Add WidgetValueTrait::value_widget_connected

Depending on the usage of value widgets, we might wish to display an
alternative UI if the input is connected.

Motivating example: consider a node with an input representing some
amount of time in the context of digital signal processing. Multiple
valuable representations of time may be desired as inputs:
* number of seconds
* number of samples
* wavelength corresponding to some frequency in Hz

In such a case we would use value_widget to display a widget for
selecting both the number and the unit, and value_widget_connected for
only selecting the unit, because the numerical value would already be
supplied by another node.

egui_node_graph/src/editor_ui.rs

@@ -537,9 +537,6 @@ where
             for (param_name, param_id) in inputs {
                 if self.graph[param_id].shown_inline {
                     let height_before = ui.min_rect().bottom();
-                    if self.graph.connection(param_id).is_some() {
-                        ui.label(param_name);
-                    } else {
                     // NOTE: We want to pass the `user_data` to
                     // `value_widget`, but we can't since that would require
                     // borrowing the graph twice. Here, we make the
@@ -548,6 +545,18 @@ where
                     // dummy value. This requires `ValueType` to implement
                     // Default, but results in a totally safe alternative.
                     let mut value = std::mem::take(&mut self.graph[param_id].value);
+
+                    if self.graph.connection(param_id).is_some() {
+                        let node_responses = value.value_widget_connected(
+                            &param_name,
+                            self.node_id,
+                            ui,
+                            user_state,
+                            &self.graph[self.node_id].user_data,
+                        );
+
+                        responses.extend(node_responses.into_iter().map(NodeResponse::User));
+                    } else {
                         let node_responses = value.value_widget(
                             &param_name,
                             self.node_id,
@@ -555,9 +564,12 @@ where
                             user_state,
                             &self.graph[self.node_id].user_data,
                         );
-                        self.graph[param_id].value = value;
+
                         responses.extend(node_responses.into_iter().map(NodeResponse::User));
                     }
+
+                    self.graph[param_id].value = value;
+
                     let height_after = ui.min_rect().bottom();
                     input_port_heights.push((height_before + height_after) / 2.0);
                 }

egui_node_graph/src/traits.rs

@@ -13,10 +13,11 @@ pub trait WidgetValueTrait: Default {
     type Response;
     type UserState;
     type NodeData;
+
     /// This method will be called for each input parameter with a widget. The
     /// return value is a vector of custom response objects which can be used
     /// to implement handling of side effects. If unsure, the response Vec can
-    /// be empty.
+    /// be empty. It isn't called if the input is connected.
     fn value_widget(
         &mut self,
         param_name: &str,
@@ -25,6 +26,24 @@ pub trait WidgetValueTrait: Default {
         user_state: &mut Self::UserState,
         node_data: &Self::NodeData,
     ) -> Vec<Self::Response>;
+
+    /// This method will be called for each input parameter with a widget. The
+    /// return value is a vector of custom response objects which can be used
+    /// to implement handling of side effects. If unsure, the response Vec can
+    /// be empty. This differs from [`WidgetValueTrait::value_widget`]
+    /// by being called if and only if the input has a connection.
+    fn value_widget_connected(
+        &mut self,
+        param_name: &str,
+        _node_id: NodeId,
+        ui: &mut egui::Ui,
+        _user_state: &mut Self::UserState,
+        _node_data: &Self::NodeData,
+    ) -> Vec<Self::Response> {
+        ui.label(param_name);
+
+        Default::default()
+    }
 }
 
 /// This trait must be implemented by the `DataType` generic parameter of the