +536,10 @@ static ListBase *scene_collection_listbase_find(ListBase *lb, SceneCollection *s
 void BKE_layer_collection_reinsert_after(
         const Scene *scene, SceneLayer *sl, LayerCollection *lc_reinsert, LayerCollection *lc_after)
 {
+	/* TODO this function probably needs to be rewritten completely to support all cases
+	 * (reinserting master collection, reinsert into different hierarchy levels, etc) */
+	TODO_LAYER_OPERATORS;
+
 	SceneCollection *sc_master = BKE_collection_master(scene);
 	SceneCollection *sc_reinsert = lc_reinsert->scene_collection;
 	ListBase *lc_reinsert_lb = layer_collection_listbase_find(&sl->layer_collections, lc_reinsert);
@@ -564,6 +568,13 @@ void BKE_layer_collection_reinsert_after(
 	BKE_scene_layer_engine_settings_collection_recalculate(sl, lc_reinsert);
 }
 
+void BKE_layer_collection_reinsert_into(LayerCollection *lc_reinsert, LayerCollection *lc_into)
+{
+	/* TODO this is missing */
+	TODO_LAYER_OPERATORS;
+	UNUSED_VARS(lc_reinsert, lc_into);
+}
+
 /**
  * Link a collection to a renderlayer
  * The collection needs to be created separately
diff --git a/source/blender/editors/space_outliner/outliner_draw.c b/source/blender/editors/space_outliner/outliner_draw.c
index 990ad296bb9..816740b8769 100644
--- a/source/blender/editors/space_outliner/outliner_draw.c
+++ b/source/blender/editors/space_outliner/outliner_draw.c
@@ -1540,7 +1540,7 @@ static int outliner_count_visible_childs(const SpaceOops *soops, const TreeEleme
 static void outliner_draw_tree_element_floating(const SpaceOops *soops, const ARegion *ar,
                                                 const TreeElement *te_floating)
 {
-	const TreeElement *te_insert = te_floating->drag_data->insert_te;
+	const TreeElement *te_insert = te_floating->drag_data->insert_handle;
 	const ListBase *lb_parent = te_floating->parent ? &te_floating->parent->subtree : &soops->tree;
 	const TreeElement *te_insert_fallback = te_insert ? te_insert : lb_parent->first;
 	const int line_width = 2;
@@ -1560,14 +1560,30 @@ static void outliner_draw_tree_element_floating(const SpaceOops *soops, const AR
 
 	UI_GetThemeColorShade4ubv(TH_BACK, -40, col);
 	immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
-	immUniformColor4ubv(col);
-	glLineWidth(line_width);
+	glEnable(GL_BLEND);
+
+	if (!te_insert || (te_floating->drag_data->insert_type == TE_INSERT_AFTER)) {
+		immUniformColor4ubv(col);
+		glLineWidth(line_width);
 
-	immBegin(PRIM_LINE_STRIP, 2);
-	immVertex2f(pos, 0, coord_y);
-	immVertex2f(pos, ar->v2d.cur.xmax, coord_y);
-	immEnd();
+		immBegin(PRIM_LINE_STRIP, 2);
+		immVertex2f(pos, 0, coord_y);
+		immVertex2f(pos, ar->v2d.cur.xmax, coord_y);
+		immEnd();
+	}
+	else {
+		BLI_assert(te_floating->drag_data->insert_type == TE_INSERT_INTO);
+		immUniformColor4ub(UNPACK3(col), col[3] * 0.5f);
+
+		immBegin(PRIM_QUADS, 4);
+		immVertex2f(pos, 0, coord_y);
+		immVertex2f(pos, 0, coord_y + UI_UNIT_Y);
+		immVertex2f(pos, ar->v2d.cur.xmax, coord_y + UI_UNIT_Y);
+		immVertex2f(pos, ar->v2d.cur.xmax, coord_y);
+		immEnd();
+	}
 
+	glDisable(GL_BLEND);
 	immUnbindProgram();
 }
 
diff --git a/source/blender/editors/space_outliner/outliner_intern.h b/source/blender/editors/space_outliner/outliner_intern.h
index 731b5041bd9..4cee0e44fca 100644
--- a/source/blender/editors/space_outliner/outliner_intern.h
+++ b/source/blender/editors/space_outliner/outliner_intern.h
@@ -49,6 +49,12 @@ struct EditBone;
 struct wmKeyConfig;
 
