Add ZOffset

crates/egui/src/layers.rs

@@ -117,8 +117,11 @@ impl AreaLayerId {
     }
 }
 
-/// Represents the relative order an element should be displayed
-/// Lower values render first, and therefore appear below higher values
+// ----------------------------------------------------------------------------
+
+/// Represents the relative order an element should be displayed.
+///
+/// Lower values render first, and therefore appear below higher values.
 #[derive(Clone, Copy, Debug, Hash, Eq, PartialEq, Ord, PartialOrd)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct ZOrder(pub i32);
@@ -155,6 +158,27 @@ impl Default for ZOrder {
     }
 }
 
+impl std::ops::Add<ZOffset> for ZOrder {
+    type Output = ZOrder;
+
+    fn add(self, offset: ZOffset) -> Self::Output {
+        Self(self.0.saturating_add(offset))
+    }
+}
+
+impl std::ops::AddAssign<ZOffset> for ZOrder {
+    fn add_assign(&mut self, offset: ZOffset) {
+        self.0 = self.0.saturating_add(offset);
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+/// Offset within a [`ZOrder`].
+pub type ZOffset = i32;
+
+// ----------------------------------------------------------------------------
+
 /// An identifier for a paint layer which supports Z-indexing
 ///
 /// This says: draw on [`AreaLayerId`] with index z. This only affects the display

crates/egui/src/lib.rs

@@ -361,7 +361,7 @@ pub use {
     grid::Grid,
     id::{Id, IdMap},
     input_state::{InputState, MultiTouchInfo, PointerState},
-    layers::{AreaLayerId, Order},
+    layers::{AreaLayerId, Order, ZOffset, ZOrder},
     layout::*,
     memory::{Memory, Options},
     painter::Painter,

crates/egui/src/painter.rs

@@ -3,7 +3,7 @@ use std::sync::Arc;
 
 use crate::{
     emath::{Align2, Pos2, Rect, Vec2},
-    layers::{PaintList, ShapeIdx, ZLayer, ZOrder},
+    layers::{PaintList, ShapeIdx, ZLayer, ZOffset, ZOrder},
     AreaLayerId, Color32, Context, FontId,
 };
 use epaint::{
@@ -42,28 +42,25 @@ impl Painter {
         }
     }
 
-    /// Redirect where you are painting.
-    #[must_use]
-    pub fn with_z_layer(self, z_layer: ZLayer) -> Self {
-        Self {
-            ctx: self.ctx,
-            z_layer,
-            clip_rect: self.clip_rect,
-            fade_to_color: None,
-        }
-    }
-
     /// Redirect where you are painting with default z-index
     #[must_use]
-    pub fn with_layer_id(self, layer: AreaLayerId) -> Self {
-        self.with_z_layer(ZLayer::from_area_layer(layer))
+    pub fn with_layer_id(mut self, layer: AreaLayerId) -> Self {
+        self.z_layer = ZLayer::from_area_layer(layer);
+        self
     }
 
     /// Redirect z-index
     #[must_use]
-    pub fn with_z(self, z: ZOrder) -> Self {
-        let layer = self.z_layer.area_layer.with_z(z);
-        self.with_z_layer(layer)
+    pub fn with_z(mut self, z: ZOrder) -> Self {
+        self.z_layer.z = z;
+        self
+    }
+
+    /// Modify z-index
+    #[must_use]
+    pub fn with_z_offset(mut self, z_offset: ZOffset) -> Self {
+        self.z_layer.z += z_offset;
+        self
     }
 
     /// Create a painter for a sub-region of this [`Painter`].