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use crate::peg::{Peg, Pegged};

#[doc(hidden)]
#[derive(Default)]
pub struct Listeners<T: 'static> {
    ls: Vec<Pegged<Listener<T>>>,
}

#[doc(hidden)]
pub type Listener<T> = Box<dyn FnMut(Option<&T>)>;

impl<T> Listeners<T> {
    pub fn new() -> Self {
        Listeners { ls: vec![] }
    }

    /// Add a new listener.
    pub fn add<F: FnMut(Option<&T>) + 'static>(&mut self, listener: F) -> Peg {
        let boxed: Listener<T> = Box::new(listener);
        let (val, peg) = Pegged::new_pair(boxed);
        self.ls.push(val);
        peg
    }

    pub fn clear(&mut self) {
        self.ls.clear();
    }

    /// Iterate over listeners and receive them one by one in a callback.
    pub fn iter(&mut self, mut cb: impl FnMut(&mut Listener<T>)) {
        self.ls.retain(|p| {
            p.with_value(|v| {
                if let Some(v) = v {
                    cb(v);
                    true
                } else {
                    false
                }
            })
        });
    }
}

/// A subscription is a receipt for adding a listener to a stream. Can be used to stop listening.
///
/// ## Subscription lifetimes
///
/// Every combinator _subscribes_ to events from its parent stream. It is basically the
/// same as calling `.subscribe()` but with an important twist. Xi reference counts
/// the number of children alive to determine when to unsubscribe.
///
/// Example:
/// ```
/// use xi::{Sink, Stream};
///
/// let sink: Sink<u32> = Stream::sink();
/// let stream = sink.stream();
///
/// // map is subscribed (once) to stream
/// let map = stream.map(|v| v * 2);
/// let map2 = map.clone();
///
/// drop(map);
/// drop(map2);
/// // map is unsubscribed from stream
/// ```
///
/// This is different to regular subscriptions where we must explicitly call `.unsubscribe()`
/// on the returned subscription instance.
///
/// Example:
/// ```
/// use xi::{Sink, Stream};
///
/// let sink: Sink<u32> = Stream::sink();
/// let stream = sink.stream();
///
/// // subscribed to stream
/// let sub = stream.subscribe(|v| if let Some(v) = v {
///     println!("{}", v)
/// });
///
/// drop(sub);
/// // still subscribed to stream
/// ```
///
#[derive(Clone)]
pub struct Subscription {
    peg: Peg,
}

impl Subscription {
    pub(crate) fn new(peg: Peg) -> Self {
        Subscription { peg }
    }

    /// Stops listening to the stream.
    pub fn unsubscribe(&self) {
        self.peg.unpeg()
    }
}