Node callback for handling link insertion and swapping of occupied inputs.

Nodes have a feature for moving existing links to unoccupied sockets when connecting
to an already used input. This is based on the standard legacy socket types (value/float,
vector, color/rgba) and works reasonably well for shader, compositor and texture nodes.

For new pynode systems, however, the hardcoded nature of that feature has major drawbacks:
* It does not take different type systems into account, leading to meaningless connections
  when sockets are swapped and making the feature useless or outright debilitating.
* Advanced socket behaviors would be possible with a registerable callback, e.g. creating
  extensible input lists that move existing connections down to make room for a new link.

Now any handling of new links is done via the 'insert_links' callback, which can also be
registered through the RNA API. For the legacy shader/compo/tex nodes the behavior is the
same, using a C callback.

Note on the 'use_swap' flag: this has been removed because it was meaningless anyway:
It was disabled only for the insert-node-on-link feature, which works only for
completely unconnected nodes anyway, so there would be nothing to swap in the first place.

source/blender/blenkernel/BKE_node.h

@@ -206,6 +206,8 @@ typedef struct bNodeType {
 	/* can this node be added to a node tree */
 	int (*poll_instance)(struct bNode *node, struct bNodeTree *nodetree);
 	
+	/* optional handling of link insertion */
+	void (*insert_link)(struct bNodeTree *ntree, struct bNode *node, struct bNodeLink *link);
 	/* Update the internal links list, for muting and disconnect operators. */
 	void (*update_internal_links)(struct bNodeTree *, struct bNode *node);
 	

source/blender/editors/space_node/node_relationships.c

@@ -29,8 +29,6 @@
  *  \ingroup spnode
  */
 
-#include <ctype.h>
-
 #include "MEM_guardedalloc.h"
 
 #include "DNA_node_types.h"
@@ -420,7 +418,6 @@ static void node_link_update_header(bContext *C, bNodeLinkDrag *UNUSED(nldrag))
 #undef HEADER_LENGTH
 }
 
-/* update link_count fields to avoid repeated link counting */
 static int node_count_links(bNodeTree *ntree, bNodeSocket *sock)
 {
 	bNodeLink *link;
@@ -434,64 +431,13 @@ static int node_count_links(bNodeTree *ntree, bNodeSocket *sock)
 	return count;
 }
 
-/* test if two sockets are interchangeable
+static void node_remove_extra_links(SpaceNode *snode, bNodeLink *link)
- * XXX this could be made into a tree-type callback for flexibility
- */
-static bool node_link_socket_match(bNodeSocket *a, bNodeSocket *b)
-{
-	/* tests if alphabetic prefix matches
-	 * this allows for imperfect matches, such as numeric suffixes,
-	 * like Color1/Color2
-	 */
-	int prefix_len = 0;
-	char *ca = a->name, *cb = b->name;
-	for (; *ca != '\0' && *cb != '\0'; ++ca, ++cb) {
-		/* end of common prefix? */
-		if (*ca != *cb) {
-			/* prefix delimited by non-alphabetic char */
-			if (isalpha(*ca) || isalpha(*cb))
-				return false;
-			break;
-		}
-		++prefix_len;
-	}
-	return prefix_len > 0;
-}
-
-/* find an eligible socket for linking */
-static bNodeSocket *node_find_linkable_socket(bNodeTree *ntree, bNode *node, bNodeSocket *cur, bool use_swap)
-{
-	int cur_link_count = node_count_links(ntree, cur);
-	if (cur_link_count <= cur->limit) {
-		/* current socket is fine, use it */
-		return cur;
-	}
-	else if (use_swap) {
-		/* link swapping: try to find a free slot with a matching name */
-		
-		bNodeSocket *first = cur->in_out == SOCK_IN ? node->inputs.first : node->outputs.first;
-		bNodeSocket *sock;
-		
-		sock = cur->next ? cur->next : first; /* wrap around the list end */
-		while (sock != cur) {
-			if (!nodeSocketIsHidden(sock) && node_link_socket_match(sock, cur)) {
-				int link_count = node_count_links(ntree, sock);
-				/* take +1 into account since we would add a new link */
-				if (link_count + 1 <= sock->limit)
-					return sock; /* found a valid free socket we can swap to */
-			}
-			
-			sock = sock->next ? sock->next : first; /* wrap around the list end */
-		}
-	}
-	return NULL;
-}
-
-static void node_remove_extra_links(SpaceNode *snode, bNodeLink *link, bool use_swap)
 {
 	bNodeTree *ntree = snode->edittree;
 	bNodeSocket *from = link->fromsock, *to = link->tosock;
 	bNodeLink *tlink, *tlink_next;
+	int to_count = node_count_links(ntree, to);
+	int from_count = node_count_links(ntree, from);
 	