 
+typedef enum TreeElementInsertType {
+	/* no INSERT_BEFORE needed for now */
+	TE_INSERT_AFTER,
+	TE_INSERT_INTO,
+} TreeElementInsertType;
+
 typedef enum TreeTraversalAction {
 	/* Continue traversal regularly, don't skip children. */
 	TRAVERSE_CONTINUE = 0,
@@ -63,7 +69,7 @@ typedef enum TreeTraversalAction {
  * Passing scene right now, may be better to allow some custom data.
  */
 typedef void (*TreeElementReinsertFunc)(const struct Scene *scene, struct TreeElement *insert_element,
-                                        struct TreeElement *insert_after);
+                                        struct TreeElement *insert_handle, TreeElementInsertType action);
 typedef TreeTraversalAction (*TreeTraversalFunc)(struct TreeElement *te, void *customdata);
 
 
@@ -84,8 +90,9 @@ typedef struct TreeElement {
 	TreeElementReinsertFunc reinsert;
 
 	struct {
+		TreeElementInsertType insert_type;
 		/* the element after which we may insert the dragged one (NULL to insert at top) */
-		struct TreeElement *insert_te;
+		struct TreeElement *insert_handle;
 	} *drag_data;
 } TreeElement;
 
diff --git a/source/blender/editors/space_outliner/outliner_ops.c b/source/blender/editors/space_outliner/outliner_ops.c
index 5187cc66f58..3db0b6ffa08 100644
--- a/source/blender/editors/space_outliner/outliner_ops.c
+++ b/source/blender/editors/space_outliner/outliner_ops.c
@@ -87,24 +87,40 @@ static void outliner_item_drag_end(OutlinerItemDrag *op_drag_data)
 	MEM_freeN(op_drag_data);
 }
 
-static void outliner_item_drag_handle(OutlinerItemDrag *op_drag_data, ARegion *ar, const wmEvent *event)
+static void outliner_item_drag_handle(ARegion *ar, const wmEvent *event, OutlinerItemDrag *op_drag_data)
 {
 	TreeElement *dragged_te = op_drag_data->dragged_te;
 	const int delta_mouse_y = event->y - op_drag_data->init_mouse_xy[1];
 	const int cmp_coord = (int)UI_view2d_region_to_view_y(&ar->v2d, event->mval[1]);
+	const float margin = UI_UNIT_Y * (1.0f / 3);
 
 	/* by default we don't change the item position */
-	dragged_te->drag_data->insert_te = dragged_te;
+	dragged_te->drag_data->insert_handle = dragged_te;
 
 	if (delta_mouse_y > 0) {
-		for (TreeElement *te = dragged_te->prev; te && (cmp_coord >= (te->ys + (UI_UNIT_Y * 0.5f))); te = te->prev) {
-			/* will be NULL if we want to insert as first element */
-			dragged_te->drag_data->insert_te = te->prev;
+		for (TreeElement *te = dragged_te->prev; te && (cmp_coord >= (te->ys + margin)); te = te->prev) {
+			if (cmp_coord > (te->ys + (2 * margin))) {
+				dragged_te->drag_data->insert_type = TE_INSERT_AFTER;
+				/* will be NULL if we want to insert as first element */
+				dragged_te->drag_data->insert_handle = te->prev;
+			}
+			else {
+				dragged_te->drag_data->insert_type = TE_INSERT_INTO;
+				dragged_te->drag_data->insert_handle = te;
+			}
 		}
 	}
 	else {
-		for (TreeElement *te = dragged_te->next; te && (cmp_coord <= (te->ys + (UI_UNIT_Y * 0.5f))); te = te->next) {
-			dragged_te->drag_data->insert_te = te;
+		for (TreeElement *te = dragged_te->next; te && (cmp_coord <= (te->ys + UI_UNIT_Y - margin)); te = te->next) {
+			if (cmp_coord < (te->ys + margin)) {
+				dragged_te->drag_data->insert_type = TE_INSERT_AFTER;
+				dragged_te->drag_data->insert_handle = te;
+				BLI_assert(te->prev != NULL);
+			}
+			else {
+				dragged_te->drag_data->insert_type = TE_INSERT_INTO;
+				dragged_te->drag_data->insert_handle = te;
+			}
 		}
 	}
 }
@@ -112,7 +128,7 @@ static void outliner_item_drag_handle(OutlinerItemDrag *op_drag_data, ARegion *a
 static bool outliner_item_drag_drop_apply(const Scene *scene, OutlinerItemDrag *op_drag_data)
 {
 	TreeElement *dragged_te = op_drag_data->dragged_te;
-	TreeElement *insert_after = dragged_te->drag_data->insert_te;
+	TreeElement *insert_after = dragged_te->drag_data->insert_handle;
 
