# tokio::sync::Notify - Rust
API documentation for the Rust `Notify` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, Notify

# Struct tokio::sync::Notify [−] [src]

    pub struct Notify { /* fields omitted */ }

Notify a single task to wake up.

`Notify` provides a basic mechanism to notify a single task of an event. `Notify` itself does not carry any data. Instead, it is to be used to signal another task to perform an operation.

`Notify` can be thought of as a `Semaphore` starting with 0 permits. `notified().await` waits for a permit to become available, and `notify_one()` sets a permit if there currently are no available permits.

The synchronization details of `Notify` are similar to `thread::park` and `Thread::unpark` from std. A `Notify` value contains a single permit. `notified().await` waits for the permit to be made available, consumes the permit, and resumes. `notify_one()` sets the permit, waking a pending task if there is one.

If `notify_one()` is called before `notified().await`, then the next call to `notified().await` will complete immediately, consuming the permit. Any subsequent calls to `notified().await` will wait for a new permit.

If `notify_one()` is called multiple times before `notified().await`, only a single permit is stored. The next call to `notified().await` will complete immediately, but the one after will wait for a new permit.

# Examples

Basic usage.

    use tokio::sync::Notify;
    use std::sync::Arc;

    #[tokio::main]
    async fn main() {
        let notify = Arc::new(Notify::new());
        let notify2 = notify.clone();

        tokio::spawn(async move {
            notify2.notified().await;
            println!("received notification");
        });

        println!("sending notification");
        notify.notify_one();
    }

Unbound mpsc channel.

    use tokio::sync::Notify;

    use std::collections::VecDeque;
    use std::sync::Mutex;

    struct Channel<T> {
        values: Mutex<VecDeque<T>>,
        notify: Notify,
    }

    impl<T> Channel<T> {
        pub fn send(&self, value: T) {
            self.values.lock().unwrap()
                .push_back(value);

            // Notify the consumer a value is available
            self.notify.notify_one();
        }

        pub async fn recv(&self) -> T {
            loop {
                // Drain values
                if let Some(value) = self.values.lock().unwrap().pop_front() {
                    return value;
                }

                // Wait for values to be available
                self.notify.notified().await;
            }
        }
    }

## Implementations

### `impl Notify` [src]

#### `pub fn new() -> Notify` [src]

Create a new `Notify`, initialized without a permit.

# Examples

    use tokio::sync::Notify;

    let notify = Notify::new();

#### `pub const fn const_new() -> Notify` [src]

Create a new `Notify`, initialized without a permit.

# Examples

    use tokio::sync::Notify;

    static NOTIFY: Notify = Notify::const_new();

#### `pub fn notified(&self) -> Notified<'_>` [src]

Wait for a notification.

Equivalent to:

ⓘ

    async fn notified(&self);

Each `Notify` value holds a single permit. If a permit is available from an earlier call to `notify_one()`, then `notified().await` will complete immediately, consuming that permit. Otherwise, `notified().await` waits for a permit to be made available by the next call to `notify_one()`.

# Examples

    use tokio::sync::Notify;
    use std::sync::Arc;

    #[tokio::main]
    async fn main() {
        let notify = Arc::new(Notify::new());
        let notify2 = notify.clone();

        tokio::spawn(async move {
            notify2.notified().await;
            println!("received notification");
        });

        println!("sending notification");
        notify.notify_one();
    }

#### `pub fn notify_one(&self)` [src]

Notifies a waiting task

If a task is currently waiting, that task is notified. Otherwise, a permit is stored in this `Notify` value and the next call to `notified().await` will complete immediately consuming the permit made available by this call to `notify_one()`.

At most one permit may be stored by `Notify`. Many sequential calls to `notify_one` will result in a single permit being stored. The next call to `notified().await` will complete immediately, but the one after that will wait.

# Examples

    use tokio::sync::Notify;
    use std::sync::Arc;

    #[tokio::main]
    async fn main() {
        let notify = Arc::new(Notify::new());
        let notify2 = notify.clone();

        tokio::spawn(async move {
            notify2.notified().await;
            println!("received notification");
        });

        println!("sending notification");
        notify.notify_one();
    }

#### `pub fn notify_waiters(&self)` [src]

Notifies all waiting tasks

If a task is currently waiting, that task is notified. Unlike with `notify_one()`, no permit is stored to be used by the next call to `notified().await`. The purpose of this method is to notify all already registered waiters. Registering for notification is done by acquiring an instance of the `Notified` future via calling `notified()`.

# Examples

    use tokio::sync::Notify;
    use std::sync::Arc;

    #[tokio::main]
    async fn main() {
        let notify = Arc::new(Notify::new());
        let notify2 = notify.clone();

        let notified1 = notify.notified();
        let notified2 = notify.notified();

        let handle = tokio::spawn(async move {
            println!("sending notifications");
            notify2.notify_waiters();
        });

        notified1.await;
        notified2.await;
        println!("received notifications");
    }

## Trait Implementations

### `impl Debug for Notify` [src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl Default for Notify` [src]

#### `fn default() -> Notify` [src]

Returns the "default value" for a type. Read more

## Auto Trait Implementations

### `impl !RefUnwindSafe for Notify` [src]

### `impl Send for Notify` [src]

### `impl Sync for Notify` [src]

### `impl Unpin for Notify` [src]

### `impl UnwindSafe for Notify` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.