/*
 * ***** BEGIN GPL LICENSE BLOCK *****
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 * The Original Code is Copyright (C) 2004 Blender Foundation.
 * All rights reserved.
 *
 * The Original Code is: all of this file.
 *
 * Contributor(s): Joshua Leung
 *
 * ***** END GPL LICENSE BLOCK *****
 */

/** \file blender/editors/space_outliner/outliner_dragdrop.c
 *  \ingroup spoutliner
 */

#include <string.h>

#include "MEM_guardedalloc.h"

#include "DNA_collection_types.h"
#include "DNA_material_types.h"
#include "DNA_object_types.h"
#include "DNA_space_types.h"

#include "BLI_listbase.h"
#include "BLI_string.h"

#include "BLT_translation.h"

#include "BKE_collection.h"
#include "BKE_context.h"
#include "BKE_layer.h"
#include "BKE_library.h"
#include "BKE_main.h"
#include "BKE_material.h"
#include "BKE_object.h"
#include "BKE_report.h"
#include "BKE_scene.h"

#include "DEG_depsgraph.h"
#include "DEG_depsgraph_build.h"

#include "ED_object.h"
#include "ED_outliner.h"
#include "ED_screen.h"

#include "UI_interface.h"
#include "UI_resources.h"
#include "UI_view2d.h"

#include "GPU_state.h"

#include "RNA_access.h"
#include "RNA_define.h"
#include "RNA_enum_types.h"

#include "WM_api.h"
#include "WM_types.h"

#include "outliner_intern.h"

/* ******************** Drop Target Find *********************** */

static TreeElement *outliner_dropzone_element(TreeElement *te, const float fmval[2], const bool children)
{
	if ((fmval[1] > te->ys) && (fmval[1] < (te->ys + UI_UNIT_Y))) {
		/* name and first icon */
		if ((fmval[0] > te->xs + UI_UNIT_X) && (fmval[0] < te->xend))
			return te;
	}
	/* Not it.  Let's look at its children. */
	if (children && (TREESTORE(te)->flag & TSE_CLOSED) == 0 && (te->subtree.first)) {
		for (te = te->subtree.first; te; te = te->next) {
			TreeElement *te_valid = outliner_dropzone_element(te, fmval, children);
			if (te_valid)
				return te_valid;
		}
	}
	return NULL;
}

/* Find tree element to drop into. */
static TreeElement *outliner_dropzone_find(const SpaceOops *soops, const float fmval[2], const bool children)
{
	TreeElement *te;

	for (te = soops->tree.first; te; te = te->next) {
		TreeElement *te_valid = outliner_dropzone_element(te, fmval, children);
		if (te_valid)
			return te_valid;
	}
	return NULL;
}

static TreeElement *outliner_drop_find(bContext *C, const wmEvent *event)
{
	ARegion *ar = CTX_wm_region(C);
	SpaceOops *soops = CTX_wm_space_outliner(C);
	float fmval[2];
	UI_view2d_region_to_view(&ar->v2d, event->mval[0], event->mval[1], &fmval[0], &fmval[1]);

	return outliner_dropzone_find(soops, fmval, true);
}

static ID *outliner_ID_drop_find(bContext *C, const wmEvent *event, short idcode)
{
	TreeElement *te = outliner_drop_find(C, event);
	TreeStoreElem *tselem = (te) ? TREESTORE(te) : NULL;

	if (te && te->idcode == idcode && tselem->type == 0) {
		return tselem->id;
	}
	else {
		return NULL;
	}
}

/* Find tree element to drop into, with additional before and after reorder support. */
static TreeElement *outliner_drop_insert_find(
        bContext *C, const wmEvent *event,
        TreeElementInsertType *r_insert_type)
{
	SpaceOops *soops = CTX_wm_space_outliner(C);
	ARegion *ar = CTX_wm_region(C);
	TreeElement *te_hovered;
	float view_mval[2];

