Add details for forkEvent (facebook#403)

slorber · hramos · commit b2d9cce3b18a · 2018-06-15T11:33:50.000-07:00
it was unclear to me if `event` could actually be a considered as an opaque structure, ie if it would work as expected for any kind of thing the user can actually provide as a `onScroll` listener for example. It should be clear that it's safe to call with null or regular js listener cc @sahrens
diff --git a/docs/animated.md b/docs/animated.md
@@ -367,7 +367,7 @@ Config is an object that may have the following options:
 static forkEvent(event, listener)
 ```
 
-Advanced imperative API for snooping on animated events that are passed in through props. Use values directly where possible.
+Advanced imperative API for snooping on animated events that are passed in through props. It permits to add a new javascript listener to an existing `AnimatedEvent`. If `animatedEvent` is a simple javascript listener, it will merge the 2 listeners into a single one, and if `animatedEvent` is null/undefined, it will assign the javascript listener directly. Use values directly where possible.
 
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