added tweening engine

src/external_state/timer.rs

@@ -19,11 +19,11 @@ impl Timer {
 
     /// Returns true if the time_start + timer_len > Instant::now
     /// or if the timer is stopped
-    pub fn is_complete(&mut self) -> bool {
+    pub fn is_complete(&self) -> bool {
         if let Some(start) = self.timer_start {
             start + self.timer_len > Instant::now()
         } else {
-            false
+            true
         }
     }
 
@@ -39,4 +39,64 @@ impl Timer {
             }
         }
     }
+    
+    // Get the percentage complete. Returns 100.0 if the timer is complete or 
+    // hasn't been started
+    pub fn percentage_complete(&self) -> f32 {
+        if self.is_complete() || self.timer_start.is_none() {
+            100.0
+        } else {
+            if let Some(start) = self.timer_start {
+                (start.elapsed().as_secs_f32() / self.timer_len.as_secs_f32()) * 100.0
+            } else {
+                100.0
+            }
+        }
+    }
 }
+
+pub struct TweeningEngine {
+    timer: Timer,
+    start_value: f32,
+    end_value: f32,
+}
+
+impl TweeningEngine {
+    pub fn new(start_value: f32, end_value: f32) -> Self {
+        TweeningEngine {
+            timer: Timer::default(),
+            start_value,
+            end_value,
+        }
+    }
+
+    pub fn starting(mut self, start_value: f32) -> Self {
+        self.start_value = start_value;
+        self
+    }
+    pub fn ending(mut self, end_value: f32) -> Self {
+        self.end_value = end_value;
+        self
+    }
+    pub fn duration(mut self, dur: Duration) -> Self {
+        self.timer.set_len(dur);
+        self
+    }
+
+    pub fn chain_start(mut self) -> Self {
+        self.timer.start();
+        self
+    }
+    pub fn start(&mut self) {
+        self.timer.start();
+    }
+
+    pub fn is_complete(&self) -> bool {
+        self.timer.is_complete()
+    }
+
+    pub fn get_value(&self) -> f32 {
+        self.start_value + (self.end_value - self.start_value) * self.timer.percentage_complete()
+    }
+}
+