 	for (tlink = ntree->links.first; tlink; tlink = tlink_next) {
 		tlink_next = tlink->next;
@@ -499,28 +445,18 @@ static void node_remove_extra_links(SpaceNode *snode, bNodeLink *link, bool use_
 			continue;
 		
 		if (tlink && tlink->fromsock == from) {
-			bNodeSocket *new_from = node_find_linkable_socket(ntree, tlink->fromnode, from, use_swap);
+			if (from_count > from->limit) {
-			if (new_from && new_from != from) {
-				/* redirect existing link */
-				tlink->fromsock = new_from;
-			}
-			else if (!new_from) {
-				/* no possible replacement, remove tlink */
 				nodeRemLink(ntree, tlink);
 				tlink = NULL;
+				--from_count;
 			}
 		}
 		
 		if (tlink && tlink->tosock == to) {
-			bNodeSocket *new_to = node_find_linkable_socket(ntree, tlink->tonode, to, use_swap);
+			if (to_count > to->limit) {
-			if (new_to && new_to != to) {
-				/* redirect existing link */
-				tlink->tosock = new_to;
-			}
-			else if (!new_to) {
-				/* no possible replacement, remove tlink */
 				nodeRemLink(ntree, tlink);
 				tlink = NULL;
+				--to_count;
 			}
 		}
 	}
@@ -537,6 +473,14 @@ static void node_link_exit(bContext *C, wmOperator *op, bool apply_links)
 		bNodeLink *link = linkdata->data;
 		
 		if (apply_links && link->tosock && link->fromsock) {
+			/* before actually adding the link,
+			 * let nodes perform special link insertion handling
+			 */
+			if (link->fromnode->typeinfo->insert_link)
+				link->fromnode->typeinfo->insert_link(ntree, link->fromnode, link);
+			if (link->tonode->typeinfo->insert_link)
+				link->tonode->typeinfo->insert_link(ntree, link->tonode, link);
+			
 			/* add link to the node tree */
 			BLI_addtail(&ntree->links, link);
 			
@@ -546,7 +490,7 @@ static void node_link_exit(bContext *C, wmOperator *op, bool apply_links)
 			link->tonode->update |= NODE_UPDATE;
 			
 			/* we might need to remove a link */
-			node_remove_extra_links(snode, link, true);
+			node_remove_extra_links(snode, link);
 		}
 		else
 			nodeRemLink(ntree, link);
@@ -1746,7 +1690,7 @@ void ED_node_link_insert(ScrArea *sa)
 			
 			link->tonode = select;
 			link->tosock = best_input;
-			node_remove_extra_links(snode, link, false);
+			node_remove_extra_links(snode, link);
 			link->flag &= ~NODE_LINKFLAG_HILITE;
 			
 			nodeAddLink(snode->edittree, select, best_output, node, sockto);

source/blender/makesrna/intern/rna_nodetree.c

@@ -1182,6 +1182,24 @@ static void rna_Node_update_reg(bNodeTree *ntree, bNode *node)
 	RNA_parameter_list_free(&list);
 }
 
+static void rna_Node_insert_link(bNodeTree *ntree, bNode *node, bNodeLink *link)
+{
+	extern FunctionRNA rna_Node_insert_link_func;
+
+	PointerRNA ptr;
+	ParameterList list;
+	FunctionRNA *func;
+
+	RNA_pointer_create((ID *)ntree, node->typeinfo->ext.srna, node, &ptr);
+	func = &rna_Node_insert_link_func;
+
+	RNA_parameter_list_create(&list, &ptr, func);
+	RNA_parameter_set_lookup(&list, "link", &link);
+	node->typeinfo->ext.call(NULL, &ptr, func, &list);
+
+	RNA_parameter_list_free(&list);
+}
+
 static void rna_Node_init(const bContext *C, PointerRNA *ptr)
 {
 	extern FunctionRNA rna_Node_init_func;
@@ -1384,12 +1402,13 @@ static bNodeType *rna_Node_register_base(Main *bmain, ReportList *reports, Struc
 	nt->poll = (have_function[0]) ? rna_Node_poll : NULL;
 	nt->poll_instance = (have_function[1]) ? rna_Node_poll_instance : rna_Node_poll_instance_default;
 	nt->updatefunc = (have_function[2]) ? rna_Node_update_reg : NULL;
-	nt->initfunc_api = (have_function[3]) ? rna_Node_init : NULL;
+	nt->insert_link = (have_function[3]) ? rna_Node_insert_link : NULL;
-	nt->copyfunc_api = (have_function[4]) ? rna_Node_copy : NULL;
+	nt->initfunc_api = (have_function[4]) ? rna_Node_init : NULL;
-	nt->freefunc_api = (have_function[5]) ? rna_Node_free : NULL;
+	nt->copyfunc_api = (have_function[5]) ? rna_Node_copy : NULL;
-	nt->draw_buttons = (have_function[6]) ? rna_Node_draw_buttons : NULL;
+	nt->freefunc_api = (have_function[6]) ? rna_Node_free : NULL;
-	nt->draw_buttons_ex = (have_function[7]) ? rna_Node_draw_buttons_ext : NULL;
+	nt->draw_buttons = (have_function[7]) ? rna_Node_draw_buttons : NULL;
-	nt->labelfunc = (have_function[8]) ? rna_Node_draw_label : NULL;
+	nt->draw_buttons_ex = (have_function[8]) ? rna_Node_draw_buttons_ext : NULL;
+	nt->labelfunc = (have_function[9]) ? rna_Node_draw_label : NULL;
 	
