Summary:
This PR adds support for Animated tracking to Animated Native Driver implementation on Android and iOS.
Animated tracking allows for animation to be started with a "dynamic" end value. Instead of passing a fixed number as end value we can pass a reference to another Animated.Value. Then when that value changes, the animation will be reconfigured to drive the animation to the new destination point. What is important is that animation will keep its state in the process of updating "toValue". That is if it is a spring animation and the end value changes while the previous animation still hasn't settled the new animation will start from the current position and will inherit current velocity. This makes end value transitions very smooth.
Animated tracking is available in JS implementation of Animated library but not in the native implementation. Therefore until now, it wasn't possible to utilize native driver when using animated tracking. Offloading animation from JS thread turns out to be crucial for gesture driven animations. This PR is a step forward towards feature parity between JS and native implementations of Animated.
Here is a link to example video that shows how tracking can be used to implement chat heads effect: https://twitter.com/kzzzf/status/958362032650244101
In addition this PR fixes an issue with frames animation driver on Android that because of rounding issues was taking one extra frame to start. Because of that change I had to update a number of Android unit tests that were relying on that behavior and running that one additional animation step prior to performing checks.
As a part of this PR I'm adding three unit tests for each of the platforms that verifies most important aspects of this implementation. Please refer to the code and look at the test cases top level comments to learn what they do.
I'm also adding a section to "Native Animated Example" screen in RNTester app that provides a test case for tracking. In the example we have blue square that fallows the red line drawn on screen. Line uses Animated.Value for it's position while square is connected via tracking spring animation to that value. So it is ought to follow the line. When user taps in the area surrounding the button new position for the red line is selected at random and the value updates. Then we can watch blue screen animate to that position.
You can also refer to this video that I use to demonstrate how tracking can be linked with native gesture events using react-native-gesture-handler lib: https://twitter.com/kzzzf/status/958362032650244101
[GENERAL][FEATURE][Native Animated] - Added support for animated tracking to native driver. Now you can use `useNativeDriver` flag with animations that track other Animated.Values
Closes https://github.com/facebook/react-native/pull/17896
Differential Revision: D6974170
Pulled By: hramos
fbshipit-source-id: 50e918b36ee10f80c1deb866c955661d4cc2619b
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