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React Native Animated for Beginners



This is just a helper function to map arbitrary data to Animated values. Check out the docs for their explanation http://facebook.github.io/react-native/docs/animated.html#event.

There are 2 main cases when you want to use Animated.event. When dealing withPanResponder events, and or dealing with onScroll events. You could write up your own function and map things yourself but use what's provided for you.

Example of PanResponder

 onPanResponderMove: Animated.event([
   null,                // raw event arg ignored
   {dx: this._pan.x, dy: this._pan.y},    // gestureState arg

What is happening here is that the onPanResponderMove calls it's function with 2 params. We are ignoring the raw event, which is the first argument, thus we pass null. The second argument is the gestureState which provides some helpful values, in our case dx and dy. Which are deltas (aka the changes) in how far the user has moved their finger from the beginning of the touch.

We pass in our 2 animated values from an Animated.ValueXY which our Animated.event will take that configuration, traverse it, find the Animated values and call setValue for us. Effectively when the user moves their finger our Animated values will be the delta positions.

This could then be passed into a translateX/translateY

The other case is Scroll. Maybe you want to trigger certain background colors? effects? etc on scroll. Well hooking into the onScroll of a ScrollView or ListView with an Animated.event can help with that.

In our case here we're listening to how far the x is being scrolled.

   [{nativeEvent: {contentOffset: {x: this._scrollX}}}]
   {listener},          // Optional async listener

Additionally you can define a callback listener as the second argument to Animated.event.