embassy_sync/
signal.rs

1//! A synchronization primitive for passing the latest value to a task.
2use core::cell::Cell;
3use core::future::{poll_fn, Future};
4use core::task::{Context, Poll, Waker};
5
6use crate::blocking_mutex::raw::RawMutex;
7use crate::blocking_mutex::Mutex;
8
9/// Single-slot signaling primitive for a _single_ consumer.
10///
11/// This is similar to a [`Channel`](crate::channel::Channel) with a buffer size of 1, except
12/// "sending" to it (calling [`Signal::signal`]) when full will overwrite the previous value instead
13/// of waiting for the receiver to pop the previous value.
14///
15/// It is useful for sending data between tasks when the receiver only cares about
16/// the latest data, and therefore it's fine to "lose" messages. This is often the case for "state"
17/// updates.
18///
19/// For more advanced use cases, you might want to use [`Channel`](crate::channel::Channel) instead.
20/// For multiple consumers, use [`Watch`](crate::watch::Watch) instead.
21///
22/// Signals are generally declared as `static`s and then borrowed as required.
23///
24/// ```
25/// use embassy_sync::signal::Signal;
26/// use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;
27///
28/// enum SomeCommand {
29///   On,
30///   Off,
31/// }
32///
33/// static SOME_SIGNAL: Signal<CriticalSectionRawMutex, SomeCommand> = Signal::new();
34/// ```
35pub struct Signal<M, T>
36where
37    M: RawMutex,
38{
39    state: Mutex<M, Cell<State<T>>>,
40}
41
42enum State<T> {
43    None,
44    Waiting(Waker),
45    Signaled(T),
46}
47
48impl<M, T> Signal<M, T>
49where
50    M: RawMutex,
51{
52    /// Create a new `Signal`.
53    pub const fn new() -> Self {
54        Self {
55            state: Mutex::new(Cell::new(State::None)),
56        }
57    }
58}
59
60impl<M, T> Default for Signal<M, T>
61where
62    M: RawMutex,
63{
64    fn default() -> Self {
65        Self::new()
66    }
67}
68
69impl<M, T> Signal<M, T>
70where
71    M: RawMutex,
72{
73    /// Mark this Signal as signaled.
74    pub fn signal(&self, val: T) {
75        self.state.lock(|cell| {
76            let state = cell.replace(State::Signaled(val));
77            if let State::Waiting(waker) = state {
78                waker.wake();
79            }
80        })
81    }
82
83    /// Remove the queued value in this `Signal`, if any.
84    pub fn reset(&self) {
85        self.state.lock(|cell| cell.set(State::None));
86    }
87
88    fn poll_wait(&self, cx: &mut Context<'_>) -> Poll<T> {
89        self.state.lock(|cell| {
90            let state = cell.replace(State::None);
91            match state {
92                State::None => {
93                    cell.set(State::Waiting(cx.waker().clone()));
94                    Poll::Pending
95                }
96                State::Waiting(w) if w.will_wake(cx.waker()) => {
97                    cell.set(State::Waiting(w));
98                    Poll::Pending
99                }
100                State::Waiting(w) => {
101                    cell.set(State::Waiting(cx.waker().clone()));
102                    w.wake();
103                    Poll::Pending
104                }
105                State::Signaled(res) => Poll::Ready(res),
106            }
107        })
108    }
109
110    /// Future that completes when this Signal has been signaled, taking the value out of the signal.
111    pub fn wait(&self) -> impl Future<Output = T> + '_ {
112        poll_fn(move |cx| self.poll_wait(cx))
113    }
114
115    /// non-blocking method to try and take the signal value.
116    pub fn try_take(&self) -> Option<T> {
117        self.state.lock(|cell| {
118            let state = cell.replace(State::None);
119            match state {
120                State::Signaled(res) => Some(res),
121                state => {
122                    cell.set(state);
123                    None
124                }
125            }
126        })
127    }
128
129    /// non-blocking method to check whether this signal has been signaled. This does not clear the signal.  
130    pub fn signaled(&self) -> bool {
131        self.state.lock(|cell| {
132            let state = cell.replace(State::None);
133
134            let res = matches!(state, State::Signaled(_));
135
136            cell.set(state);
137
138            res
139        })
140    }
141}
embassy_sync/signal.rs

embassy_sync/
signal.rs
