feat: implement `Cached` widget

2 files changed, 346 insertions, 0 deletions

diff --git a/lazy/src/cached.rs b/lazy/src/cached.rs
new file mode 100644
index 00000000..c046179f
--- /dev/null
+++ b/lazy/src/cached.rs
@@ -0,0 +1,344 @@
+use iced_native::event;
+use iced_native::layout::{self, Layout};
+use iced_native::mouse;
+use iced_native::overlay;
+use iced_native::renderer;
+use iced_native::{Clipboard, Hasher, Length, Point, Rectangle, Shell, Size};
+use iced_native::widget::tree::{self, Tree};
+use iced_native::{Element, Widget};
+
+use ouroboros::self_referencing;
+use std::cell::{Ref, RefCell, RefMut};
+use std::hash::{Hash, Hasher as H};
+use std::marker::PhantomData;
+use std::rc::Rc;
+
+#[allow(missing_debug_implementations)]
+pub struct Cached<'a, Message, Renderer, Dependency, View> {
+    dependency: Dependency,
+    view: Box<dyn Fn() -> View + 'a>,
+    element: RefCell<Option<Rc<RefCell<Element<'static, Message, Renderer>>>>>,
+}
+
+impl<'a, Message, Renderer, Dependency, View>
+    Cached<'a, Message, Renderer, Dependency, View>
+where
+    Dependency: Hash + 'a,
+    View: Into<Element<'static, Message, Renderer>>,
+{
+    pub fn new(dependency: Dependency, view: impl Fn() -> View + 'a) -> Self {
+        Self {
+            dependency,
+            view: Box::new(view),
+            element: RefCell::new(None),
+        }
+    }
+
+    fn with_element<T>(
+        &self,
+        f: impl FnOnce(Ref<Element<Message, Renderer>>) -> T,
+    ) -> T {
+        f(self.element.borrow().as_ref().unwrap().borrow())
+    }
+
+    fn with_element_mut<T>(
+        &self,
+        f: impl FnOnce(RefMut<Element<Message, Renderer>>) -> T,
+    ) -> T {
+        f(self.element.borrow().as_ref().unwrap().borrow_mut())
+    }
+}
+
+struct Internal<Message, Renderer> {
+    element: Rc<RefCell<Element<'static, Message, Renderer>>>,
+    hash: u64,
+}
+
+impl<'a, Message, Renderer, Dependency, View> Widget<Message, Renderer>
+    for Cached<'a, Message, Renderer, Dependency, View>
+where
+    View: Into<Element<'static, Message, Renderer>> + 'static,
+    Dependency: Hash + 'a,
+    Message: 'static,
+    Renderer: iced_native::Renderer + 'static,
+{
+    fn tag(&self) -> tree::Tag {
+        struct Tag<T>(T);
+        tree::Tag::of::<Tag<View>>()
+    }
+
+    fn state(&self) -> tree::State {
+        let mut hasher = Hasher::default();
+        self.dependency.hash(&mut hasher);
+        let hash = hasher.finish();
+
+        let element = Rc::new(RefCell::new((self.view)().into()));
+
+        (*self.element.borrow_mut()) = Some(element.clone());
+
+        tree::State::new(Internal { element, hash })
+    }
+
+    fn children(&self) -> Vec<Tree> {
+        vec![Tree::new(
+            self.element.borrow().as_ref().unwrap().borrow().as_widget(),
+        )]
+    }
+
+    fn diff(&self, tree: &mut Tree) {
+        let current = tree.state.downcast_mut::<Internal<Message, Renderer>>();
+
+        let mut hasher = Hasher::default();
+        self.dependency.hash(&mut hasher);
+        let new_hash = hasher.finish();
+
+        if current.hash != new_hash {
+            current.hash = new_hash;
+
+            let element = (self.view)().into();
+            current.element = Rc::new(RefCell::new(element));
+        }
+
+        (*self.element.borrow_mut()) = Some(current.element.clone());
+        tree.diff_children(std::slice::from_ref(
+            &self.element.borrow().as_ref().unwrap().borrow().as_widget(),
+        ));
+    }
+
+    fn width(&self) -> Length {
+        self.with_element(|element| element.as_widget().width())
+    }
+
+    fn height(&self) -> Length {
+        self.with_element(|element| element.as_widget().height())
+    }
+
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.with_element(|element| {
+            element.as_widget().layout(renderer, limits)
+        })
+    }
+
+    fn on_event(
+        &mut self,
+        tree: &mut Tree,
+        event: iced_native::Event,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        renderer: &Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, Message>,
+    ) -> event::Status {
+        self.with_element_mut(|mut element| {
+            element.as_widget_mut().on_event(
+                &mut tree.children[0],
+                event,
+                layout,
+                cursor_position,
+                renderer,
+                clipboard,
+                shell,
+            )
+        })
+    }
+
+    fn mouse_interaction(
+        &self,
+        tree: &Tree,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+        renderer: &Renderer,
+    ) -> mouse::Interaction {
+        self.with_element(|element| {
+            element.as_widget().mouse_interaction(
+                &tree.children[0],
+                layout,
+                cursor_position,
+                viewport,
+                renderer,
+            )
+        })
+    }
+
+    fn draw(
+        &self,
+        tree: &Tree,
+        renderer: &mut Renderer,
