Introduce `canvas::Cache` grouping

Caches with the same `Group` will share their text
atlas!

5 files changed, 200 insertions, 71 deletions

diff --git a/graphics/src/cache.rs b/graphics/src/cache.rs
new file mode 100644
index 00000000..7106c392
--- /dev/null
+++ b/graphics/src/cache.rs
@@ -0,0 +1,158 @@
+//! Cache computations and efficiently reuse them.
+use std::cell::RefCell;
+use std::fmt;
+use std::sync::atomic::{self, AtomicU64};
+
+/// A simple cache that stores generated values to avoid recomputation.
+///
+/// Keeps track of the last generated value after clearing.
+pub struct Cache<T> {
+    group: Group,
+    state: RefCell<State<T>>,
+}
+
+impl<T> Cache<T> {
+    /// Creates a new empty [`Cache`].
+    pub fn new() -> Self {
+        Cache {
+            group: Group::unique(),
+            state: RefCell::new(State::Empty { previous: None }),
+        }
+    }
+
+    /// Creates a new empty [`Cache`] with the given [`Group`].
+    ///
+    /// Caches within the same group may reuse internal rendering storage.
+    ///
+    /// You should generally group caches that are likely to change
+    /// together.
+    pub fn with_group(group: Group) -> Self {
+        Cache {
+            group,
+            state: RefCell::new(State::Empty { previous: None }),
+        }
+    }
+
+    /// Returns the [`Group`] of the [`Cache`].
+    pub fn group(&self) -> Group {
+        self.group
+    }
+
+    /// Puts the given value in the [`Cache`].
+    ///
+    /// Notice that, given this is a cache, a mutable reference is not
+    /// necessary to call this method. You can safely update the cache in
+    /// rendering code.
+    pub fn put(&self, value: T) {
+        *self.state.borrow_mut() = State::Filled { current: value };
+    }
+
+    /// Returns a reference cell to the internal [`State`] of the [`Cache`].
+    pub fn state(&self) -> &RefCell<State<T>> {
+        &self.state
+    }
+
+    /// Clears the [`Cache`].
+    pub fn clear(&self)
+    where
+        T: Clone,
+    {
+        use std::ops::Deref;
+
+        let previous = match self.state.borrow().deref() {
+            State::Empty { previous } => previous.clone(),
+            State::Filled { current } => Some(current.clone()),
+        };
+
+        *self.state.borrow_mut() = State::Empty { previous };
+    }
+}
+
+/// A cache group.
+///
+/// Caches that share the same group generally change together.
+///
+/// A cache group can be used to implement certain performance
+/// optimizations during rendering, like batching or sharing atlases.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Group(u64);
+
+impl Group {
+    /// Generates a new unique cache [`Group`].
+    pub fn unique() -> Self {
+        static NEXT: AtomicU64 = AtomicU64::new(0);
+
+        Self(NEXT.fetch_add(1, atomic::Ordering::Relaxed))
+    }
+}
+
+impl<T> fmt::Debug for Cache<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        use std::ops::Deref;
+
+        let state = self.state.borrow();
+
+        match state.deref() {
+            State::Empty { previous } => {
+                write!(f, "Cache::Empty {{ previous: {previous:?} }}")
+            }
+            State::Filled { current } => {
+                write!(f, "Cache::Filled {{ current: {current:?} }}")
+            }
+        }
+    }
+}
+
+impl<T> Default for Cache<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// The state of a [`Cache`].
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum State<T> {
+    /// The [`Cache`] is empty.
+    Empty {
+        /// The previous value of the [`Cache`].
+        previous: Option<T>,
+    },
+    /// The [`Cache`] is filled.
+    Filled {
+        /// The current value of the [`Cache`]
+        current: T,
+    },
+}
+
+/// A piece of data that can be cached.
+pub trait Cached: Sized {
+    /// The type of cache produced.
+    type Cache: Clone;
+
+    /// Loads the [`Cache`] into a proper instance.
+    ///
+    /// [`Cache`]: Self::Cache
+    fn load(cache: &Self::Cache) -> Self;
+
+    /// Caches this value, producing its corresponding [`Cache`].
+    ///
+    /// [`Cache`]: Self::Cache
+    fn cache(self, group: Group, previous: Option<Self::Cache>) -> Self::Cache;
+}
+
+#[cfg(debug_assertions)]
+impl Cached for () {
+    type Cache = ();
+
+    fn load(_cache: &Self::Cache) -> Self {}
+
+    fn cache(
+        self,
+        _group: Group,
+        _previous: Option<Self::Cache>,
+    ) -> Self::Cache {
+    }
+}
diff --git a/graphics/src/cached.rs b/graphics/src/cached.rs
deleted file mode 100644
index c0e4e029..00000000
--- a/graphics/src/cached.rs
+++ /dev/null
@@ -1,24 +0,0 @@
-/// A piece of data that can be cached.
-pub trait Cached: Sized {
-    /// The type of cache produced.
-    type Cache: Clone;
-
-    /// Loads the [`Cache`] into a proper instance.
-    ///
-    /// [`Cache`]: Self::Cache
-    fn load(cache: &Self::Cache) -> Self;
-
-    /// Caches this value, producing its corresponding [`Cache`].
-    ///
-    /// [`Cache`]: Self::Cache
-    fn cache(self, previous: Option<Self::Cache>) -> Self::Cache;
-}
-
-#[cfg(debug_assertions)]
-impl Cached for () {
-    type Cache = ();
-
-    fn load(_cache: &Self::Cache) -> Self {}
-
-    fn cache(self, _previous: Option<Self::Cache>) -> Self::Cache {}
-}
diff --git a/graphics/src/geometry.rs b/graphics/src/geometry.rs
index dd07097f..ab4a7a36 100644
--- a/graphics/src/geometry.rs
+++ b/graphics/src/geometry.rs
@@ -18,8 +18,8 @@ pub use text::Text;
 
 pub use crate::gradient::{self, Gradient};
 
