util: add generic event listener framework

src/utils/event_listener.rs

@@ -0,0 +1,69 @@
+use {
+    crate::utils::linkedlist::{LinkedList, LinkedListIter, LinkedNode},
+    std::{
+        cell::Cell,
+        rc::{Rc, Weak},
+    },
+};
+
+pub struct EventSource<T: ?Sized> {
+    listeners: LinkedList<Weak<T>>,
+    on_attach: Cell<Option<Box<dyn FnOnce()>>>,
+}
+
+pub struct EventListener<T: ?Sized> {
+    link: LinkedNode<Weak<T>>,
+}
+
+impl<T: ?Sized> Default for EventSource<T> {
+    fn default() -> Self {
+        Self {
+            listeners: Default::default(),
+            on_attach: Default::default(),
+        }
+    }
+}
+
+impl<T: ?Sized> EventSource<T> {
+    pub fn iter(&self) -> EventSourceIter<T> {
+        EventSourceIter {
+            iter: self.listeners.iter(),
+        }
+    }
+}
+
+pub struct EventSourceIter<T: ?Sized> {
+    iter: LinkedListIter<Weak<T>>,
+}
+
+impl<T: ?Sized> Iterator for EventSourceIter<T> {
+    type Item = Rc<T>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        for weak in self.iter.by_ref() {
+            if let Some(t) = weak.upgrade() {
+                return Some(t);
+            }
+        }
+        None
+    }
+}
+
+impl<T: ?Sized> EventListener<T> {
+    pub fn new(t: Weak<T>) -> Self {
+        Self {
+            link: LinkedNode::detached(t),
+        }
+    }
+
+    pub fn attach(&self, source: &EventSource<T>) {
+        source.listeners.add_last_existing(&self.link);
+        if let Some(on_attach) = source.on_attach.take() {
+            on_attach();
+        }
+    }
+
+    pub fn detach(&self) {
+        self.link.detach();
+    }
+}