The [`Animated`](docs/animated.html) API is designed to make it very easy to concisely express a wide variety of interesting animation and interaction patterns in a very performant way.
`Animated` focuses on declarative relationships between inputs and outputs, with configurable transforms in between, and simple `start`/`stop` methods to control time-based animation execution.
`Animated` exports four animatable component types: `View`, `Text`, `Image`, and `ScrollView`, but you can also create your own using `Animated.createAnimatedComponent()`.
For example, a container view that fades in when it is mounted may look like this:
In the `FadeInView` constructor, a new `Animated.Value` called `fadeAnim` is initialized as part of `state`.
The opacity property on the `View` is mapped to this animated value.
Behind the scenes, the numeric value is extracted and used to set opacity.
When the component mounts, the opacity is set to 0.
Then, an easing animation is started on the `fadeAnim` animated value,
which will update all of its dependent mappings (in this case, just the opacity) on each frame as the value animates to the final value of 1.
This is done in an optimized way that is faster than calling `setState` and re-rendering.
Because the entire configuration is declarative, we will be able to implement further optimizations that serialize the configuration and runs the animation on a high-priority thread.
### Configuring animations
Animations are heavily configurable. Custom and predefined easing functions, delays, durations, decay factors, spring constants, and more can all be tweaked depending on the type of animation.
`Animated` provides several animation types, the most commonly used one being [`Animated.timing()`](docs/animated.html#timing).
It supports animating a value over time using one of various predefined easing functions, or you can use your own.
Easing functions are typically used in animation to convey gradual acceleration and deceleration of objects.
By default, `timing` will use a easeInOut curve that conveys gradual acceleration to full speed and concludes by gradually decelerating to a stop.
You can specify a different easing function by passing a `easing` parameter.
Custom `duration` or even a `delay` before the animation starts is also supported.
For example, if we want to create a 2-second long animation of an object that slightly backs up before moving to its final position:
Take a look at the [Configuring animations](docs/animated.html#configuring-animations) section of the `Animated` API reference to learn more about all the config parameters supported by the built-in animations.
### Composing animations
Animations can be combined and played in sequence or in parallel.
Sequential animations can play immediately after the previous animation has finished,
or they can start after a specified delay.
The `Animated` API provides several methods, such as `sequence()` and `delay()`,
each of which simply take an array of animations to execute and automatically calls `start()`/`stop()` as needed.
For example, the following animation coasts to a stop, then it springs back while twirling in parallel:
If one animation is stopped or interrupted, then all other animations in the group are also stopped.
`Animated.parallel` has a `stopTogether` option that can be set to `false` to disable this.
You can find a full list of composition methods in the [Composing animations](docs/animated.html#composing-animations) section of the `Animated` API reference.
You can [combine two animated values](docs/animated.html#combining-animated-values) via addition, multiplication, division, or modulo to make a new animated value.
There are some cases where an animated value needs to invert another animated value for calculation.
An example is inverting a scale (2x --> 0.5x):
```javascript
const a = Animated.Value(1);
const b = Animated.divide(1, a);
Animated.spring(a, {
toValue: 2,
}).start();
```
### Interpolation
Each property can be run through an interpolation first.
An interpolation maps input ranges to output ranges,
typically using a linear interpolation but also supports easing functions.
By default, it will extrapolate the curve beyond the ranges given, but you can also have it clamp the output value.
A simple mapping to convert a 0-1 range to a 0-100 range would be:
For example, you may want to think about your `Animated.Value` as going from 0 to 1,
but animate the position from 150px to 0px and the opacity from 0 to 1.
This can easily be done by modifying `style` from the example above like so:
```javascript
style={{
opacity: this.state.fadeAnim, // Binds directly
transform: [{
translateY: this.state.fadeAnim.interpolate({
inputRange: [0, 1],
outputRange: [150, 0] // 0 : 150, 0.5 : 75, 1 : 0
}),
}],
}}
```
[`interpolate()`](docs/animated.html#interpolate) supports multiple range segments as well, which is handy for defining dead zones and other handy tricks.
For example, to get an negation relationship at -300 that goes to 0 at -100, then back up to 1 at 0, and then back down to zero at 100 followed by a dead-zone that remains at 0 for everything beyond that, you could do:
`interpolate()` also supports mapping to strings, allowing you to animate colors as well as values with units. For example, if you wanted to animate a rotation you could do:
Just set the `toValue` of an animation to another animated value instead of a plain number.
For example, a "Chat Heads" animation like the one used by Messenger on Android could be implemented with a `spring()` pinned on another animated value, or with `timing()` and a `duration` of 0 for rigid tracking.
You may notice that there is no obvious way to read the current value while animating.
This is because the value may only be known in the native runtime due to optimizations.
If you need to run JavaScript in response to the current value, there are two approaches:
-`spring.stopAnimation(callback)` will stop the animation and invoke `callback` with the final value. This is useful when making gesture transitions.
-`spring.addListener(callback)` will invoke `callback` asynchronously while the animation is running, providing a recent value.
This is useful for triggering state changes,
for example snapping a bobble to a new option as the user drags it closer,
because these larger state changes are less sensitive to a few frames of lag compared to continuous gestures like panning which need to run at 60 fps.
`Animated` is designed to be fully serializable so that animations can be run in a high performance way, independent of the normal JavaScript event loop.
This does influence the API, so keep that in mind when it seems a little trickier to do something compared to a fully synchronous system.
Check out `Animated.Value.addListener` as a way to work around some of these limitations,
but use it sparingly since it might have performance implications in the future.
### Using the native driver
The `Animated` API is designed to be serializable.
By using the [native driver](http://facebook.github.io/react-native/blog/2017/02/14/using-native-driver-for-animated.html),
we send everything about the animation to native before starting the animation,
allowing native code to perform the animation on the UI thread without having to go through the bridge on every frame.
Once the animation has started, the JS thread can be blocked without affecting the animation.
Using the native driver for normal animations is quite simple.
Just add `useNativeDriver: true` to the animation config when starting it.
```javascript
Animated.timing(this.state.animatedValue, {
toValue: 1,
duration: 500,
useNativeDriver: true, // <--Addthis
}).start();
```
Animated values are only compatible with one driver so if you use native driver when starting an animation on a value,
make sure every animation on that value also uses the native driver.
The native driver also works with `Animated.event`.
This is specially useful for animations that follow the scroll position as without the native driver,
the animation will always run a frame behind the gesture due to the async nature of React Native.
You can also take a look at the [source code](https://github.com/facebook/react-native/blob/master/RNTester/js/NativeAnimationsExample.js) to learn how these examples were produced.
