Provide instructions for resource change notifications in tools (#9283)

(cherry picked from commit e6c8e51)

tutorials/plugins/running_code_in_the_editor.rst

@@ -245,6 +245,174 @@ angle add a setter ``set(new_speed)`` which is executed with the input from the
     to run in the editor too. Autoload nodes cannot be accessed in the editor at
     all.
 
+Getting notified when resources change
+--------------------------------------
+
+Some times you want your tool to use a resource. However, when you change a
+property of that resource in the editor, the ``set()`` method of your tool will
+not be called.
+
+.. tabs::
+ .. code-tab:: gdscript GDScript
+
+    @tool
+    class_name MyTool
+    extends Node
+
+    @export var resource: MyResource:
+        set(new_resource):
+            resource = new_resource
+            _on_resource_set()
+
+    # This will only be called when you create, delete, or paste a resource.
+    # You will not get an update when tweaking properties of it.
+    func _on_resource_set():
+        print("My resource was set!")
+
+ .. code-tab:: csharp
+
+    using Godot;
+
+    [Tool]
+    public partial class MyTool : Node
+    {
+        private MyResource _resource;
+
+        [Export]
+        public MyResource Resource
+        {
+            get => _resource;
+            set
+            {
+                _resource = value;
+                OnResourceSet();
+            }
+        }
+    }
+
+    // This will only be called when you create, delete, or paste a resource.
+    // You will not get an update when tweaking properties of it.
+    private void OnResourceSet()
+    {
+        GD.Print("My resource was set!");
+    }
+
+To get around this problem you first have to make your resource a tool and make it
+emit the ``changed`` signal whenever a property is set:
+
+.. tabs::
+ .. code-tab:: gdscript GDScript
+
+    # Make Your Resource a tool.
+    @tool
+    class_name MyResource
+    extends Resource
+
+    @export var property = 1:
+        set(new_setting):
+            property = new_setting
+            # Emit a signal when the property is changed.
+            changed.emit()
+
+ .. code-tab:: csharp
+
+    using Godot;
+
+    [Tool]
+    public partial class MyResource : Resource
+    {
+        private float _property = 1;
+
+        [Export]
+        public float Property
+        {
+            get => _property;
+            set
+            {
+                _property = value;
+                // Emit a signal when the property is changed.
+                EmitChanged();
+            }
+        }
+    }
+
+You then want to connect the signal when a new resource is set:
+
+.. tabs::
+ .. code-tab:: gdscript GDScript
+
+    @tool
+    class_name MyTool
+    extends Node
+
+    @export var resource: MyResource:
+        set(new_resource):
+            resource = new_resource
+            # Connect the changed signal as soon as a new resource is being added.
+            resource.changed.connect(_on_resource_changed)
+
+    func _on_resource_changed():
+        print("My resource just changed!")
+
+ .. code-tab:: csharp
+
+    using Godot;
+
+    [Tool]
+    public partial class MyTool : Node
+    {
+        private MyResource _resource;
+
+        [Export]
+        public MyResource Resource
+        {
+            get => _resource;
+            set
+            {
+                _resource = value;
+                // Connect the changed signal as soon as a new resource is being added.
+                _resource.Changed += OnResourceChanged;
+            }
+        }
+    }
+
+    private void OnResourceChanged()
+    {
+        GD.Print("My resource just changed!");
+    }
+
+Lastly, you should to disconnect the signal as the old resource being used and changed somewhere else
+would cause unneeded updates.
+
+.. tabs::
+ .. code-tab:: gdscript GDScript
+
+    @export var resource: MyResource:
+        set(new_resource):
+            # Disconnect the signal if the previous resource was not null.
+            if resource != null:
+                resource.changed.disconnect(_on_resource_changed)
+            resource = new_resource
+            resource.changed.connect(_on_resource_changed)
+
+ .. code-tab:: csharp
+
+    [Export]
+    public MyResource Resource
+    {
+        get => _resource;
+        set
+        {
+            // Disconnect the signal if the previous resource was not null.
+            if (_resource != null)
+            {
+                _resource.Changed -= OnResourceChanged;
+            }
+            _resource = value;
+            _resource.Changed += OnResourceChanged;
+        }
+    }
+
 Reporting node configuration warnings
 -------------------------------------