	UI_view2d_region_to_view(&ar->v2d, event->mval[0], event->mval[1], &view_mval[0], &view_mval[1]);
	te_hovered = outliner_find_item_at_y(soops, &soops->tree, view_mval[1]);

	if (te_hovered) {
		/* mouse hovers an element (ignoring x-axis), now find out how to insert the dragged item exactly */
		const float margin = UI_UNIT_Y * (1.0f / 4);

		if (view_mval[1] < (te_hovered->ys + margin)) {
			if (TSELEM_OPEN(TREESTORE(te_hovered), soops)) {
				/* inserting after a open item means we insert into it, but as first child */
				if (BLI_listbase_is_empty(&te_hovered->subtree)) {
					*r_insert_type = TE_INSERT_INTO;
					return te_hovered;
				}
				else {
					*r_insert_type = TE_INSERT_BEFORE;
					return te_hovered->subtree.first;
				}
			}
			else {
				*r_insert_type = TE_INSERT_AFTER;
				return te_hovered;
			}
		}
		else if (view_mval[1] > (te_hovered->ys + (3 * margin))) {
			*r_insert_type = TE_INSERT_BEFORE;
			return te_hovered;
		}
		else {
			*r_insert_type = TE_INSERT_INTO;
			return te_hovered;
		}
	}
	else {
		/* mouse doesn't hover any item (ignoring x-axis), so it's either above list bounds or below. */
		TreeElement *first = soops->tree.first;
		TreeElement *last = soops->tree.last;

		if (view_mval[1] < last->ys) {
			*r_insert_type = TE_INSERT_AFTER;
			return last;
		}
		else if (view_mval[1] > (first->ys + UI_UNIT_Y)) {
			*r_insert_type = TE_INSERT_BEFORE;
			return first;
		}
		else {
			BLI_assert(0);
			return NULL;
		}
	}
}

static Collection *outliner_collection_from_tree_element_and_parents(TreeElement *te, TreeElement **r_te)
{
	while (te != NULL) {
		Collection *collection = outliner_collection_from_tree_element(te);
		if (collection) {
			*r_te = te;
			return collection;
		}
		te = te->parent;
	}
	return NULL;
}

static TreeElement *outliner_drop_insert_collection_find(
        bContext *C, const wmEvent *event,
        TreeElementInsertType *r_insert_type)
{
	TreeElement *te = outliner_drop_insert_find(C, event, r_insert_type);
	if (!te) return NULL;

	TreeElement *collection_te;
	Collection *collection = outliner_collection_from_tree_element_and_parents(te, &collection_te);
	if (!collection) return NULL;

	if (collection_te != te) {
		*r_insert_type = TE_INSERT_INTO;
	}

	/* We can't insert before/after master collection. */
	if (collection->flag & COLLECTION_IS_MASTER) {
		*r_insert_type = TE_INSERT_INTO;
	}

	return collection_te;
}

/* ******************** Parent Drop Operator *********************** */

static bool parent_drop_allowed(SpaceOops *soops, TreeElement *te, Object *potential_child)
{
	TreeStoreElem *tselem = TREESTORE(te);
	if (te->idcode != ID_OB || tselem->type != 0) {
		return false;
	}

	Object *potential_parent = (Object *)tselem->id;

	if (potential_parent == potential_child) return false;
	if (BKE_object_is_child_recursive(potential_child, potential_parent)) return false;
	if (potential_parent == potential_child->parent) return false;

	/* check that parent/child are both in the same scene */
	Scene *scene = (Scene *)outliner_search_back(soops, te, ID_SCE);

	/* currently outliner organized in a way that if there's no parent scene
		* element for object it means that all displayed objects belong to
		* active scene and parenting them is allowed (sergey)
		*/
	if (scene) {
		for (ViewLayer *view_layer = scene->view_layers.first;
		     view_layer;
		     view_layer = view_layer->next)
		{
			if (BKE_view_layer_base_find(view_layer, potential_child)) {
				return true;
			}
		}
		return false;
	}
	else {
		return true;
	}
}

static bool allow_parenting_without_modifier_key(SpaceOops *soops)
{
	switch (soops->outlinevis) {
		case SO_VIEW_LAYER:
			return soops->filter & SO_FILTER_NO_COLLECTION;
		case SO_SCENES:
			return true;
		default:
			return false;
	}
}

static bool parent_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event, const char **UNUSED(tooltip))
{
	SpaceOops *soops = CTX_wm_space_outliner(C);

	bool changed = outliner_flag_set(&soops->tree, TSE_DRAG_ANY, false);
	if (changed) ED_region_tag_redraw_no_rebuild(CTX_wm_region(C));

