Mutable: Conditionally modify values in place

Signed-off-by: Uwe Klotz <uwe.klotz@slowtec.de>

src/signal/mutable.rs

@@ -1,6 +1,7 @@
 use super::Signal;
 use std;
 use std::fmt;
+use std::panic;
 use std::pin::Pin;
 use std::marker::Unpin;
 use std::ops::{Deref, DerefMut};
@@ -310,6 +311,40 @@ impl<A> Mutable<A> {
         }
     }
 
+    /// Conditionally modify the value in place
+    ///
+    /// Borrows the value mutably and modifies it in place.
+    ///
+    /// The value is only considered as modified if the modifying closure
+    /// returned `true`. Otherwise the value is assumed to be unmodified
+    /// despite the mutable borrow before.
+    ///
+    /// Returns the result of the closure, i.e. if the value has been modified.
+    pub fn modify<F>(&self, modify: F) -> bool
+    where
+        F: FnOnce(&mut A) -> bool,
+    {
+        let mut locked_state = self.state().lock.write().unwrap();
+
+        let result =
+            panic::catch_unwind(panic::AssertUnwindSafe(|| modify(&mut locked_state.value)));
+        // If modify() panicked forward the panic to the caller without poisoning the lock.
+        match result {
+            Ok(modified) => {
+                if modified {
+                    locked_state.notify(modified);
+                }
+                modified
+            }
+            Err(panicked) => {
+                // Drop the lock to avoid poisoning it.
+                drop(locked_state);
+                // Forward the panic to the caller.
+                panic::resume_unwind(panicked);
+            }
+        }
+    }
+
     // TODO lots of unit tests to verify that it only notifies when the object is mutated
     // TODO return Result ?
     // TODO should this inline ?

tests/signal.rs

@@ -530,3 +530,54 @@ fn test_throttle_timing() {
         assert_eq!(output.poll_change(cx), Poll::Ready(None));
     });
 }
+
+#[test]
+fn test_modify() {
+    #[derive(Debug, Clone, Copy, PartialEq)]
+    struct State {
+        counter: usize,
+    }
+
+    let inc_counter_if_odd = |state: &mut State| {
+        if state.counter % 2 == 1 {
+            state.counter += 1;
+            return true;
+        }
+        false
+    };
+
+    util::with_noop_context(|cx| {
+        let state = Mutable::new(State { counter: 1 });
+        let initial_state = state.get();
+        let modified_state = {
+            let mut modified_state = initial_state;
+            assert!(inc_counter_if_odd(&mut modified_state));
+            modified_state
+        };
+        assert_ne!(initial_state, modified_state);
+
+        let signal = state.signal();
+        pin_mut!(signal);
+
+        // Initial value
+        assert_eq!(
+            signal.as_mut().poll_change(cx),
+            Poll::Ready(Some(initial_state))
+        );
+
+        // Modified
+        assert_eq!(signal.as_mut().poll_change(cx), Poll::Pending);
+        assert!(state.modify(inc_counter_if_odd));
+        assert_eq!(state.get(), modified_state);
+        assert_eq!(
+            signal.as_mut().poll_change(cx),
+            Poll::Ready(Some(modified_state))
+        );
+
+        // Unmodified
+        assert_eq!(signal.as_mut().poll_change(cx), Poll::Pending);
+        assert!(!state.modify(inc_counter_if_odd));
+        assert_eq!(state.get(), modified_state);
+        assert_eq!(signal.as_mut().poll_change(cx), Poll::Pending);
+    });
+}