Factor composition start and end frames (work area) into progress calculation

lottie/src/main/java/com/airbnb/lottie/AnimatableTransform.java

@@ -156,5 +156,4 @@ private static void throwMissingTransform(String missingProperty) {
       throw new IllegalArgumentException("Missing transform for " + missingProperty);
     }
   }
-
 }

lottie/src/main/java/com/airbnb/lottie/Keyframe.java

@@ -50,6 +50,9 @@ static void setEndFrames(List<? extends Keyframe<?>> keyframes) {
   @SuppressWarnings("WeakerAccess") final float startFrame;
   @SuppressWarnings("WeakerAccess") @Nullable Float endFrame;
 
+  private float startProgress = Float.MIN_VALUE;
+  private float endProgress = Float.MIN_VALUE;
+
   public Keyframe(LottieComposition composition, @Nullable T startValue, @Nullable T endValue,
       @Nullable Interpolator interpolator, float startFrame, @Nullable Float endFrame) {
     this.composition = composition;
@@ -60,15 +63,25 @@ public Keyframe(LottieComposition composition, @Nullable T startValue, @Nullable
     this.endFrame = endFrame;
   }
 
-  @FloatRange(from = 0f, to = 1f)
   float getStartProgress() {
-    return startFrame / composition.getDurationFrames();
+    if (startProgress == Float.MIN_VALUE) {
+      startProgress = (startFrame  - composition.getStartFrame()) / composition.getDurationFrames();
+    }
+    return startProgress;
   }
 
-  @FloatRange(from = 0f, to = 1f)
   float getEndProgress() {
-    //noinspection Range
-    return endFrame == null ? 1f : endFrame / composition.getDurationFrames();
+    if (endProgress == Float.MIN_VALUE) {
+      if (endFrame == null) {
+        endProgress = 1f;
+      } else {
+        float startProgress = getStartProgress();
+        float durationFrames = endFrame - startFrame;
+        float durationProgress = durationFrames / composition.getDurationFrames();
+        endProgress = startProgress + durationProgress;
+      }
+    }
+    return endProgress;
   }
 
   boolean isStatic() {

lottie/src/main/java/com/airbnb/lottie/LottieComposition.java

@@ -94,7 +94,7 @@ Layer layerModelForId(long id) {
 
   @SuppressWarnings("WeakerAccess") public long getDuration() {
     long frameDuration = endFrame - startFrame;
-    return (long) (frameDuration / (float) frameRate * 1000);
+    return (long) (frameDuration / frameRate * 1000);
   }
 
   int getMajorVersion() {
@@ -109,6 +109,10 @@ int getPatchVersion() {
     return patchVersion;
   }
 
+  long getStartFrame() {
+    return startFrame;
+  }
+
   long getEndFrame() {
     return endFrame;
   }
@@ -139,7 +143,7 @@ Map<String, LottieImageAsset> getImages() {
   }
 
   float getDurationFrames() {
-    return getDuration() * (float) frameRate / 1000f;
+    return getDuration() * frameRate / 1000f;
   }