 	if (insert_after == dragged_te) {
 		/* No need to do anything */
@@ -123,7 +139,7 @@ static bool outliner_item_drag_drop_apply(const Scene *scene, OutlinerItemDrag *
 		/* Not sure yet what the best way to handle reordering elements of different types
 		 * (and stored in different lists). For collection display mode this is enough. */
 		if (!insert_after || (insert_after->reinsert == dragged_te->reinsert)) {
-			dragged_te->reinsert(scene, dragged_te, insert_after);
+			dragged_te->reinsert(scene, dragged_te, insert_after, dragged_te->drag_data->insert_type);
 		}
 	}
 
@@ -157,7 +173,7 @@ static int outliner_item_drag_drop_modal(bContext *C, wmOperator *op, const wmEv
 			redraw = true;
 			break;
 		case MOUSEMOVE:
-			outliner_item_drag_handle(op_drag_data, ar, event);
+			outliner_item_drag_handle(ar, event, op_drag_data);
 			redraw = true;
 			break;
 	}
@@ -186,7 +202,7 @@ static int outliner_item_drag_drop_invoke(bContext *C, wmOperator *op, const wmE
 	op->customdata = outliner_item_drag_data_create(te, &event->x);
 	te->drag_data = MEM_callocN(sizeof(*te->drag_data), __func__);
 	/* by default we don't change the item position */
-	te->drag_data->insert_te = te;
+	te->drag_data->insert_handle = te;
 	/* unset highlighted tree element, dragged one will be highlighted instead */
 	outliner_set_flag(&soops->tree, TSE_HIGHLIGHTED, false);
 
diff --git a/source/blender/editors/space_outliner/outliner_tree.c b/source/blender/editors/space_outliner/outliner_tree.c
index 8a024e0cc82..0cf76a671a4 100644
--- a/source/blender/editors/space_outliner/outliner_tree.c
+++ b/source/blender/editors/space_outliner/outliner_tree.c
@@ -1287,13 +1287,19 @@ static void outliner_add_orphaned_datablocks(Main *mainvar, SpaceOops *soops)
 	}
 }
 
-static void outliner_collections_reorder(const Scene *scene, TreeElement *insert_element, TreeElement *insert_after)
+static void outliner_collections_reorder(const Scene *scene, TreeElement *insert_element, TreeElement *insert_handle,
+                                         TreeElementInsertType action)
 {
 	SceneLayer *sl = BKE_scene_layer_render_active(scene);
 	LayerCollection *insert_coll = insert_element->directdata;
-	LayerCollection *insert_after_coll = insert_after ? insert_after->directdata : NULL;
+	LayerCollection *insert_handle_coll = insert_handle ? insert_handle->directdata : NULL;
 
-	BKE_layer_collection_reinsert_after(scene, sl, insert_coll, insert_after_coll);
+	if (action == TE_INSERT_AFTER) {
+		BKE_layer_collection_reinsert_after(scene, sl, insert_coll, insert_handle_coll);
+	}
+	else if (action == TE_INSERT_INTO) {
+		BKE_layer_collection_reinsert_into(insert_coll, insert_handle_coll);
+	}
 }
 
 static void outliner_add_layer_collections_recursive(SpaceOops *soops, ListBase *tree, Scene *scene,
diff --git a/source/blender/editors/space_outliner/outliner_utils.c b/source/blender/editors/space_outliner/outliner_utils.c
index 0e3a4e89fdf..f123b240efc 100644
--- a/source/blender/editors/space_outliner/outliner_utils.c
+++ b/source/blender/editors/space_outliner/outliner_utils.c
@@ -49,7 +49,7 @@ TreeElement *outliner_find_item_at_y(const SpaceOops *soops, const ListBase *tre
 {
 	for (TreeElement *te_iter = tree->first; te_iter; te_iter = te_iter->next) {
 		if (view_co_y < (te_iter->ys + UI_UNIT_Y)) {
-			if (view_co_y > te_iter->ys) {
+			if (view_co_y >= te_iter->ys) {
 				/* co_y is inside this element */
 				return te_iter;
 			}