split part 11 into two, and explain interior mutability and Cell and RefCell in the...

[rust-101.git] / solutions / src / callbacks.rs

diff --git a/solutions/src/callbacks.rs b/solutions/src/callbacks.rs
new file mode 100644 (file)
index 0000000..93fcb17
--- /dev/null
+++ b/solutions/src/callbacks.rs
@@ -0,0 +1,66 @@
+use std::rc::Rc;
+use std::cell::RefCell;
+
+#[derive(Clone)]
+pub struct Callbacks {
+    callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
+}
+
+impl Callbacks {
+    pub fn new() -> Self {
+        Callbacks { callbacks: Vec::new() }                      /*@*/
+    }
+
+    pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
+        let cell = Rc::new(RefCell::new(callback));
+        self.callbacks.push(cell);                                  /*@*/
+    }
+
+    pub fn call(&self, val: i32) {
+        for callback in self.callbacks.iter() {
+            // We have to *explicitly* borrow the contents of a `RefCell`.
+            //@ At run-time, the cell will keep track of the number of outstanding shared and mutable borrows,
+            //@ and panic if the rules are violated. Since this function is the only one that borrow the
+            //@ environments of the closures, and this function requires a *mutable* borrow of `self`, we know this cannot
+            //@ happen. <br />
+            //@ For this check to be performed, `closure` is a *guard*: Rather than a normal borrow, `borrow_mut` returns
+            //@ a smart pointer (`RefMut`, in this case) that waits until is goes out of scope, and then
+            //@ appropriately updates the number of active borrows.
+            //@ 
+            //@ The function would still typecheck with an immutable borrow of `self` (since we are
+            //@ relying on the interior mutability of `self`), but then it could happen that a callback
+            //@ will in turn trigger another round of callbacks, so that `call` would indirectly call itself.
+            //@ This is called reentrancy. It would imply that we borrow the closure a second time, and
+            //@ panic at run-time. I hope this also makes it clear that there's absolutely no hope of Rust
+            //@ performing these checks statically, at compile-time: It would have to detect reentrancy!
+            let mut closure = callback.borrow_mut();
+            // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
+            // dereference the smart pointer and obtain a mutable borrow of the target.
+            (&mut *closure)(val);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::rc::Rc;
+    use std::cell::RefCell;
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn test_reentrant() {
+        let c = Rc::new(RefCell::new(Callbacks::new()));
+        c.borrow_mut().register(|val| println!("Callback called: {}", val) );
+
+        // If we change the two "borrow" below to "borrow_mut", you can get a panic even with a "call" that requires a
+        // mutable borrow. However, that panic is then triggered by our own, external `RefCell` (so it's kind of our fault),
+        // rather than being triggered by the `RefCell` in the `Callbacks`.
+        {
+            let c2 = c.clone();
+            c.borrow_mut().register(move |val| c2.borrow().call(val+val) );
+        }
+
+        c.borrow().call(42);
+    }
+}
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