addListener

Animation is asynchronous, but in some cases you may want to keep track of what the animated values are. For example you may want to animate a progress value from 0 to 1 while a user is holding down a button, and if it reaches 1, then on release you might fire an action.

With addListener that is possible.

Some basic example code:

this._animatedValue = new Animated.Value(0);

this._animatedValue.addListener(({value}) => this._value = value);

Animated.timing(this._animatedValue, {
    toValue: 100,
    duration: 500
}).start();

Synchronous Value?

Many people may be wondering if they can get the value synchronously. The answer is yes, no, and well you shouldn't.
Previously you could call getAnimatedValue() or access the internal value of the Animated.Value. However, this is bad practice. The current implementation of Animated is implemented in the JavaScript world, but there are plans to move it from the JavaScript world to the native world for performance reasons.

Thus, accessing the animated value synchronously is frowned upon. It is a little more verbose but attaching a listener is the recommended way. Unfortunately, at the moment, you cannot attach a listener to interpolated values.

IF YOU ATTACH A LISTENER DO NOT FORGET TO REMOVE IT

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