+        theme: &Renderer::Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        self.with_element(|element| {
+            element.as_widget().draw(
+                &tree.children[0],
+                renderer,
+                theme,
+                style,
+                layout,
+                cursor_position,
+                viewport,
+            )
+        })
+    }
+
+    fn overlay<'b>(
+        &'b self,
+        tree: &'b mut Tree,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+    ) -> Option<overlay::Element<'_, Message, Renderer>> {
+        let overlay = OverlayBuilder {
+            cached: self,
+            tree: &mut tree.children[0],
+            types: PhantomData,
+            element_ref_builder: |cached| cached.element.borrow(),
+            element_builder: |element_ref| {
+                element_ref.as_ref().unwrap().borrow()
+            },
+            overlay_builder: |element, tree| {
+                element.as_widget().overlay(tree, layout, renderer)
+            },
+        }
+        .build();
+
+        let has_overlay = overlay.with_overlay(|overlay| {
+            overlay.as_ref().map(overlay::Element::position)
+        });
+
+        has_overlay
+            .map(|position| overlay::Element::new(position, Box::new(overlay)))
+    }
+}
+
+#[self_referencing]
+struct Overlay<'a, 'b, Message, Renderer, Dependency, View> {
+    cached: &'a Cached<'b, Message, Renderer, Dependency, View>,
+    tree: &'a mut Tree,
+    types: PhantomData<(Message, Dependency, View)>,
+
+    #[borrows(cached)]
+    #[covariant]
+    element_ref:
+        Ref<'this, Option<Rc<RefCell<Element<'static, Message, Renderer>>>>>,
+
+    #[borrows(element_ref)]
+    #[covariant]
+    element: Ref<'this, Element<'static, Message, Renderer>>,
+
+    #[borrows(element, mut tree)]
+    #[covariant]
+    overlay: Option<overlay::Element<'this, Message, Renderer>>,
+}
+
+impl<'a, 'b, Message, Renderer, Dependency, View>
+    Overlay<'a, 'b, Message, Renderer, Dependency, View>
+{
+    fn with_overlay_maybe<T>(
+        &self,
+        f: impl FnOnce(&overlay::Element<'_, Message, Renderer>) -> T,
+    ) -> Option<T> {
+        self.borrow_overlay().as_ref().map(f)
+    }
+
+    fn with_overlay_mut_maybe<T>(
+        &mut self,
+        f: impl FnOnce(&mut overlay::Element<'_, Message, Renderer>) -> T,
+    ) -> Option<T> {
+        self.with_overlay_mut(|overlay| overlay.as_mut().map(f))
+    }
+}
+
+impl<'a, 'b, Message, Renderer, Dependency, View>
+    overlay::Overlay<Message, Renderer>
+    for Overlay<'a, 'b, Message, Renderer, Dependency, View>
+where
+    Renderer: iced_native::Renderer,
+{
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        bounds: Size,
+        position: Point,
+    ) -> layout::Node {
+        self.with_overlay_maybe(|overlay| {
+            let vector = position - overlay.position();
+
+            overlay.layout(renderer, bounds).translate(vector)
+        })
+        .unwrap_or_default()
+    }
+
+    fn draw(
+        &self,
+        renderer: &mut Renderer,
+        theme: &Renderer::Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+    ) {
+        let _ = self.with_overlay_maybe(|overlay| {
+            overlay.draw(renderer, theme, style, layout, cursor_position);
+        });
+    }
+
+    fn mouse_interaction(
+        &self,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+        renderer: &Renderer,
+    ) -> mouse::Interaction {
+        self.with_overlay_maybe(|overlay| {
+            overlay.mouse_interaction(
+                layout,
+                cursor_position,
+                viewport,
+                renderer,
+            )
+        })
+        .unwrap_or_default()
+    }
+
+    fn on_event(
+        &mut self,
+        event: iced_native::Event,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        renderer: &Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, Message>,
+    ) -> event::Status {
+        self.with_overlay_mut_maybe(|overlay| {
+            overlay.on_event(
+                event,
+                layout,
+                cursor_position,
+                renderer,
+                clipboard,
+                shell,
+            )
+        })
+        .unwrap_or_else(|| iced_native::event::Status::Ignored)
+    }
+}
+
+impl<'a, Message, Renderer, Dependency, View>
+    From<Cached<'a, Message, Renderer, Dependency, View>>
+    for Element<'a, Message, Renderer>
+where
+    View: Into<Element<'static, Message, Renderer>> + 'static,
+    Renderer: iced_native::Renderer + 'static,
+    Message: 'static,
+    Dependency: Hash + 'a,
+{
+    fn from(cached: Cached<'a, Message, Renderer, Dependency, View>) -> Self {
+        Self::new(cached)
+    }
+}
diff --git a/lazy/src/lib.rs b/lazy/src/lib.rs
index 3827746c..c01b439b 100644
--- a/lazy/src/lib.rs
+++ b/lazy/src/lib.rs
@@ -17,9 +17,11 @@
     clippy::type_complexity
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
+pub mod cached;
 pub mod component;
 pub mod responsive;
 
+pub use cached::Cached;
 pub use component::Component;
 pub use responsive::Responsive;