1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151

use futures::prelude::*;
use futures::task::{Waker, Poll, Context};
use futures::stream::{BoxStream};

use std::pin::*;
use std::sync::*;

///
/// The core of a switching stream
///
struct SwitchingStreamCore<TValue> {
    /// The stream to read from
    stream: BoxStream<'static, TValue>,

    /// Set to true if the stream has been closed
    closed: bool,

    /// Set to false once the switch has been removed
    switch_available: bool,

    /// The current waker for the stream
    wake: Option<Waker>
}

///
/// A stream that relays the results from a source stream, which can be switched out if necessary
///
/// This is useful when using a stream as a source of events (eg, as part of a UI application). This can be used to change the source
/// of events being consumed by one part of an application from another part without creating a direct dependency between those two parts.
///
/// For example, in an application that implements a set of tools - a paint program for example - this could be used to direct the flow
/// of events to and from the active tool.
///
/// A switching stream will not close until both the stream it contains has finished and the corresponding switch object has been removed
/// from memory.
///
pub struct SwitchingStream<TValue> {
    core: Arc<Mutex<SwitchingStreamCore<TValue>>>
}

///
/// Used to switch the stream for a corresponding `SwitchingStream` item
///
/// This must be dropped before the corresponding `SwitchingStream` can be closed
///
pub struct StreamSwitch<TValue> {
    core: Arc<Mutex<SwitchingStreamCore<TValue>>>
}

impl<TValue> Stream for SwitchingStream<TValue> {
    type Item = TValue;

    fn poll_next(self: Pin<&mut Self>, context: &mut Context) -> Poll<Option<TValue>> {
        // Lock the core
        let mut core = self.core.lock().unwrap();

        // Remember the waker: this is used if StreamSwitch is used to change the stream that this is returning
        core.wake = Some(context.waker().clone());

        if core.closed {
            // Stream has been closed
            if !core.switch_available {
                // The switch has been removed so there will be no more streams sent here
                Poll::Ready(None)
            } else {
                // The stream is closed but the switch is available: the stream 
                Poll::Pending
            }
        } else {
            // Poll the underlying stream
            let result = core.stream.poll_next_unpin(context);

            match result {
                Poll::Ready(None) => {
                    // Core stream has been closed
                    core.closed = true;

                    if !core.switch_available {
                        // The switch object has been freed
                        Poll::Ready(None)
                    } else {
                        // The switch object may restart the core stream
                        Poll::Pending
                    }
                }

                _ => result
            }
        }
    }
}

impl<TValue> StreamSwitch<TValue> {
    ///
    /// Switches the stream being returned by the corresponding `SwitchingStream` to the specified new stream
    ///
    pub fn switch_to_stream<TStream: 'static+Send+Stream<Item=TValue>>(&self, new_stream: TStream) {
        let waker = {
            let mut core    = self.core.lock().unwrap();

            // Un-close the stream and set it to the new stream that was passed in
            core.closed     = false;
            core.stream     = new_stream.boxed();

            // Wake up the core
            core.wake.take()
        };

        waker.map(|waker| waker.wake());
    }
}

impl<TValue> Drop for StreamSwitch<TValue> {
    fn drop(&mut self) {
        let waker = {
            let mut core = self.core.lock().unwrap();

            // The switch for this core is no longer available, so the stream can close
            core.switch_available = false;

            // Wake up the core
            core.wake.take()
        };

        waker.map(|waker| waker.wake());
    }
}

///
/// Returns a switching stream and its switch, set to initially read from the stream that's passed in
///
/// A switching stream can be changed to return results from another stream at any time. It's quite useful for things like streams
/// of events in particular, where something like a user interface might want to change the source of events around.
///
/// The `StreamSwitch` can be used to change where the results for the returned `SwitchingStream` are generated from. The 
/// `SwitchingStream` will only close once both its underlying stream has finished and the switch has been dropped.
///
pub fn switchable_stream<TStream: 'static+Send+Stream>(stream: TStream) -> (SwitchingStream<TStream::Item>, StreamSwitch<TStream::Item>) {
    let core = SwitchingStreamCore {
        stream:             stream.boxed(),
        closed:             false,
        switch_available:   true,
        wake:               None
    };
    let core = Arc::new(Mutex::new(core));

    let stream = SwitchingStream    { core: Arc::clone(&core) };
    let switch = StreamSwitch       { core: core };

    (stream, switch)
}