	Object *potential_child = (Object *)WM_drag_ID(drag, ID_OB);
	if (!potential_child) return false;

	if (!allow_parenting_without_modifier_key(soops)) {
		if (!event->shift) return false;
	}

	TreeElement *te = outliner_drop_find(C, event);
	if (!te) return false;

	if (parent_drop_allowed(soops, te, potential_child)) {
		TREESTORE(te)->flag |= TSE_DRAG_INTO;
		ED_region_tag_redraw_no_rebuild(CTX_wm_region(C));
		return true;
	}

	return false;
}

static int parent_drop_exec(bContext *C, wmOperator *op)
{
	Object *par = NULL, *ob = NULL;
	Main *bmain = CTX_data_main(C);
	Scene *scene = CTX_data_scene(C);
	int partype = -1;
	char parname[MAX_NAME], childname[MAX_NAME];

	partype = RNA_enum_get(op->ptr, "type");
	RNA_string_get(op->ptr, "parent", parname);
	par = (Object *)BKE_libblock_find_name(bmain, ID_OB, parname);
	RNA_string_get(op->ptr, "child", childname);
	ob = (Object *)BKE_libblock_find_name(bmain, ID_OB, childname);

	if (ID_IS_LINKED(ob)) {
		BKE_report(op->reports, RPT_INFO, "Can't edit library linked object");
		return OPERATOR_CANCELLED;
	}

	ED_object_parent_set(op->reports, C, scene, ob, par, partype, false, false, NULL);

	DEG_relations_tag_update(bmain);
	WM_event_add_notifier(C, NC_OBJECT | ND_TRANSFORM, NULL);
	WM_event_add_notifier(C, NC_OBJECT | ND_PARENT, NULL);

	return OPERATOR_FINISHED;
}

static int parent_drop_invoke(bContext *C, wmOperator *op, const wmEvent *event)
{
	Main *bmain = CTX_data_main(C);
	SpaceOops *soops = CTX_wm_space_outliner(C);
	TreeElement *te = outliner_drop_find(C, event);
	TreeStoreElem *tselem = te ? TREESTORE(te) : NULL;

	if (!(te && te->idcode == ID_OB && tselem->type == 0)) {
		return OPERATOR_CANCELLED;
	}

	Object *par = (Object *)tselem->id;
	Object *ob = (Object *)WM_drag_ID_from_event(event, ID_OB);

	if (ELEM(NULL, ob, par)) {
		return OPERATOR_CANCELLED;
	}
	if (ob == par) {
		return OPERATOR_CANCELLED;
	}
	if (ID_IS_LINKED(ob)) {
		BKE_report(op->reports, RPT_INFO, "Can't edit library linked object");
		return OPERATOR_CANCELLED;
	}

	char childname[MAX_NAME];
	char parname[MAX_NAME];
	STRNCPY(childname, ob->id.name + 2);
	STRNCPY(parname, par->id.name + 2);
	RNA_string_set(op->ptr, "child", childname);
	RNA_string_set(op->ptr, "parent", parname);

	Scene *scene = (Scene *)outliner_search_back(soops, te, ID_SCE);

	if (scene == NULL) {
		/* currently outlier organized in a way, that if there's no parent scene
		 * element for object it means that all displayed objects belong to
		 * active scene and parenting them is allowed (sergey)
		 */

		scene = CTX_data_scene(C);
	}

	if ((par->type != OB_ARMATURE) && (par->type != OB_CURVE) && (par->type != OB_LATTICE)) {
		int partype = 0;
		if (ED_object_parent_set(op->reports, C, scene, ob, par, partype, false, false, NULL)) {
			DEG_relations_tag_update(bmain);
			WM_event_add_notifier(C, NC_OBJECT | ND_TRANSFORM, NULL);
			WM_event_add_notifier(C, NC_OBJECT | ND_PARENT, NULL);
		}
	}
	else {
		/* Menu creation */
		wmOperatorType *ot = WM_operatortype_find("OUTLINER_OT_parent_drop", false);
		uiPopupMenu *pup = UI_popup_menu_begin(C, IFACE_("Set Parent To"), ICON_NONE);
		uiLayout *layout = UI_popup_menu_layout(pup);
		PointerRNA ptr;

