Macro listener

macro_rules! listener {
    ($event:expr => $listener:ident) => { ... };
}

Create a stack-based event listener for an Event.

EventListener allocates the listener on the heap. While this works for most use cases, in practice this heap allocation can be expensive for repeated uses. This method allows for allocating the listener on the stack instead.

There are limitations to using this macro instead of the EventListener type, however. Firstly, it is significantly less flexible. The listener is locked to the current stack frame, meaning that it can’t be returned or put into a place where it would go out of scope. For instance, this will not work:

use event_listener::{Event, Listener, listener};

fn get_listener(event: &Event) -> impl Listener {
    listener!(event => cant_return_this);
    cant_return_this
}

In addition, the types involved in creating this listener are not able to be named. Therefore it cannot be used in hand-rolled futures or similar structures.

The type created by this macro implements Listener, allowing it to be used in cases where EventListener would normally be used.

Example

To use this macro, replace cases where you would normally use this…

let listener = event.listen();

…with this:

listener!(event => listener);

Here is the top level example from this crate’s documentation, but using listener instead of EventListener.

use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::thread;
use std::time::Duration;
use std::usize;
use event_listener::{Event, listener, IntoNotification, Listener};

let flag = Arc::new(AtomicBool::new(false));
let event = Arc::new(Event::new());

// Spawn a thread that will set the flag after 1 second.
thread::spawn({
    let flag = flag.clone();
    let event = event.clone();
    move || {
        // Wait for a second.
        thread::sleep(Duration::from_secs(1));

        // Set the flag.
        flag.store(true, Ordering::SeqCst);

        // Notify all listeners that the flag has been set.
        event.notify(usize::MAX);
    }
});

// Wait until the flag is set.
loop {
    // Check the flag.
    if flag.load(Ordering::SeqCst) {
        break;
    }

    // Start listening for events.
    // NEW: Changed to a stack-based listener.
    listener!(event => listener);

    // Check the flag again after creating the listener.
    if flag.load(Ordering::SeqCst) {
        break;
    }

    // Wait for a notification and continue the loop.
    listener.wait();
}