 	/* sanitize size values in case not all have been registered */
 	if (nt->maxwidth < nt->minwidth)
@@ -7706,6 +7725,13 @@ static void rna_def_node(BlenderRNA *brna)
 	RNA_def_function_ui_description(func, "Update on editor changes");
 	RNA_def_function_flag(func, FUNC_USE_SELF_ID | FUNC_REGISTER_OPTIONAL | FUNC_ALLOW_WRITE);
 	
+	/* insert_link */
+	func = RNA_def_function(srna, "insert_link", NULL);
+	RNA_def_function_ui_description(func, "Handle creation of a link to or from the node");
+	RNA_def_function_flag(func, FUNC_USE_SELF_ID | FUNC_REGISTER_OPTIONAL);
+	parm = RNA_def_pointer(func, "link", "NodeLink", "Link", "Node link that will be inserted");
+	RNA_def_property_flag(parm, PROP_REQUIRED | PROP_NEVER_NULL);
+	
 	/* init */
 	func = RNA_def_function(srna, "init", NULL);
 	RNA_def_function_ui_description(func, "Initialize a new instance of this node");

source/blender/nodes/composite/node_composite_util.c

@@ -55,5 +55,6 @@ void cmp_node_type_base(bNodeType *ntype, int type, const char *name, short ncla
 	
 	ntype->poll = cmp_node_poll_default;
 	ntype->updatefunc = cmp_node_update_default;
+	ntype->insert_link = node_insert_link_default;
 	ntype->update_internal_links = node_update_internal_links_default;
 }

source/blender/nodes/composite/nodes/node_composite_common.c

@@ -51,6 +51,7 @@ void register_node_type_cmp_group(void)
 	ntype.type = NODE_GROUP;
 	ntype.poll = cmp_node_poll_default;
 	ntype.poll_instance = node_group_poll_instance;
+	ntype.insert_link = node_insert_link_default;
 	ntype.update_internal_links = node_update_internal_links_default;
 	ntype.ext.srna = RNA_struct_find("CompositorNodeGroup");
 	BLI_assert(ntype.ext.srna != NULL);

source/blender/nodes/intern/node_util.c

@@ -29,6 +29,7 @@
  *  \ingroup nodes
  */
 
+#include <ctype.h>
 #include <limits.h>
 #include <string.h>
 
@@ -112,6 +113,95 @@ void node_filter_label(bNodeTree *UNUSED(ntree), bNode *node, char *label, int m
 }
 