		/* Cannot use uiItemEnumO()... have multiple properties to set. */
		uiItemFullO_ptr(layout, ot, IFACE_("Object"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
		RNA_string_set(&ptr, "parent", parname);
		RNA_string_set(&ptr, "child", childname);
		RNA_enum_set(&ptr, "type", PAR_OBJECT);

		/* par becomes parent, make the associated menus */
		if (par->type == OB_ARMATURE) {
			uiItemFullO_ptr(layout, ot, IFACE_("Armature Deform"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_ARMATURE);

			uiItemFullO_ptr(layout, ot, IFACE_("   With Empty Groups"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_ARMATURE_NAME);

			uiItemFullO_ptr(layout, ot, IFACE_("   With Envelope Weights"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_ARMATURE_ENVELOPE);

			uiItemFullO_ptr(layout, ot, IFACE_("   With Automatic Weights"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_ARMATURE_AUTO);

			uiItemFullO_ptr(layout, ot, IFACE_("Bone"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_BONE);
		}
		else if (par->type == OB_CURVE) {
			uiItemFullO_ptr(layout, ot, IFACE_("Curve Deform"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_CURVE);

			uiItemFullO_ptr(layout, ot, IFACE_("Follow Path"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_FOLLOW);

			uiItemFullO_ptr(layout, ot, IFACE_("Path Constraint"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_PATH_CONST);
		}
		else if (par->type == OB_LATTICE) {
			uiItemFullO_ptr(layout, ot, IFACE_("Lattice Deform"), 0, NULL, WM_OP_EXEC_DEFAULT, 0, &ptr);
			RNA_string_set(&ptr, "parent", parname);
			RNA_string_set(&ptr, "child", childname);
			RNA_enum_set(&ptr, "type", PAR_LATTICE);
		}

		UI_popup_menu_end(C, pup);

		return OPERATOR_INTERFACE;
	}

	return OPERATOR_FINISHED;
}

void OUTLINER_OT_parent_drop(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Drop to Set Parent";
	ot->description = "Drag to parent in Outliner";
	ot->idname = "OUTLINER_OT_parent_drop";

	/* api callbacks */
	ot->invoke = parent_drop_invoke;
	ot->exec = parent_drop_exec;

	ot->poll = ED_operator_outliner_active;

	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;

	/* properties */
	RNA_def_string(ot->srna, "child", "Object", MAX_NAME, "Child", "Child Object");
	RNA_def_string(ot->srna, "parent", "Object", MAX_NAME, "Parent", "Parent Object");
	RNA_def_enum(ot->srna, "type", prop_make_parent_types, 0, "Type", "");
}

/* ******************** Parent Clear Operator *********************** */

static bool parent_clear_poll(bContext *C, wmDrag *drag, const wmEvent *event, const char **UNUSED(tooltip))
{
	SpaceOops *soops = CTX_wm_space_outliner(C);

	if (!allow_parenting_without_modifier_key(soops)) {
		if (!event->shift) return false;
	}

	Object *ob = (Object *)WM_drag_ID(drag, ID_OB);
	if (!ob) return false;
	if (!ob->parent) return false;

	TreeElement *te = outliner_drop_find(C, event);
	if (te) {
		TreeStoreElem *tselem = TREESTORE(te);
		ID *id = tselem->id;
		if (!id) return true;

		switch (GS(id->name)) {
			case ID_OB:
				return ELEM(tselem->type, TSE_MODIFIER_BASE, TSE_CONSTRAINT_BASE);
			case ID_GR:
				return event->shift;
			default:
				return true;
		}
	}
	else {
		return true;
	}
}

static int parent_clear_invoke(bContext *C, wmOperator *UNUSED(op), const wmEvent *event)
{
	Main *bmain = CTX_data_main(C);
	Object *ob = (Object *)WM_drag_ID_from_event(event, ID_OB);

	if (ob == NULL) {
		return OPERATOR_CANCELLED;
	}

	ED_object_parent_clear(ob, 0);

