Rename Sleep to Delay (#270)

This patch renames `Sleep` from tokio-timer and the tokio facade to
`Delay`. Given that the future does not actually put anything to sleep,
the `Delay` name feels more appropriate.

Fixes #263

tokio-timer/src/delay.rs

@@ -0,0 +1,108 @@
+use Error;
+use timer::Registration;
+
+use futures::{Future, Poll};
+
+use std::time::Instant;
+
+/// A future that completes at a specified instant in time.
+///
+/// Instances of `Delay` perform no work and complete with `()` once the
+/// specified deadline has been reached.
+///
+/// `Delay` has a resolution of one millisecond and should not be used for tasks
+/// that require high-resolution timers.
+///
+/// [`new`]: #method.new
+#[derive(Debug)]
+pub struct Delay {
+    /// The instant at which the future completes.
+    deadline: Instant,
+
+    /// The link between the `Delay` instance at the timer that drives it.
+    ///
+    /// When `Delay` is created with `new`, this is initialized to `None` and is
+    /// lazily set in `poll`. When `poll` is called, the default for the current
+    /// execution context is used (obtained via `Handle::current`).
+    ///
+    /// When `delay` is created with `new_with_registration`, the value is set.
+    ///
+    /// Once `registration` is set to `Some`, it is never changed.
+    registration: Option<Registration>,
+}
+
+impl Delay {
+    /// Create a new `Delay` instance that elapses at `deadline`.
+    ///
+    /// Only millisecond level resolution is guaranteed. There is no guarantee
+    /// as to how the sub-millisecond portion of `deadline` will be handled.
+    /// `Delay` should not be used for high-resolution timer use cases.
+    pub fn new(deadline: Instant) -> Delay {
+        Delay {
+            deadline,
+            registration: None,
+        }
+    }
+
+    pub(crate) fn new_with_registration(
+        deadline: Instant,
+        registration: Registration) -> Delay
+    {
+        Delay {
+            deadline,
+            registration: Some(registration),
+        }
+    }
+
+    /// Returns the instant at which the future will complete.
+    pub fn deadline(&self) -> Instant {
+        self.deadline
+    }
+
+    /// Returns true if the `Delay` has elapsed
+    ///
+    /// A `Delay` is elapsed when the requested duration has elapsed.
+    pub fn is_elapsed(&self) -> bool {
+        self.registration.as_ref()
+            .map(|r| r.is_elapsed())
+            .unwrap_or(false)
+    }
+
+    /// Reset the `Delay` instance to a new deadline.
+    ///
+    /// Calling this function allows changing the instant at which the `Delay`
+    /// future completes without having to create new associated state.
+    ///
+    /// This function can be called both before and after the future has
+    /// completed.
+    pub fn reset(&mut self, deadline: Instant) {
+        self.deadline = deadline;
+
+        if let Some(registration) = self.registration.as_ref() {
+            registration.reset(deadline);
+        }
+    }
+
+    /// Register the delay with the timer instance for the current execution
+    /// context.
+    fn register(&mut self) {
+        if self.registration.is_some() {
+            return;
+        }
+
+        self.registration = Some(Registration::new(self.deadline));
+    }
+}
+
+impl Future for Delay {
+    type Item = ();
+    type Error = Error;
+
+    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+        // Ensure the `Delay` instance is associated with a timer.
+        self.register();
+
+        self.registration.as_ref().unwrap()
+            .poll_elapsed()
+    }
+}