 
+/*** Link Insertion ***/
+
+/* test if two sockets are interchangeable */
+static bool node_link_socket_match(bNodeSocket *a, bNodeSocket *b)
+{
+	/* tests if alphabetic prefix matches
+	 * this allows for imperfect matches, such as numeric suffixes,
+	 * like Color1/Color2
+	 */
+	int prefix_len = 0;
+	char *ca = a->name, *cb = b->name;
+	for (; *ca != '\0' && *cb != '\0'; ++ca, ++cb) {
+		/* end of common prefix? */
+		if (*ca != *cb) {
+			/* prefix delimited by non-alphabetic char */
+			if (isalpha(*ca) || isalpha(*cb))
+				return false;
+			break;
+		}
+		++prefix_len;
+	}
+	return prefix_len > 0;
+}
+
+static int node_count_links(bNodeTree *ntree, bNodeSocket *sock)
+{
+	bNodeLink *link;
+	int count = 0;
+	for (link = ntree->links.first; link; link = link->next) {
+		if (link->fromsock == sock)
+			++count;
+		if (link->tosock == sock)
+			++count;
+	}
+	return count;
+}
+
+/* find an eligible socket for linking */
+static bNodeSocket *node_find_linkable_socket(bNodeTree *ntree, bNode *node, bNodeSocket *cur)
+{
+	/* link swapping: try to find a free slot with a matching name */
+	
+	bNodeSocket *first = cur->in_out == SOCK_IN ? node->inputs.first : node->outputs.first;
+	bNodeSocket *sock;
+	
+	sock = cur->next ? cur->next : first; /* wrap around the list end */
+	while (sock != cur) {
+		if (!nodeSocketIsHidden(sock) && node_link_socket_match(sock, cur)) {
+			int link_count = node_count_links(ntree, sock);
+			/* take +1 into account since we would add a new link */
+			if (link_count + 1 <= sock->limit)
+				return sock; /* found a valid free socket we can swap to */
+		}
+		
+		sock = sock->next ? sock->next : first; /* wrap around the list end */
+	}
+	return NULL;
+}
+
+void node_insert_link_default(bNodeTree *ntree, bNode *node, bNodeLink *link)
+{
+	bNodeSocket *sock = link->tosock;
+	bNodeLink *tlink, *tlink_next;
+	
+	/* inputs can have one link only, outputs can have unlimited links */
+	if (node != link->tonode)
+		return;
+	
+	for (tlink = ntree->links.first; tlink; tlink = tlink_next) {
+		bNodeSocket *new_sock;
+		tlink_next = tlink->next;
+		
+		if (sock != tlink->tosock)
+			continue;
+		
+		new_sock = node_find_linkable_socket(ntree, node, sock);
+		if (new_sock && new_sock != sock) {
+			/* redirect existing link */
+			tlink->tosock = new_sock;
+		}
+		else if (!new_sock) {
+			/* no possible replacement, remove tlink */
+			nodeRemLink(ntree, tlink);
+			tlink = NULL;
+		}
+	}
+}
+
+
 /**** Internal Links (mute and disconnect) ****/
 
 /* common datatype priorities, works for compositor, shader and texture nodes alike

source/blender/nodes/intern/node_util.h

@@ -76,6 +76,9 @@ void node_math_label(struct bNodeTree *ntree, struct bNode *node, char *label, i
 void node_vect_math_label(struct bNodeTree *ntree, struct bNode *node, char *label, int maxlen);
 void node_filter_label(struct bNodeTree *ntree, struct bNode *node, char *label, int maxlen);
 
+
+/*** Link Handling */
+void node_insert_link_default(struct bNodeTree *ntree, struct bNode *node, struct bNodeLink *link);
 void node_update_internal_links_default(struct bNodeTree *ntree, struct bNode *node);
 
 float node_socket_get_float(struct bNodeTree *ntree, struct bNode *node, struct bNodeSocket *sock);

source/blender/nodes/shader/node_shader_util.c

@@ -47,6 +47,7 @@ void sh_node_type_base(struct bNodeType *ntype, int type, const char *name, shor
 	node_type_base(ntype, type, name, nclass, flag);
 	
 	ntype->poll = sh_node_poll_default;
+	ntype->insert_link = node_insert_link_default;
 	ntype->update_internal_links = node_update_internal_links_default;
 }
 

source/blender/nodes/shader/nodes/node_shader_common.c

@@ -236,6 +236,7 @@ void register_node_type_sh_group(void)
 	ntype.type = NODE_GROUP;
 	ntype.poll = sh_node_poll_default;
 	ntype.poll_instance = node_group_poll_instance;
+	ntype.insert_link = node_insert_link_default;
 	ntype.update_internal_links = node_update_internal_links_default;
 	ntype.ext.srna = RNA_struct_find("ShaderNodeGroup");
 	BLI_assert(ntype.ext.srna != NULL);

source/blender/nodes/texture/node_texture_util.c

@@ -60,6 +60,7 @@ void tex_node_type_base(struct bNodeType *ntype, int type, const char *name, sho
 	node_type_base(ntype, type, name, nclass, flag);
 	
 	ntype->poll = tex_node_poll_default;
+	ntype->insert_link = node_insert_link_default;
 	ntype->update_internal_links = node_update_internal_links_default;
 }
 

source/blender/nodes/texture/nodes/node_texture_common.c

@@ -166,6 +166,7 @@ void register_node_type_tex_group(void)
 	ntype.type = NODE_GROUP;
 	ntype.poll = tex_node_poll_default;
 	ntype.poll_instance = node_group_poll_instance;
+	ntype.insert_link = node_insert_link_default;
 	ntype.update_internal_links = node_update_internal_links_default;
 	ntype.ext.srna = RNA_struct_find("TextureNodeGroup");
 	BLI_assert(ntype.ext.srna != NULL);