Add observer functions

 - When there is a setter and an observer, value is not changed
   before calling the observer
 - Partially fixes godotengine#6491

core/class_db.h

@@ -105,6 +105,7 @@ class ClassDB {
 		int index;
 		StringName setter;
 		StringName getter;
+		StringName observer;
 		MethodBind *_setptr;
 		MethodBind *_getptr;
 		Variant::Type type;

modules/gdscript/gdscript.cpp

@@ -951,13 +951,28 @@ bool GDScriptInstance::set(const StringName &p_name, const Variant &p_value) {
 				Variant::CallError err;
 				call(E->get().setter, &val, 1, err);
 				if (err.error == Variant::CallError::CALL_OK) {
-					return true; //function exists, call was successful
+					if (E->get().observer) {
+						// call observer without changing data beforehand
+						call(E->get().observer, NULL, 0, err);
+						if (err.error != Variant::CallError::CALL_OK) {
+							return false; // error in calling observer function
+						}
+					}
+					return true; // function exists, call was successful
 				}
+
 			} else {
 				if (!E->get().data_type.is_type(p_value)) {
 					return false; // Type mismatch
 				}
 				members.write[E->get().index] = p_value;
+				if (E->get().observer) {
+					Variant::CallError err;
+					call(E->get().observer, NULL, 0, err);
+					if (err.error == Variant::CallError::CALL_OK) {
+						return true;
+					}
+				}
 			}
 			return true;
 		}
@@ -1790,6 +1805,7 @@ void GDScriptLanguage::get_reserved_words(List<String> *p_words) const {
 		"func",
 		"preload",
 		"setget",
+		"observer",
 		"signal",
 		"tool",
 		"yield",

modules/gdscript/gdscript.h

@@ -63,6 +63,7 @@ class GDScript : public Script {
 		int index;
 		StringName setter;
 		StringName getter;
+		StringName observer;
 		MultiplayerAPI::RPCMode rpc_mode;
 		GDScriptDataType data_type;
 	};

modules/gdscript/gdscript_compiler.cpp

@@ -1895,6 +1895,7 @@ Error GDScriptCompiler::_parse_class_level(GDScript *p_script, const GDScriptPar
 		minfo.index = p_script->member_indices.size();
 		minfo.setter = p_class->variables[i].setter;
 		minfo.getter = p_class->variables[i].getter;
+		minfo.observer = p_class->variables[i].observer;
 		minfo.rpc_mode = p_class->variables[i].rpc_mode;
 		minfo.data_type = _gdtype_from_datatype(p_class->variables[i].data_type);
 

modules/gdscript/gdscript_editor.cpp

@@ -2181,7 +2181,7 @@ static void _find_identifiers(const GDScriptCompletionContext &p_context, bool p
 
 	static const char *_keywords[] = {
 		"and", "in", "not", "or", "false", "PI", "TAU", "INF", "NAN", "self", "true", "as", "assert",
-		"breakpoint", "class", "extends", "is", "func", "preload", "setget", "signal", "tool", "yield",
+		"breakpoint", "class", "extends", "is", "func", "preload", "setget", "observer", "signal", "tool", "yield",
 		"const", "enum", "export", "onready", "static", "var", "break", "continue", "if", "elif",
 		"else", "for", "pass", "return", "match", "while", "remote", "sync", "master", "puppet", "slave",
 		"remotesync", "mastersync", "puppetsync",

modules/gdscript/gdscript_parser.cpp

@@ -4904,6 +4904,18 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 					}
 				}
 
+				if (tokenizer->get_token() == GDScriptTokenizer::TK_PR_OBSERVER) {
+					// observer before setget
+					tokenizer->advance();
+
+					if (!tokenizer->is_token_literal()) {
+						_set_error("Expected an identifier for the observer function after \"observer\".");
+					}
+
+					member.observer = tokenizer->get_token_literal();
+					tokenizer->advance();
+				}
+
 				if (tokenizer->get_token() == GDScriptTokenizer::TK_PR_SETGET) {
 
 					tokenizer->advance();
@@ -4930,6 +4942,23 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 						member.getter = tokenizer->get_token_literal();
 						tokenizer->advance();
 					}
+
+					if (tokenizer->get_token() == GDScriptTokenizer::TK_PR_OBSERVER) {
+						// check if observer already set
+						if (!(member.observer == StringName())) {
+							_set_error("Cannot set observer twice");
+							return;
+						}
+
+						// observer after setget section
+						tokenizer->advance();
+
+						if (!tokenizer->is_token_literal()) {
+							_set_error("Expected an identifier for the observer function after \"observer\".");
+						}
+						member.observer = tokenizer->get_token_literal();
+						tokenizer->advance();
+					}
 				}
 
 				p_class->variables.push_back(member);
@@ -7690,12 +7719,25 @@ void GDScriptParser::_check_class_level_types(ClassNode *p_class) {
 			}
 		}
 
