Fix granular drag when clicking with shift held

nih_plug_iced/src/widgets/param_slider.rs

@@ -9,10 +9,13 @@ use nih_plug::prelude::{GuiContext, Param, ParamSetter};
 use super::util;
 use super::ParamMessage;
 
+/// When shift+dragging a parameter, one pixel dragged corresponds to this much change in the
+/// noramlized parameter.
+const GRANULAR_DRAG_MULTIPLIER: f32 = 0.1;
+
 /// A slider that integrates with NIH-plug's [`Param`] types.
 ///
 /// TODO: There are currently no styling options at all
-/// TODO: Handle Shift+drag for granular drag
 /// TODO: Handle Alt+click for text entry
 pub struct ParamSlider<'a, P: Param, Renderer: text::Renderer> {
     state: &'a mut State,
@@ -31,8 +34,11 @@ pub struct ParamSlider<'a, P: Param, Renderer: text::Renderer> {
 /// State for a [`ParamSlider`].
 #[derive(Debug, Default)]
 pub struct State {
-    drag_active: bool,
     keyboard_modifiers: keyboard::Modifiers,
+    drag_active: bool,
+    /// We keep track of the start coordinate holding down Shift while dragging for higher precision
+    /// dragging. This is a `None` value when granular dragging is not active.
+    granular_drag_start_x: Option<f32>,
     /// Track clicks for double clicks.
     last_click: Option<mouse::Click>,
     /// Will be set to `true` if we just reset the parameter since you could otherwise reset the
@@ -117,25 +123,35 @@ impl<'a, P: Param, Renderer: text::Renderer> Widget<ParamMessage, Renderer>
             | Event::Touch(touch::Event::FingerPressed { .. }) => {
                 if bounds.contains(cursor_position) {
                     shell.publish(ParamMessage::BeginSetParameter(self.param.as_ptr()));
+
                     self.state.drag_active = true;
 
-                    // Immediately trigger a parameter update if the value would be different, or
-                    // reset the parameter if Ctrl is held or the parameter is being double clicked
                     let click = mouse::Click::new(cursor_position, self.state.last_click);
                     self.state.last_click = Some(click);
-                    if self.state.keyboard_modifiers.control()
+                    if self.state.keyboard_modifiers.command()
                         || matches!(click.kind(), mouse::click::Kind::Double)
                     {
+                        // Immediately trigger a parameter update if the value would be different, or
+                        // reset the parameter if Ctrl is held or the parameter is being double clicked
                         self.set_normalized_value(
                             shell,
                             self.setter.default_normalized_param_value(self.param),
                         );
                         self.state.ignore_changes = true;
+                    } else if self.state.keyboard_modifiers.shift() {
+                        // When holding down shift while clicking on a parameter we want to
+                        // granuarly edit the parameter without jumping to a new value
+                        self.state.granular_drag_start_x =
+                            Some(util::remap_rect_x_t(&bounds, self.param.normalized_value()));
+
+                        self.state.ignore_changes = false;
                     } else {
                         self.set_normalized_value(
                             shell,
                             util::remap_rect_x_coordinate(&bounds, cursor_position.x),
                         );
+                        self.state.granular_drag_start_x = None;
+
                         self.state.ignore_changes = false;
                     }
 
@@ -146,6 +162,7 @@ impl<'a, P: Param, Renderer: text::Renderer> Widget<ParamMessage, Renderer>
             | Event::Touch(touch::Event::FingerLifted { .. } | touch::Event::FingerLost { .. }) => {
                 if bounds.contains(cursor_position) {
                     shell.publish(ParamMessage::EndSetParameter(self.param.as_ptr()));
+
                     self.state.drag_active = false;
 
                     return event::Status::Captured;
@@ -158,10 +175,30 @@ impl<'a, P: Param, Renderer: text::Renderer> Widget<ParamMessage, Renderer>
                     && self.state.drag_active
                     && bounds.contains(cursor_position)
                 {
-                    self.set_normalized_value(
+                    // If shift is being held then the drag should be more granular instead of
-                        shell,
+                    // absolute
-                        util::remap_rect_x_coordinate(&bounds, cursor_position.x),
+                    if self.state.keyboard_modifiers.shift() {
-                    );
+                        let drag_start_x = *self
+                            .state
+                            .granular_drag_start_x
+                            .get_or_insert(cursor_position.x);
+
+                        self.set_normalized_value(
+                            shell,
+                            util::remap_rect_x_coordinate(
+                                &bounds,
+                                drag_start_x
+                                    + (cursor_position.x - drag_start_x) * GRANULAR_DRAG_MULTIPLIER,
+                            ),
+                        );
+                    } else {
+                        self.state.granular_drag_start_x = None;
+
+                        self.set_normalized_value(
+                            shell,
+                            util::remap_rect_x_coordinate(&bounds, cursor_position.x),
+                        );
+                    }
 
                     return event::Status::Captured;
                 }