## Event listener primitives

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This crate provides a low-level primitive for building Node.js-like event listeners.

The 3 primitives are [`Bag`] that is a container for `Fn()` event handlers, [`BagOnce`] the same for `FnOnce()` event handlers and [`HandlerId`] that will remove event handler from the bag on drop.

Trivial example:
```rust
use event_listener_primitives::{Bag, HandlerId};

fn main() {
    let bag = Bag::default();

    let handler_id = bag.add(move || {
        println!("Hello")
    });

    bag.call_simple();
}
```

Close to real-world usage example:
```rust
use event_listener_primitives::{Bag, HandlerId};
fn main() {
    let bag = Bag::default();
    let handler_id = bag.add(Box::new(move || {
        println!("Hello")
    }));
    bag.call_simple();
}
```
Close to real-world usage example:
```rust
use event_listener_primitives::{Bag, BagOnce, HandlerId};
use std::sync::Arc;
#[derive(Default)]
struct Handlers {
    bar: Bag<Box<dyn Fn() + Send + Sync + 'static>>,
    closed: BagOnce<Box<dyn FnOnce() + Send + Sync + 'static>>,
}
struct Inner {
    handlers: Arc<Handlers>,
}
impl Drop for Inner {
    fn drop(&mut self) {
        self.handlers.closed.call_simple();
    }
}
#[derive(Clone)]
pub struct Foo {
    inner: Arc<Inner>,
}
impl Foo {
    pub fn new() -> Self {
        let handlers = Arc::<Handlers>::default();
        let inner = Arc::new(Inner { handlers });
        Self { inner }
    }
    pub fn do_bar(&self) {
        // Do things...
        self.inner.handlers.bar.call_simple();
    }
    pub fn do_other_bar(&self) {
        // Do things...
        self.inner.handlers.bar.call(|callback| {
            callback();
        });
    }
    pub fn on_bar<F: Fn() + Send + Sync + 'static>(&self, callback: F) -> HandlerId {
        self.inner.handlers.bar.add(Box::new(callback))
    }
    pub fn on_closed<F: FnOnce() + Send + Sync + 'static>(&self, callback: F) -> HandlerId {
        self.inner.handlers.closed.add(Box::new(callback))
    }
}
fn main() {
    let foo = Foo::new();
    let on_bar_handler_id = foo.on_bar(|| {
        println!("On bar");
    });
    foo
        .on_closed(|| {
            println!("On closed");
        })
        .detach();
    // This will trigger "bar" callback just fine since its handler ID is not dropped yet
    foo.do_bar();
    drop(on_bar_handler_id);
    // This will not trigger "bar" callback since its handler ID was already dropped
    foo.do_other_bar();
    // This will trigger "closed" callback though since we've detached handler ID
    drop(foo);
    println!("Done");
}
```

The output will be:
```text
On bar
On closed
Done
```

## Contribution
Feel free to create issues and send pull requests, they are highly appreciated!

## License
Zero-Clause BSD

https://opensource.org/licenses/0BSD

https://tldrlegal.com/license/bsd-0-clause-license