	DEG_relations_tag_update(bmain);
	WM_event_add_notifier(C, NC_OBJECT | ND_TRANSFORM, NULL);
	WM_event_add_notifier(C, NC_OBJECT | ND_PARENT, NULL);
	return OPERATOR_FINISHED;
}

void OUTLINER_OT_parent_clear(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Drop to Clear Parent";
	ot->description = "Drag to clear parent in Outliner";
	ot->idname = "OUTLINER_OT_parent_clear";

	/* api callbacks */
	ot->invoke = parent_clear_invoke;

	ot->poll = ED_operator_outliner_active;

	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;
}

/* ******************** Scene Drop Operator *********************** */

static bool scene_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event, const char **UNUSED(tooltip))
{
	/* Ensure item under cursor is valid drop target */
	Object *ob = (Object *)WM_drag_ID(drag, ID_OB);
	return (ob && (outliner_ID_drop_find(C, event, ID_SCE) != NULL));
}

static int scene_drop_invoke(bContext *C, wmOperator *UNUSED(op), const wmEvent *event)
{
	Main *bmain = CTX_data_main(C);
	Scene *scene = (Scene *)outliner_ID_drop_find(C, event, ID_SCE);
	Object *ob = (Object *)WM_drag_ID_from_event(event, ID_OB);

	if (ELEM(NULL, ob, scene) || ID_IS_LINKED(scene)) {
		return OPERATOR_CANCELLED;
	}

	if (BKE_scene_has_object(scene, ob)) {
		return OPERATOR_CANCELLED;
	}

	Collection *collection;
	if (scene != CTX_data_scene(C)) {
		/* when linking to an inactive scene link to the master collection */
		collection = BKE_collection_master(scene);
	}
	else {
		collection = CTX_data_collection(C);
	}

	BKE_collection_object_add(bmain, collection, ob);

	for (ViewLayer *view_layer = scene->view_layers.first; view_layer; view_layer = view_layer->next) {
		Base *base = BKE_view_layer_base_find(view_layer, ob);
		if (base) {
			ED_object_base_select(base, BA_SELECT);
		}
	}

	DEG_relations_tag_update(bmain);

	DEG_id_tag_update(&scene->id, ID_RECALC_SELECT);
	WM_main_add_notifier(NC_SCENE | ND_OB_SELECT, scene);

	return OPERATOR_FINISHED;
}

void OUTLINER_OT_scene_drop(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Drop Object to Scene";
	ot->description = "Drag object to scene in Outliner";
	ot->idname = "OUTLINER_OT_scene_drop";

	/* api callbacks */
	ot->invoke = scene_drop_invoke;

	ot->poll = ED_operator_outliner_active;

	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;
}

/* ******************** Material Drop Operator *********************** */

static bool material_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event, const char **UNUSED(tooltip))
{
	/* Ensure item under cursor is valid drop target */
	Material *ma = (Material *)WM_drag_ID(drag, ID_MA);
	return (ma && (outliner_ID_drop_find(C, event, ID_OB) != NULL));
}

static int material_drop_invoke(bContext *C, wmOperator *UNUSED(op), const wmEvent *event)
{
	Main *bmain = CTX_data_main(C);
	Object *ob = (Object *)outliner_ID_drop_find(C, event, ID_OB);
	Material *ma = (Material *)WM_drag_ID_from_event(event, ID_MA);

	if (ELEM(NULL, ob, ma)) {
		return OPERATOR_CANCELLED;
	}

	assign_material(bmain, ob, ma, ob->totcol + 1, BKE_MAT_ASSIGN_USERPREF);

	WM_event_add_notifier(C, NC_SPACE | ND_SPACE_VIEW3D, CTX_wm_view3d(C));
	WM_event_add_notifier(C, NC_MATERIAL | ND_SHADING_LINKS, ma);

	return OPERATOR_FINISHED;
}

void OUTLINER_OT_material_drop(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Drop Material on Object";
	ot->description = "Drag material to object in Outliner";
	ot->idname = "OUTLINER_OT_material_drop";