-		// Setter and getter
-		if (v.setter == StringName() && v.getter == StringName()) continue;
+		// Setter, getter and observer
+		if (v.setter == StringName() && v.getter == StringName() && v.observer == StringName()) continue;
 
 		bool found_getter = false;
 		bool found_setter = false;
+		bool found_observer = false;
 		for (int j = 0; j < p_class->functions.size(); j++) {
+			if (v.observer == p_class->functions[j]->name) {
+				found_observer = true;
+				FunctionNode *observer = p_class->functions[j];
+
+				if (observer->get_required_argument_count() != 0) {
+					_set_error("The observer function needs to receive exactly 0 arguments. See \"" + observer->name +
+									   "()\" definition at line " + itos(observer->line) + ".",
+							v.line);
+					return;
+				}
+				continue;
+			}
 			if (v.setter == p_class->functions[j]->name) {
 				found_setter = true;
 				FunctionNode *setter = p_class->functions[j];
@@ -7734,10 +7776,10 @@ void GDScriptParser::_check_class_level_types(ClassNode *p_class) {
 					return;
 				}
 			}
-			if (found_getter && found_setter) break;
+			if (found_getter && found_setter && found_observer) break;
 		}
 
-		if ((found_getter || v.getter == StringName()) && (found_setter || v.setter == StringName())) continue;
+		if ((found_getter || v.getter == StringName()) && (found_setter || v.setter == StringName()) && (found_observer || v.observer == StringName())) continue;
 
 		// Check for static functions
 		for (int j = 0; j < p_class->static_functions.size(); j++) {
@@ -7751,6 +7793,10 @@ void GDScriptParser::_check_class_level_types(ClassNode *p_class) {
 				_set_error("The getter can't be a static function. See \"" + getter->name + "()\" definition at line " + itos(getter->line) + ".", v.line);
 				return;
 			}
+			if (v.observer == p_class->static_functions[j]->name) {
+				FunctionNode *observer = p_class->static_functions[j];
+				_set_error("The observer function can't be a static function. See \"" + observer->name + "()\" definitino at line " + itos(observer->line) + ".", v.line);
+			}
 		}
 
 		if (!found_setter && v.setter != StringName()) {
@@ -7762,6 +7808,11 @@ void GDScriptParser::_check_class_level_types(ClassNode *p_class) {
 			_set_error("The getter function isn't defined.", v.line);
 			return;
 		}
+
+		if (!found_observer && v.observer != StringName()) {
+			_set_error("The observer function isn't defined.", v.line);
+			return;
+		}
 	}
 
 	// Inner classes

modules/gdscript/gdscript_parser.h

@@ -164,6 +164,7 @@ class GDScriptParser {
 			DataType data_type;
 			StringName setter;
 			StringName getter;
+			StringName observer;
 			int line;
 			Node *expression;
 			OperatorNode *initial_assignment;

modules/gdscript/gdscript_tokenizer.cpp

@@ -96,6 +96,7 @@ const char *GDScriptTokenizer::token_names[TK_MAX] = {
 	"static",
 	"export",
 	"setget",
+	"observer",
 	"const",
 	"var",
 	"as",
@@ -196,6 +197,7 @@ static const _kws _keyword_list[] = {
 	{ GDScriptTokenizer::TK_PR_TOOL, "tool" },
 	{ GDScriptTokenizer::TK_PR_STATIC, "static" },
 	{ GDScriptTokenizer::TK_PR_EXPORT, "export" },
+	{ GDScriptTokenizer::TK_PR_OBSERVER, "observer" },
 	{ GDScriptTokenizer::TK_PR_SETGET, "setget" },
 	{ GDScriptTokenizer::TK_PR_VAR, "var" },
 	{ GDScriptTokenizer::TK_PR_AS, "as" },
@@ -251,6 +253,7 @@ bool GDScriptTokenizer::is_token_literal(int p_offset, bool variable_safe) const
 		case TK_PR_STATIC:
 		case TK_PR_EXPORT:
 		case TK_PR_SETGET:
+		case TK_PR_OBSERVER:
 		case TK_PR_SIGNAL:
 		case TK_PR_REMOTE:
 		case TK_PR_MASTER:

modules/gdscript/gdscript_tokenizer.h

@@ -102,6 +102,7 @@ class GDScriptTokenizer {
 		TK_PR_STATIC,
 		TK_PR_EXPORT,
 		TK_PR_SETGET,
+		TK_PR_OBSERVER,
 		TK_PR_CONST,
 		TK_PR_VAR,
 		TK_PR_AS,