Add Runtime concept to abstract caching

src/runtime.rs

@@ -0,0 +1,103 @@
+use crate::{Column, Element, Event, Layout, MouseCursor, Point};
+
+use std::hash::Hasher;
+use stretch::result;
+
+pub struct Runtime {
+    cache: Cache,
+    events: Vec<Event>,
+    cursor_position: Point,
+}
+
+impl Runtime {
+    pub fn new() -> Runtime {
+        // We use this as a placeholder to initialize the cache.
+        // This way, we can avoid the overhead of using an `Option`
+        // in `compute`.
+        let root: Element<'_, (), ()> = Column::new().into();
+
+        let hasher = &mut crate::Hasher::default();
+        root.hash(hasher);
+
+        Runtime {
+            cache: Cache {
+                hash: hasher.finish(),
+                layout: root.compute_layout(&()),
+            },
+            events: Vec::new(),
+            cursor_position: Point::new(0.0, 0.0),
+        }
+    }
+
+    pub fn on_event(&mut self, event: Event) {
+        self.events.push(event);
+    }
+
+    pub fn compute<'a, Message, Renderer>(
+        &'a mut self,
+        root: Element<'a, Message, Renderer>,
+        renderer: &Renderer,
+    ) -> Interface<'a, Message, Renderer> {
+        let hasher = &mut crate::Hasher::default();
+        root.hash(hasher);
+
+        let hash = hasher.finish();
+
+        if hash != self.cache.hash {
+            self.cache = Cache {
+                hash,
+                layout: root.compute_layout(renderer),
+            };
+        }
+
+        Interface {
+            root,
+            layout: &self.cache.layout,
+            events: &mut self.events,
+            cursor_position: self.cursor_position,
+        }
+    }
+}
+
+struct Cache {
+    hash: u64,
+    layout: result::Layout,
+}
+
+pub struct Interface<'a, Message, Renderer> {
+    root: Element<'a, Message, Renderer>,
+    layout: &'a result::Layout,
+    events: &'a mut Vec<Event>,
+    cursor_position: Point,
+}
+
+impl<'a, Message, Renderer> Interface<'a, Message, Renderer> {
+    pub fn update(&mut self) -> Vec<Message> {
+        let mut messages = Vec::new();
+
+        for event in self.events.drain(..) {
+            self.root.widget.on_event(
+                event,
+                Layout::new(&self.layout),
+                self.cursor_position,
+                &mut messages,
+            );
+        }
+
+        messages
+    }
+
+    pub fn draw(
+        &self,
+        renderer: &mut Renderer,
+        cursor_position: Point,
+    ) -> MouseCursor {
+        let cursor = self.root.widget.draw(
+            renderer,
+            Layout::new(self.layout),
+            cursor_position,
+        );
+
+        cursor
+    }
+}