style: Add a new Timer structure to the shared style context, and basic infrastructure for controlling animations.

components/style/timer.rs

@@ -0,0 +1,58 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use time;
+
+/// The `TimerMode` is used to determine what time should the `Timer` return,
+/// either a fixed value (in the `Test` mode), or the actual time (in the
+/// `Current` mode).
+#[derive(Debug, Clone)]
+enum TimerMode {
+    Test(f64),
+    Current,
+}
+
+/// A `Timer` struct that takes care of giving the current time for animations.
+///
+/// This is needed to be allowed to hook the time in the animations' test-mode.
+#[derive(Debug, Clone)]
+pub struct Timer {
+    mode: TimerMode,
+}
+
+impl Timer {
+    /// Creates a new "normal" timer, i.e., a "Current" mode timer.
+    #[inline]
+    pub fn new() -> Self {
+        Timer {
+            mode: TimerMode::Current,
+        }
+    }
+
+    /// Creates a new "test mode" timer, with initial time 0.
+    #[inline]
+    pub fn test_mode() -> Self {
+        Timer {
+            mode: TimerMode::Test(0.),
+        }
+    }
+
+    /// Returns the current time, at least from the caller's perspective. In
+    /// test mode returns whatever the value is.
+    pub fn seconds(&self) -> f64 {
+        match self.mode {
+            TimerMode::Test(test_value) => test_value,
+            TimerMode::Current => time::precise_time_s(),
+        }
+    }
+
+    /// Increments the current clock. Panics if the clock is not on test mode.
+    pub fn increment(&mut self, by: f64) {
+        match self.mode {
+            TimerMode::Test(ref mut val)
+                => *val += by,
+            TimerMode::Current
+                => panic!("Timer::increment called for a non-test mode timer. This is a bug."),
+        }
+    }
+}