Add ui.allocate_painter helper

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * Turn off `Window` title bars with `window.title_bar(false)`.
 * `ImageButton` - `ui.add(ImageButton::new(...))`.
 * `ui.vertical_centered` and `ui.vertical_centered_justified`.
+* `ui.allocate_painter` helper.
 * Mouse-over explanation to duplicate ID warning.
 * You can now easily constrain Egui to a portion of the screen using `RawInput::screen_rect`.
 * You can now control the minimum and maixumum number of decimals to show in a `Slider` or `DragValue`.

egui/src/demos/demo_window.rs

@@ -243,7 +243,7 @@ impl Default for Painting {
     fn default() -> Self {
         Self {
             lines: Default::default(),
-            stroke: Stroke::new(1.0, LIGHT_GRAY),
+            stroke: Stroke::new(1.0, LIGHT_BLUE),
         }
     }
 }
@@ -265,11 +265,10 @@ impl Painting {
     }
 
     fn content(&mut self, ui: &mut Ui) {
-        let rect = ui.allocate_space(ui.available_size_before_wrap_finite());
-        let response = ui.interact(rect, ui.id(), Sense::drag());
-        let rect = response.rect;
-        let clip_rect = ui.clip_rect().intersect(rect); // Make sure we don't paint out of bounds
-        let painter = Painter::new(ui.ctx().clone(), ui.layer_id(), clip_rect);
+        let painter = ui.allocate_painter(ui.available_size_before_wrap_finite());
+        let rect = painter.clip_rect();
+        let id = ui.make_position_id();
+        let response = ui.interact(rect, id, Sense::drag());
 
         if self.lines.is_empty() {
             self.lines.push(vec![]);

egui/src/ui.rs

@@ -525,6 +525,13 @@ impl Ui {
         let response = self.interact_hover(final_child_rect);
         (ret, response)
     }
+
+    /// Convenience function to get a region to paint on
+    pub fn allocate_painter(&mut self, desired_size: Vec2) -> Painter {
+        let rect = self.allocate_space(desired_size);
+        let clip_rect = self.clip_rect().intersect(rect); // Make sure we don't paint out of bounds
+        Painter::new(self.ctx().clone(), self.layer_id(), clip_rect)
+    }
 }
 
 /// # Adding widgets