+use crate::cache::Cached;
 use crate::core::{self, Size};
-use crate::Cached;
 
 /// A renderer capable of drawing some [`Self::Geometry`].
 pub trait Renderer: core::Renderer {
diff --git a/graphics/src/geometry/cache.rs b/graphics/src/geometry/cache.rs
index 665e996b..d70cee0b 100644
--- a/graphics/src/geometry/cache.rs
+++ b/graphics/src/geometry/cache.rs
@@ -1,8 +1,8 @@
+use crate::cache::{self, Cached};
 use crate::core::Size;
 use crate::geometry::{self, Frame};
-use crate::Cached;
 
-use std::cell::RefCell;
+pub use cache::Group;
 
 /// A simple cache that stores generated geometry to avoid recomputation.
 ///
@@ -12,7 +12,13 @@ pub struct Cache<Renderer>
 where
     Renderer: geometry::Renderer,
 {
-    state: RefCell<State<Renderer::Geometry>>,
+    raw: crate::Cache<Data<<Renderer::Geometry as Cached>::Cache>>,
+}
+
+#[derive(Debug, Clone)]
+struct Data<T> {
+    bounds: Size,
+    geometry: T,
 }
 
 impl<Renderer> Cache<Renderer>
@@ -22,20 +28,25 @@ where
     /// Creates a new empty [`Cache`].
     pub fn new() -> Self {
         Cache {
-            state: RefCell::new(State::Empty { previous: None }),
+            raw: cache::Cache::new(),
+        }
+    }
+
+    /// Creates a new empty [`Cache`] with the given [`Group`].
+    ///
+    /// Caches within the same group may reuse internal rendering storage.
+    ///
+    /// You should generally group caches that are likely to change
+    /// together.
+    pub fn with_group(group: Group) -> Self {
+        Cache {
+            raw: crate::Cache::with_group(group),
         }
     }
 
     /// Clears the [`Cache`], forcing a redraw the next time it is used.
     pub fn clear(&self) {
-        use std::ops::Deref;
-
-        let previous = match self.state.borrow().deref() {
-            State::Empty { previous } => previous.clone(),
-            State::Filled { geometry, .. } => Some(geometry.clone()),
-        };
-
-        *self.state.borrow_mut() = State::Empty { previous };
+        self.raw.clear();
     }
 
     /// Draws geometry using the provided closure and stores it in the
@@ -56,27 +67,30 @@ where
     ) -> Renderer::Geometry {
         use std::ops::Deref;
 
-        let previous = match self.state.borrow().deref() {
-            State::Empty { previous } => previous.clone(),
-            State::Filled {
-                bounds: cached_bounds,
-                geometry,
-            } => {
-                if *cached_bounds == bounds {
-                    return Cached::load(geometry);
+        let state = self.raw.state();
+
+        let previous = match state.borrow().deref() {
+            cache::State::Empty { previous } => {
+                previous.as_ref().map(|data| data.geometry.clone())
+            }
+            cache::State::Filled { current } => {
+                if current.bounds == bounds {
+                    return Cached::load(&current.geometry);
                 }
 
-                Some(geometry.clone())
+                Some(current.geometry.clone())
             }
         };
 
         let mut frame = Frame::new(renderer, bounds);
         draw_fn(&mut frame);
 
-        let geometry = frame.into_geometry().cache(previous);
+        let geometry = frame.into_geometry().cache(self.raw.group(), previous);
         let result = Cached::load(&geometry);
 
-        *self.state.borrow_mut() = State::Filled { bounds, geometry };
+        *state.borrow_mut() = cache::State::Filled {
+            current: Data { bounds, geometry },
+        };
 
         result
     }
@@ -85,16 +99,10 @@ where
 impl<Renderer> std::fmt::Debug for Cache<Renderer>
 where
     Renderer: geometry::Renderer,
+    <Renderer::Geometry as Cached>::Cache: std::fmt::Debug,
 {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let state = self.state.borrow();
-
-        match *state {
-            State::Empty { .. } => write!(f, "Cache::Empty"),
-            State::Filled { bounds, .. } => {
-                write!(f, "Cache::Filled {{ bounds: {bounds:?} }}")
-            }
-        }
+        write!(f, "{:?}", &self.raw)
     }
 }
 
@@ -106,16 +114,3 @@ where
         Self::new()
     }
 }
-
-enum State<Geometry>
-where
-    Geometry: Cached,
-{
-    Empty {
-        previous: Option<Geometry::Cache>,
-    },
-    Filled {
-        bounds: Size,
-        geometry: Geometry::Cache,
-    },
-}
diff --git a/graphics/src/lib.rs b/graphics/src/lib.rs
index 865ebd97..b5ef55e7 100644
--- a/graphics/src/lib.rs
+++ b/graphics/src/lib.rs
@@ -9,10 +9,10 @@
 )]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 mod antialiasing;
-mod cached;
 mod settings;
 mod viewport;
 
+pub mod cache;
 pub mod color;
 pub mod compositor;
 pub mod damage;
@@ -27,7 +27,7 @@ pub mod text;
 pub mod geometry;
 
 pub use antialiasing::Antialiasing;
-pub use cached::Cached;
+pub use cache::Cache;
 pub use compositor::Compositor;
 pub use error::Error;
 pub use gradient::Gradient;