Add Animated.useCode hook as an alternative to Animated.Code (#237)

* add useCode hook as an alternative to Animated.Code

* Add useCode documentation to README

Added a short description of Animated.useCode to the README, with both a function signature and an example of its use in a component.

README.md

@@ -220,6 +220,21 @@ block([
 }/>
 ```
 
+## `Animated.useCode`
+
+The `useCode` hook acts as an alternative to the `Animated.Code` component.
+```js
+Animated.useCode(node, deps)
+```
+It's passed an animated node and an array of dependencies, and updates that node both when the component mounts and every time a value in that array changes. It does nothing on versions of React Native that don't support hooks (<0.59).
+```js
+const [offset, setOffset] = React.useState(20);
+Animated.useCode(
+  set(transX1, add(_transX, offset)),
+  [offset]
+);
+```
+
 ## Event handling with reanimated nodes
 
 `react-native-reanimated`'s new syntax is possible to be used with `Animated.event`. Instead of providing only a mapping from event fields to animated nodes, it is allowed to write a function that takes reanimated values map as an input and return a block (or any other reanimated function) that will be then used to handle the event.

react-native-reanimated.d.ts

@@ -278,6 +278,12 @@ declare module 'react-native-reanimated' {
       config: DecayConfig,
     ): BackwardCompatibleWrapper;
 
+    // hooks
+    export function useCode(
+      exec: AnimatedNode<number>,
+      deps: Array<any>,
+    ): void
+
     // configuration
 
     // `addWhitelistedNativeProps` will likely be removed soon, and so is

src/derived/index.js

@@ -9,3 +9,4 @@ export { default as min } from './min';
 export { default as onChange } from './onChange';
 export { default as floor } from './floor';
 export { default as ceil } from './ceil';
+export { default as useCode } from './useCode';

src/derived/useCode.js

@@ -0,0 +1,15 @@
+import React from 'react';
+import { always } from '../base';
+
+function useCode(exec, deps) {
+  if (typeof React.useEffect === 'function') {
+    React.useEffect(() => {
+      const animatedAlways = always(exec);
+      animatedAlways.__attach();
+      return () => {
+        animatedAlways.__detach();
+      };
+    }, deps);
+  }
+}
+export default useCode;