	/* api callbacks */
	ot->invoke = material_drop_invoke;

	ot->poll = ED_operator_outliner_active;

	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;
}

/* ******************** Collection Drop Operator *********************** */

typedef struct CollectionDrop {
	Collection *from;
	Collection *to;

	TreeElement *te;
	TreeElementInsertType insert_type;
} CollectionDrop;

static Collection *collection_parent_from_ID(ID *id)
{
	/* Can't change linked parent collections. */
	if (!id || ID_IS_LINKED(id)) {
		return NULL;
	}

	/* Also support dropping into/from scene collection. */
	if (GS(id->name) == ID_SCE) {
		return ((Scene *)id)->master_collection;
	}
	else if (GS(id->name) == ID_GR) {
		return (Collection *)id;
	}

	return NULL;
}

static bool collection_drop_init(bContext *C, wmDrag *drag, const wmEvent *event, CollectionDrop *data)
{
	SpaceOops *soops = CTX_wm_space_outliner(C);

	/* Get collection to drop into. */
	TreeElementInsertType insert_type;
	TreeElement *te = outliner_drop_insert_collection_find(C, event, &insert_type);
	if (!te) {
		return false;
	}

	Collection *to_collection = outliner_collection_from_tree_element(te);
	if (ID_IS_LINKED(to_collection)) {
		return false;
	}

	/* Get drag datablocks. */
	if (drag->type != WM_DRAG_ID) {
		return false;
	}

	wmDragID *drag_id = drag->ids.first;
	if (drag_id == NULL) {
		return false;
	}

	ID *id = drag_id->id;
	if (!(id && ELEM(GS(id->name), ID_GR, ID_OB))) {
		return false;
	}

	/* Get collection to drag out of. */
	ID *parent = drag_id->from_parent;
	Collection *from_collection = collection_parent_from_ID(parent);
	if (event->ctrl || soops->outlinevis == SO_SCENES) {
		from_collection = NULL;
	}

	/* Get collections. */
	if (GS(id->name) == ID_GR) {
		if (id == &to_collection->id) {
			return false;
		}
	}
	else {
		insert_type = TE_INSERT_INTO;
	}

	data->from = from_collection;
	data->to = to_collection;
	data->te = te;
	data->insert_type = insert_type;

	return true;
}

static bool collection_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event, const char **tooltip)
{
	SpaceOops *soops = CTX_wm_space_outliner(C);
	ARegion *ar = CTX_wm_region(C);
	bool changed = outliner_flag_set(&soops->tree, TSE_HIGHLIGHTED | TSE_DRAG_ANY, false);

	CollectionDrop data;
	if (!event->shift && collection_drop_init(C, drag, event, &data)) {
		TreeElement *te = data.te;
		TreeStoreElem *tselem = TREESTORE(te);
		if (!data.from || event->ctrl) {
			tselem->flag |= TSE_DRAG_INTO;
			changed = true;
			*tooltip = IFACE_("Link inside Collection");
		}
		else {
			switch (data.insert_type) {
				case TE_INSERT_BEFORE:
					tselem->flag |= TSE_DRAG_BEFORE;
					changed = true;
					if (te->prev && outliner_is_collection_tree_element(te->prev)) {
						*tooltip = TIP_("Move between collections");
					}
					else {
						*tooltip = TIP_("Move before collection");
					}
					break;
				case TE_INSERT_AFTER:
					tselem->flag |= TSE_DRAG_AFTER;
					changed = true;
					if (te->next && outliner_is_collection_tree_element(te->next)) {
						*tooltip = TIP_("Move between collections");
					}
					else {
						*tooltip = TIP_("Move after collection");
					}
					break;
				case TE_INSERT_INTO:
					tselem->flag |= TSE_DRAG_INTO;
					changed = true;
					*tooltip = TIP_("Move inside collection (Ctrl to link, Shift to parent)");
					break;
			}
		}
		if (changed) ED_region_tag_redraw_no_rebuild(ar);
		return true;
	}
	else {
		if (changed) ED_region_tag_redraw_no_rebuild(ar);
		return false;
	}
}

static int collection_drop_invoke(bContext *C, wmOperator *UNUSED(op), const wmEvent *event)
{
	Main *bmain = CTX_data_main(C);
	Scene *scene = CTX_data_scene(C);

	if (event->custom != EVT_DATA_DRAGDROP) {
		return OPERATOR_CANCELLED;
	}

	ListBase *lb = event->customdata;
	wmDrag *drag = lb->first;

