Allow returning null from value animator callbacks (#960)

Fixes #879

7 files changed, 40 insertions, 18 deletions

diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ColorKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ColorKeyframeAnimation.java
index aeb37ff4..54e3d511 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ColorKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ColorKeyframeAnimation.java
@@ -1,7 +1,7 @@
 package com.airbnb.lottie.animation.keyframe;
 
-import com.airbnb.lottie.value.Keyframe;
 import com.airbnb.lottie.utils.GammaEvaluator;
+import com.airbnb.lottie.value.Keyframe;
 
 import java.util.List;
 
@@ -20,8 +20,11 @@ public class ColorKeyframeAnimation extends KeyframeAnimation<Integer> {
 
     if (valueCallback != null) {
       //noinspection ConstantConditions
-      return valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame, startColor,
-          endColor, keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      Integer value = valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame, startColor,
+              endColor, keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     return GammaEvaluator.evaluate(keyframeProgress, startColor, endColor);
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/FloatKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/FloatKeyframeAnimation.java
index e1dacfeb..81e58a25 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/FloatKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/FloatKeyframeAnimation.java
@@ -18,9 +18,12 @@ public class FloatKeyframeAnimation extends KeyframeAnimation<Float> {
 
     if (valueCallback != null) {
       //noinspection ConstantConditions
-      return valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
-          keyframe.startValue, keyframe.endValue,
-          keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      Float value = valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
+              keyframe.startValue, keyframe.endValue,
+              keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     return MiscUtils.lerp(keyframe.startValue, keyframe.endValue, keyframeProgress);
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/IntegerKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/IntegerKeyframeAnimation.java
index 3730adee..4db3fae2 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/IntegerKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/IntegerKeyframeAnimation.java
@@ -18,9 +18,12 @@ public class IntegerKeyframeAnimation extends KeyframeAnimation<Integer> {
 
     if (valueCallback != null) {
       //noinspection ConstantConditions
-      return valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
-          keyframe.startValue, keyframe.endValue,
-          keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      Integer value = valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
+              keyframe.startValue, keyframe.endValue,
+              keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     return MiscUtils.lerp(keyframe.startValue, keyframe.endValue, keyframeProgress);
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PathKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PathKeyframeAnimation.java
index 851ce4b7..2e0acdb7 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PathKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PathKeyframeAnimation.java
@@ -26,9 +26,12 @@ public class PathKeyframeAnimation extends KeyframeAnimation<PointF> {
     }
 
     if (valueCallback != null) {
-      return valueCallback.getValueInternal(pathKeyframe.startFrame, pathKeyframe.endFrame,
-          pathKeyframe.startValue, pathKeyframe.endValue, getLinearCurrentKeyframeProgress(),
-          keyframeProgress, getProgress());
+      PointF value = valueCallback.getValueInternal(pathKeyframe.startFrame, pathKeyframe.endFrame,
+              pathKeyframe.startValue, pathKeyframe.endValue, getLinearCurrentKeyframeProgress(),
+              keyframeProgress, getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     if (pathMeasureKeyframe != pathKeyframe) {
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PointKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PointKeyframeAnimation.java
index 81fa37f2..9bad6030 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PointKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/PointKeyframeAnimation.java
@@ -23,8 +23,11 @@ public class PointKeyframeAnimation extends KeyframeAnimation<PointF> {
 
     if (valueCallback != null) {
       //noinspection ConstantConditions
-      return valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame, startPoint,
-          endPoint, keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      PointF value = valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame, startPoint,
+              endPoint, keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     point.set(startPoint.x + keyframeProgress * (endPoint.x - startPoint.x),
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ScaleKeyframeAnimation.java b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ScaleKeyframeAnimation.java
index 2474809b..02a1bbd8 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ScaleKeyframeAnimation.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/keyframe/ScaleKeyframeAnimation.java
@@ -20,9 +20,12 @@ public class ScaleKeyframeAnimation extends KeyframeAnimation<ScaleXY> {
 
     if (valueCallback != null) {
       //noinspection ConstantConditions
-      return valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
-          startTransform, endTransform,
-          keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      ScaleXY value = valueCallback.getValueInternal(keyframe.startFrame, keyframe.endFrame,
+              startTransform, endTransform,
+              keyframeProgress, getLinearCurrentKeyframeProgress(), getProgress());
+      if (value != null) {
+        return value;
+      }
     }
 
     return new ScaleXY(
diff --git a/lottie/src/main/java/com/airbnb/lottie/value/LottieValueCallback.java b/lottie/src/main/java/com/airbnb/lottie/value/LottieValueCallback.java
index 54ca6048..a30d8f9d 100644
--- a/lottie/src/main/java/com/airbnb/lottie/value/LottieValueCallback.java
+++ b/lottie/src/main/java/com/airbnb/lottie/value/LottieValueCallback.java
@@ -12,7 +12,7 @@ import com.airbnb.lottie.animation.keyframe.BaseKeyframeAnimation;
  */
 public class LottieValueCallback<T> {
   private final LottieFrameInfo<T> frameInfo = new LottieFrameInfo<>();
-  @Nullable BaseKeyframeAnimation<?, ?> animation;
+  @Nullable private BaseKeyframeAnimation<?, ?> animation;
 
   /**
    * This can be set with {@link #setValue(Object)} to use a value instead of deferring
@@ -29,7 +29,10 @@ public class LottieValueCallback<T> {
 
   /**
    * Override this if you haven't set a static value in the constructor or with setValue.
+   *
+   * Return null to resort to the default value.
    */
+  @Nullable
   public T getValue(LottieFrameInfo<T> frameInfo) {
     return value;
   }
@@ -42,6 +45,7 @@ public class LottieValueCallback<T> {
   }
 
   @RestrictTo(RestrictTo.Scope.LIBRARY)
+  @Nullable
   public final T getValueInternal(
       float startFrame,
       float endFrame,