UI/Nodes: Improve feedback when adding node fails (e.g. on drag & drop)

This is especially useful when trying to add a node group instance, e.g. via
drag & drop from the Outliner or Asset Browser.
Previously this would just silently fail, with no information why. This is a
source of confusion, e.g. earlier, it took me a moment to realize I was
dragging a node group into itself, which failed of course.
Blender should always try to help the user with useful error messages.

Adds error messages like: "Nesting a node group inside of itself is not
allowed", "Not a compositor node tree", etc.

Adds a disabled hint return argument to node and node tree polling functions.
On error the hint is reported, or could even be shown in advance (e.g. if
checked via an operator poll option).

Differential Revision: https://developer.blender.org/D10422

Reviewed by: Jacques Lucke

2 files changed, 14 insertions, 10 deletions

diff --git a/source/blender/nodes/intern/node_common.c b/source/blender/nodes/intern/node_common.c
index 0b1ab85c059..7fc9f664df0 100644
--- a/source/blender/nodes/intern/node_common.c
+++ b/source/blender/nodes/intern/node_common.c
@@ -79,12 +79,12 @@ void node_group_label(bNodeTree *UNUSED(ntree), bNode *node, char *label, int ma
   BLI_strncpy(label, (node->id) ? node->id->name + 2 : IFACE_("Missing Data-Block"), maxlen);
 }
 
-bool node_group_poll_instance(bNode *node, bNodeTree *nodetree)
+bool node_group_poll_instance(bNode *node, bNodeTree *nodetree, const char **disabled_hint)
 {
-  if (node->typeinfo->poll(node->typeinfo, nodetree)) {
+  if (node->typeinfo->poll(node->typeinfo, nodetree, disabled_hint)) {
     bNodeTree *grouptree = (bNodeTree *)node->id;
     if (grouptree) {
-      return nodeGroupPoll(nodetree, grouptree);
+      return nodeGroupPoll(nodetree, grouptree, disabled_hint);
     }
 
     return true; /* without a linked node tree, group node is always ok */
@@ -93,25 +93,27 @@ bool node_group_poll_instance(bNode *node, bNodeTree *nodetree)
   return false;
 }
 
-int nodeGroupPoll(bNodeTree *nodetree, bNodeTree *grouptree)
+bool nodeGroupPoll(bNodeTree *nodetree, bNodeTree *grouptree, const char **r_disabled_hint)
 {
   bNode *node;
-  int valid = 1;
+  bool valid = true;
 
   /* unspecified node group, generally allowed
    * (if anything, should be avoided on operator level)
    */
   if (grouptree == NULL) {
-    return 1;
+    return true;
   }
 
   if (nodetree == grouptree) {
-    return 0;
+    *r_disabled_hint = "Nesting a node group inside of itself is not allowed";
+    return false;
   }
 
   for (node = grouptree->nodes.first; node; node = node->next) {
-    if (node->typeinfo->poll_instance && !node->typeinfo->poll_instance(node, nodetree)) {
-      valid = 0;
+    if (node->typeinfo->poll_instance &&
+        !node->typeinfo->poll_instance(node, nodetree, r_disabled_hint)) {
+      valid = false;
       break;
     }
   }
diff --git a/source/blender/nodes/intern/node_common.h b/source/blender/nodes/intern/node_common.h
index 7aad6782640..cdb7b6897b9 100644
--- a/source/blender/nodes/intern/node_common.h
+++ b/source/blender/nodes/intern/node_common.h
@@ -32,7 +32,9 @@ extern "C" {
 struct bNodeTree;
 
 void node_group_label(struct bNodeTree *ntree, struct bNode *node, char *label, int maxlen);
-bool node_group_poll_instance(struct bNode *node, struct bNodeTree *nodetree);
+bool node_group_poll_instance(struct bNode *node,
+                              struct bNodeTree *nodetree,
+                              const char **r_disabled_hint);
 
 void ntree_update_reroute_nodes(struct bNodeTree *ntree);