Add `Tween::set_direction()` for backward playback (#15)

Allow a `Tween` to play backward by setting its direction with
`Tween::set_direction()`.

CHANGELOG.md

@@ -3,6 +3,13 @@
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Add `is_forward()` and `is_backward()` convenience helpers to `TweeningDirection`.
+- Add `Tween::set_direction()` and `Tween::with_direction()` which allow configuring the playback direction of a tween, allowing to play it backward from end to start.
+
 ## [0.4.0] - 2022-04-16
 
 ### Changed

src/lib.rs

@@ -250,17 +250,37 @@ impl From<EaseFunction> for EaseMethod {
 
 /// Direction a tweening animation is playing.
 ///
-/// For all but [`TweeningType::PingPong`] this is always [`TweeningDirection::Forward`]. For the
-/// [`TweeningType::PingPong`] tweening type, this is either forward (from start to end; ping) or
-/// backward (from end to start; pong).
+/// When playing a tweenable forward, the progress values `0` and `1` are respectively mapped to
+/// the start and end bounds of the lens(es) being used. Conversely, when playing backward, this
+/// mapping is reversed, such that a progress value of `0` corresponds to the state of the target
+/// at the end bound of the lens, while a progress value of `1` corresponds to the state of that
+/// target at the start bound of the lens, effectively making the animation play backward.
+///
+/// For all but [`TweeningType::PingPong`] this is always [`TweeningDirection::Forward`], unless
+/// manually configured with [`Tween::set_direction()`] in which case the value is constant equal
+/// to the value set. For the [`TweeningType::PingPong`] tweening type, this is either forward
+/// (from start to end; ping) or backward (from end to start; pong), depending on the current
+/// iteration of the loop.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum TweeningDirection {
     /// Animation playing from start to end.
     Forward,
-    /// Animation playing from end to start.
+    /// Animation playing from end to start, in reverse.
     Backward,
 }
 
+impl TweeningDirection {
+    /// Is the direction equal to [`TweeningDirection::Forward`]?
+    pub fn is_forward(&self) -> bool {
+        *self == TweeningDirection::Forward
+    }
+
+    /// Is the direction equal to [`TweeningDirection::Backward`]?
+    pub fn is_backward(&self) -> bool {
+        *self == TweeningDirection::Backward
+    }
+}
+
 impl Default for TweeningDirection {
     fn default() -> Self {
         TweeningDirection::Forward