Fixed the issue that _onTransitionEnd might try to setState at unmoun…

Summary:
This is to fix the issue that if `Animated.parallel`'s callback - `_onTransitionEnd` being triggered twice in a really short period(say quickly double-click the Android's hardware back button),  it might try to `setState` at unmounted stage, hence cause app crash.

This will make sure `_onTransitionEnd` only fired after mounted.
Closes https://github.com/facebook/react-native/pull/10878

Differential Revision: D4167266

Pulled By: ericvicenti

fbshipit-source-id: 7361e0ea4e8481b2da3fa39f78cdc0461693631f

Libraries/NavigationExperimental/NavigationTransitioner.js

@@ -60,6 +60,7 @@ class NavigationTransitioner extends React.Component<any, Props, State> {
   _onTransitionEnd: () => void;
   _prevTransitionProps: ?NavigationTransitionProps;
   _transitionProps: NavigationTransitionProps;
+  _isMounted: boolean;
 
   props: Props;
   state: State;
@@ -94,6 +95,7 @@ class NavigationTransitioner extends React.Component<any, Props, State> {
 
     this._prevTransitionProps = null;
     this._transitionProps = buildTransitionProps(props, this.state);
+    this._isMounted = false;
   }
 
   componentWillMount(): void {
@@ -101,6 +103,14 @@ class NavigationTransitioner extends React.Component<any, Props, State> {
     this._onTransitionEnd = this._onTransitionEnd.bind(this);
   }
 
+  componentDidMount(): void {
+    this._isMounted = true;
+  }
+
+  componentWillUnmount(): void {
+    this._isMounted = false;
+  }
+
   componentWillReceiveProps(nextProps: Props): void {
     const nextScenes = NavigationScenesReducer(
       this.state.scenes,
@@ -211,6 +221,10 @@ class NavigationTransitioner extends React.Component<any, Props, State> {
   }
 
   _onTransitionEnd(): void {
+    if (!this._isMounted) {
+      return;
+    }
+
     const prevTransitionProps = this._prevTransitionProps;
     this._prevTransitionProps = null;