native decay animation

Summary: Add support for `useNativeDriver: true` to `Animated.decay`. Add example in Native Animated Example UIExplorer app.

Reviewed By: ritzau

Differential Revision: D3690127

fbshipit-source-id: eaa5e61293ed174191cec72255ea2677dbaa1757

ReactAndroid/src/main/java/com/facebook/react/animated/DecayAnimation.java

@@ -0,0 +1,54 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.animated;
+
+import com.facebook.react.bridge.ReadableMap;
+
+/**
+ * Implementation of {@link AnimationDriver} providing support for decay animations. The
+ * implementation is copied from the JS version in {@code AnimatedImplementation.js}.
+ */
+public class DecayAnimation extends AnimationDriver {
+
+  private final double mVelocity;
+  private final double mDeceleration;
+
+  private long mStartFrameTimeMillis = -1;
+  private double mFromValue;
+  private double mLastValue;
+
+  public DecayAnimation(ReadableMap config) {
+    mVelocity = config.getDouble("velocity");
+    mDeceleration = config.getDouble("deceleration");
+  }
+
+  @Override
+  public void runAnimationStep(long frameTimeNanos) {
+    long frameTimeMillis = frameTimeNanos / 1000000;
+    if (mStartFrameTimeMillis == -1) {
+      // since this is the first animation step, consider the start to be on the previous frame
+      mStartFrameTimeMillis = frameTimeMillis - 16;
+      mFromValue = mAnimatedValue.mValue;
+      mLastValue = mAnimatedValue.mValue;
+    }
+
+    final double value = mFromValue +
+      (mVelocity / (1 - mDeceleration)) *
+        (1 - Math.exp(-(1 - mDeceleration) * (frameTimeMillis - mStartFrameTimeMillis)));
+
+    if (Math.abs(mLastValue - value) < 0.1) {
+      mHasFinished = true;
+      return;
+    }
+
+    mLastValue = value;
+    mAnimatedValue.mValue = value;
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedNodesManager.java

@@ -139,6 +139,8 @@ import javax.annotation.Nullable;
       animation = new FrameBasedAnimationDriver(animationConfig);
     } else if ("spring".equals(type)) {
       animation = new SpringAnimation(animationConfig);
+    } else if ("decay".equals(type)) {
+      animation = new DecayAnimation(animationConfig);
     } else {
       throw new JSApplicationIllegalArgumentException("Unsupported animation type: " + type);
     }