Module tokio::sync::watch

A single-producer, multi-consumer channel that only retains the last sent value.

This channel is useful for watching for changes to a value from multiple points in the code base, for example, changes to configuration values.

Usage

channel returns a Sender / Receiver pair. These are the producer and consumer halves of the channel. The channel is created with an initial value.

Each Receiver independently tracks the last value seen by its caller.

To access the current value stored in the channel and mark it as seen by a given Receiver, use Receiver::borrow_and_update().

To access the current value without marking it as seen, use Receiver::borrow(). (If the value has already been marked seen, Receiver::borrow() is equivalent to Receiver::borrow_and_update().)

For more information on when to use these methods, see here.

Change notifications

The Receiver half provides an asynchronous changed method. This method is ready when a new, unseen value is sent via the Sender half.

• Receiver::changed() returns Ok(()) on receiving a new value, or Err(error::RecvError) if the Sender has been dropped.
• If the current value is unseen when calling changed, then changed will return immediately. If the current value is seen, then it will sleep until either a new message is sent via the Sender half, or the Sender is dropped.
• On completion, the changed method marks the new value as seen.
• At creation, the initial value is considered seen. In other words, Receiver::changed() will not return until a subsequent value is sent.
• New Receiver instances can be created with Sender::subscribe(). The current value at the time the Receiver is created is considered seen.

borrow_and_update versus borrow

If the receiver intends to await notifications from changed in a loop, Receiver::borrow_and_update() should be preferred over Receiver::borrow(). This avoids a potential race where a new value is sent between changed being ready and the value being read. (If Receiver::borrow() is used, the loop may run twice with the same value.)

If the receiver is only interested in the current value, and does not intend to wait for changes, then Receiver::borrow() can be used. It may be more convenient to use borrow since it’s an &self method—borrow_and_update requires &mut self.

Examples

The following example prints hello! world! .

use tokio::sync::watch;
use tokio::time::{Duration, sleep};

let (tx, mut rx) = watch::channel("hello");

tokio::spawn(async move {
    // Use the equivalent of a "do-while" loop so the initial value is
    // processed before awaiting the `changed()` future.
    loop {
        println!("{}! ", *rx.borrow_and_update());
        if rx.changed().await.is_err() {
            break;
        }
    }
});

sleep(Duration::from_millis(100)).await;
tx.send("world")?;

Closing

Sender::is_closed and Sender::closed allow the producer to detect when all Receiver handles have been dropped. This indicates that there is no further interest in the values being produced and work can be stopped.

The value in the channel will not be dropped until the sender and all receivers have been dropped.

Thread safety

Both Sender and Receiver are thread safe. They can be moved to other threads and can be used in a concurrent environment. Clones of Receiver handles may be moved to separate threads and also used concurrently.

Modules

• error
  Watch error types.

Structs

• Receiver
  Receives values from the associated Sender.
• Ref
  Returns a reference to the inner value.
• Sender
  Sends values to the associated Receiver.

Functions

• channel
  Creates a new watch channel, returning the “send” and “receive” handles.