Implement placeholder text in text boxes

crates/yakui-core/src/geometry/color.rs

@@ -78,6 +78,11 @@ impl Color {
             .into_components()
             .into()
     }
+
+    /// Blend with `other` in linear space
+    pub fn lerp(&self, other: &Color, ratio: f32) -> Self {
+        Self::from_linear(self.to_linear().lerp(other.to_linear(), ratio))
+    }
 }
 
 macro_rules! builtin_colors {

crates/yakui-widgets/src/widgets/textbox.rs

@@ -29,6 +29,8 @@ pub struct TextBox {
     pub style: TextStyle,
     pub padding: Pad,
     pub fill: Option<Color>,
+    /// Drawn when no text has been set
+    pub placeholder: String,
 }
 
 impl TextBox {
@@ -38,6 +40,7 @@ impl TextBox {
             style: TextStyle::label(),
             padding: Pad::all(8.0),
             fill: Some(colors::BACKGROUND_3),
+            placeholder: String::new(),
         }
     }
 
@@ -76,12 +79,26 @@ impl Widget for TextBoxWidget {
     fn update(&mut self, props: Self::Props) -> Self::Response {
         self.props = props;
 
-        let text = self.updated_text.as_ref().unwrap_or(&self.props.text);
+        let mut text = self.updated_text.as_ref().unwrap_or(&self.props.text);
+        let use_placeholder = text.is_empty();
+        if use_placeholder {
+            text = &self.props.placeholder;
+        }
 
         let mut render = RenderTextBox::new(text.clone());
         render.style = self.props.style.clone();
         render.selected = self.selected;
-        render.cursor = self.cursor;
+        if !use_placeholder {
+            render.cursor = self.cursor;
+        }
+        if use_placeholder {
+            // Dim towards background
+            render.style.color = self
+                .props
+                .style
+                .color
+                .lerp(&self.props.fill.unwrap_or(Color::CLEAR), 0.75);
+        }
 
         pad(self.props.padding, || {
             let res = render.show();

crates/yakui/examples/textbox.rs

@@ -2,12 +2,13 @@ use yakui::widgets::{Pad, TextBox};
 use yakui::{center, use_state};
 
 pub fn run() {
-    let text = use_state(|| "Hello\nWorld\nBar".to_owned());
+    let text = use_state(|| "".to_owned());
 
     center(|| {
         let mut my_box = TextBox::new(text.borrow().as_str());
         my_box.style.font_size = 60.0;
         my_box.padding = Pad::all(50.0);
+        my_box.placeholder = "placeholder".into();
 
         if let Some(new_text) = my_box.show().into_inner().text {
             text.set(new_text);