	CollectionDrop data;
	if (!collection_drop_init(C, drag, event, &data)) {
		return OPERATOR_CANCELLED;
	}

	/* Before/after insert handling. */
	Collection *relative = NULL;
	bool relative_after = false;

	if (ELEM(data.insert_type, TE_INSERT_BEFORE, TE_INSERT_AFTER)) {
		SpaceOops *soops = CTX_wm_space_outliner(C);

		relative = data.to;
		relative_after = (data.insert_type == TE_INSERT_AFTER);

		TreeElement *parent_te = outliner_find_parent_element(&soops->tree, NULL, data.te);
		data.to = (parent_te) ? outliner_collection_from_tree_element(parent_te) : NULL;
	}

	if (!data.to) {
		return OPERATOR_CANCELLED;
	}

	if (BKE_collection_is_empty(data.to)) {
		TREESTORE(data.te)->flag &= ~TSE_CLOSED;
	}

	for (wmDragID *drag_id = drag->ids.first; drag_id; drag_id = drag_id->next) {
		/* Ctrl enables linking, so we don't need a from collection then. */
		Collection *from = (event->ctrl) ? NULL : collection_parent_from_ID(drag_id->from_parent);

		if (GS(drag_id->id->name) == ID_OB) {
			/* Move/link object into collection. */
			Object *object = (Object *)drag_id->id;

			if (from) {
				BKE_collection_object_move(bmain, scene, data.to, from, object);
			}
			else {
				BKE_collection_object_add(bmain, data.to, object);
			}
		}
		else if (GS(drag_id->id->name) == ID_GR) {
			/* Move/link collection into collection. */
			Collection *collection = (Collection *)drag_id->id;

			if (collection != from) {
				BKE_collection_move(bmain, data.to, from, relative, relative_after, collection);
			}
		}

		if (from) {
			DEG_id_tag_update(&from->id, ID_RECALC_COPY_ON_WRITE);
		}
	}

	/* Update dependency graph. */
	DEG_id_tag_update(&data.to->id, ID_RECALC_COPY_ON_WRITE);
	DEG_relations_tag_update(bmain);
	WM_event_add_notifier(C, NC_SCENE | ND_LAYER, scene);

	return OPERATOR_FINISHED;
}

void OUTLINER_OT_collection_drop(wmOperatorType *ot)
{
	/* identifiers */
	ot->name = "Move to Collection";
	ot->description = "Drag to move to collection in Outliner";
	ot->idname = "OUTLINER_OT_collection_drop";

	/* api callbacks */
	ot->invoke = collection_drop_invoke;
	ot->poll = ED_operator_outliner_active;

	/* flags */
	ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;
}

/* ********************* Outliner Drag Operator ******************** */

static TreeElement *outliner_item_drag_element_find(SpaceOops *soops, ARegion *ar, const wmEvent *event)
{
	/* note: using EVT_TWEAK_ events to trigger dragging is fine,
	 * it sends coordinates from where dragging was started */
	const float my = UI_view2d_region_to_view_y(&ar->v2d, event->mval[1]);
	return outliner_find_item_at_y(soops, &soops->tree, my);
}

static int outliner_item_drag_drop_invoke(bContext *C, wmOperator *UNUSED(op), const wmEvent *event)
{
	ARegion *ar = CTX_wm_region(C);
	SpaceOops *soops = CTX_wm_space_outliner(C);
	TreeElement *te = outliner_item_drag_element_find(soops, ar, event);

	if (!te) {
		return (OPERATOR_CANCELLED | OPERATOR_PASS_THROUGH);
	}

	TreeElementIcon data = tree_element_get_icon(TREESTORE(te), te);
	if (!data.drag_id) {
		return (OPERATOR_CANCELLED | OPERATOR_PASS_THROUGH);
	}

	wmDrag *drag = WM_event_start_drag(C, data.icon, WM_DRAG_ID, NULL, 0.0, WM_DRAG_NOP);

	if (ELEM(GS(data.drag_id->name), ID_OB, ID_GR)) {
		/* For collections and objects we cheat and drag all selected. */

