Add repeating flag to Timer

Repeating timers will reset themselves upon finishing, carrying over any
excess time during the last tick. This fixes timer drift upon resetting.

crates/bevy_core/src/time/timer.rs

@@ -4,39 +4,58 @@ use bevy_property::Properties;
 use std::time::Duration;
 
 /// Tracks elapsed time. Enters the finished state once `duration` is reached.
+///
+/// Non repeating timers will stop tracking and stay in the finished state until reset.
+/// Repeating timers will only be in the finished state on each tick `duration` is reached or exceeded, and can still be reset at any given point.
 #[derive(Clone, Debug, Default, Properties)]
 pub struct Timer {
     pub elapsed: f32,
     pub duration: f32,
     pub finished: bool,
+    /// Stores the last tick's finished state. Is checked against in just_finished.
+    prev_finished: bool,
+    pub repeating: bool,
 }
 
 impl Timer {
-    pub fn new(duration: Duration) -> Self {
+    pub fn new(duration: Duration, repeating: bool) -> Self {
         Timer {
             duration: duration.as_secs_f32(),
+            repeating,
             ..Default::default()
         }
     }
 
-    pub fn from_seconds(seconds: f32) -> Self {
+    pub fn from_seconds(seconds: f32, repeating: bool) -> Self {
         Timer {
             duration: seconds,
+            repeating,
             ..Default::default()
         }
     }
 
     pub fn tick(&mut self, delta: f32) {
-        self.elapsed = (self.elapsed + delta).min(self.duration);
-        if self.elapsed >= self.duration {
-            self.finished = true;
+        if self.elapsed < self.duration {
+            self.elapsed += delta;
+        }
+
+        self.prev_finished = self.finished;
+        self.finished = self.elapsed >= self.duration;
+
+        if self.repeating && self.finished {
+            self.elapsed = self.elapsed % self.duration;
         }
     }
 
     pub fn reset(&mut self) {
         self.finished = false;
         self.elapsed = 0.0;
     }
+
+    /// Returns true only on the exact tick elapsed time first exceeds duration.
+    pub fn just_finished(&self) -> bool {
+        (!self.prev_finished || self.repeating) && self.finished
+    }
 }
 
 pub(crate) fn timer_system(time: Res<Time>, mut query: Query<&mut Timer>) {

crates/bevy_diagnostic/src/print_diagnostics_plugin.rs

@@ -30,7 +30,7 @@ impl Default for PrintDiagnosticsPlugin {
 impl Plugin for PrintDiagnosticsPlugin {
     fn build(&self, app: &mut bevy_app::AppBuilder) {
         app.add_resource(PrintDiagnosticsState {
-            timer: Timer::new(self.wait_duration),
+            timer: Timer::new(self.wait_duration, true),
             filter: self.filter.clone(),
         });
 
@@ -82,8 +82,6 @@ impl PrintDiagnosticsPlugin {
                     Self::print_diagnostic(diagnostic);
                 }
             }
-
-            state.timer.reset();
         }
     }
 
@@ -105,8 +103,6 @@ impl PrintDiagnosticsPlugin {
                     println!("{:#?}\n", diagnostic);
                 }
             }
-
-            state.timer.reset();
         }
     }
 }

examples/2d/sprite_sheet.rs

@@ -16,7 +16,6 @@ fn animate_sprite_system(
         if timer.finished {
             let texture_atlas = texture_atlases.get(&texture_atlas_handle).unwrap();
             sprite.index = ((sprite.index as usize + 1) % texture_atlas.textures.len()) as u32;
-            timer.reset();
         }
     }
 }
@@ -43,5 +42,5 @@ fn setup(
             scale: Scale(6.0),
             ..Default::default()
         })
-        .with(Timer::from_seconds(0.1));
+        .with(Timer::from_seconds(0.1, true));
 }

examples/app/plugin.rs

@@ -27,7 +27,7 @@ impl Plugin for PrintMessagePlugin {
     fn build(&self, app: &mut AppBuilder) {
         let state = PrintMessageState {
             message: self.message.clone(),
-            timer: Timer::new(self.wait_duration),
+            timer: Timer::new(self.wait_duration, true),
         };
         app.add_resource(state)
             .add_system(print_message_system.system());
@@ -43,6 +43,5 @@ fn print_message_system(mut state: ResMut<PrintMessageState>, time: Res<Time>) {
     state.timer.tick(time.delta_seconds);
     if state.timer.finished {
         println!("{}", state.message);
-        state.timer.reset();
     }
 }

examples/ecs/event.rs

@@ -24,7 +24,7 @@ struct EventTriggerState {
 impl Default for EventTriggerState {
     fn default() -> Self {
         EventTriggerState {
-            event_timer: Timer::from_seconds(1.0),
+            event_timer: Timer::from_seconds(1.0, true),
         }
     }
 }
@@ -40,8 +40,6 @@ fn event_trigger_system(
         my_events.send(MyEvent {
             message: "MyEvent just happened!".to_string(),
         });
-
-        state.event_timer.reset();
     }
 }
 

examples/ui/font_atlas_debug.rs

@@ -22,7 +22,7 @@ impl Default for State {
         Self {
             added: false,
             handle: Handle::default(),
-            timer: Timer::from_seconds(0.05),
+            timer: Timer::from_seconds(0.05, true),
         }
     }
 }