		/* Only drag element under mouse if it was not selected before. */
		if ((TREESTORE(te)->flag & TSE_SELECTED) == 0) {
			outliner_flag_set(&soops->tree, TSE_SELECTED, 0);
			TREESTORE(te)->flag |= TSE_SELECTED;
		}

		/* Gather all selected elements. */
		struct IDsSelectedData selected = {
			.selected_array  = {NULL, NULL},
		};

		if (GS(data.drag_id->name) == ID_OB) {
			outliner_tree_traverse(soops, &soops->tree, 0, TSE_SELECTED, outliner_find_selected_objects, &selected);
		}
		else {
			outliner_tree_traverse(soops, &soops->tree, 0, TSE_SELECTED, outliner_find_selected_collections, &selected);
		}

		LISTBASE_FOREACH (LinkData *, link, &selected.selected_array) {
			TreeElement *te_selected = (TreeElement *)link->data;
			ID *id;

			if (GS(data.drag_id->name) == ID_OB) {
				id = TREESTORE(te_selected)->id;
			}
			else {
				/* Keep collection hierarchies intact when dragging. */
				bool parent_selected = false;
				for (TreeElement *te_parent = te_selected->parent; te_parent; te_parent = te_parent->parent) {
					if (outliner_is_collection_tree_element(te_parent)) {
						if (TREESTORE(te_parent)->flag & TSE_SELECTED) {
							parent_selected = true;
							break;
						}
					}
				}

				if (parent_selected) {
					continue;
				}

				id = &outliner_collection_from_tree_element(te_selected)->id;
			}

			/* Find parent collection. */
			Collection *parent = NULL;

			if (te_selected->parent) {
				for (TreeElement *te_parent = te_selected->parent; te_parent; te_parent = te_parent->parent) {
					if (outliner_is_collection_tree_element(te_parent)) {
						parent = outliner_collection_from_tree_element(te_parent);
						break;
					}
				}
			}
			else {
				Scene *scene = CTX_data_scene(C);
				parent = BKE_collection_master(scene);
			}

			WM_drag_add_ID(drag, id, &parent->id);
		}

		BLI_freelistN(&selected.selected_array);
	}
	else {
		/* Add single ID. */
		WM_drag_add_ID(drag, data.drag_id, data.drag_parent);
	}

	return (OPERATOR_FINISHED | OPERATOR_PASS_THROUGH);
}

/* Outliner drag and drop. This operator mostly exists to support dragging
 * from outliner text instead of only from the icon, and also to show a
 * hint in the statusbar keymap. */

void OUTLINER_OT_item_drag_drop(wmOperatorType *ot)
{
	ot->name = "Drag and Drop";
	ot->idname = "OUTLINER_OT_item_drag_drop";
	ot->description = "Drag and drop element to another place";

	ot->invoke = outliner_item_drag_drop_invoke;
	ot->poll = ED_operator_outliner_active;
}

/* *************************** Drop Boxes ************************** */

/* region dropbox definition */
void outliner_dropboxes(void)
{
	ListBase *lb = WM_dropboxmap_find("Outliner", SPACE_OUTLINER, RGN_TYPE_WINDOW);

	WM_dropbox_add(lb, "OUTLINER_OT_parent_drop", parent_drop_poll, NULL);
	WM_dropbox_add(lb, "OUTLINER_OT_parent_clear", parent_clear_poll, NULL);
	WM_dropbox_add(lb, "OUTLINER_OT_scene_drop", scene_drop_poll, NULL);
	WM_dropbox_add(lb, "OUTLINER_OT_material_drop", material_drop_poll, NULL);
	WM_dropbox_add(lb, "OUTLINER_OT_collection_drop", collection_drop_poll, NULL);
}