Flow strict in ViewPagerAndroid.android.js (facebook#22134)

Summary:
Related to facebook#22100

Turn Flow strict mode on for Libraries/Components/ViewPager/ViewPagerAndroid.android.js

- [x] npm run prettier
- [x] npm run flow-check-ios
- [x] npm run flow-check-android

[GENERAL] [ENHANCEMENT] [Libraries/Components/ViewPager/ViewPagerAndroid.android.js] - Flow strict mode
Pull Request resolved: facebook#22134

Differential Revision: D12930719

Pulled By: TheSavior

fbshipit-source-id: 9519f31b7af27f0497e42fd51f18c19be3692823

Libraries/Components/ViewPager/ViewPagerAndroid.android.js

@@ -5,7 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
- * @flow
+ * @flow strict-local
  */
 
 'use strict';
@@ -19,11 +19,31 @@ const requireNativeComponent = require('requireNativeComponent');
 
 const NativeAndroidViewPager = requireNativeComponent('AndroidViewPager');
 
+import type {SyntheticEvent} from 'CoreEventTypes';
 import type {ViewStyleProp} from 'StyleSheet';
 
 const VIEWPAGER_REF = 'viewPager';
 
-type Event = Object;
+type PageScrollState = 'idle' | 'dragging' | 'settling';
+
+type PageScrollEvent = SyntheticEvent<
+  $ReadOnly<{|
+    position: number,
+    offset: number,
+  |}>,
+>;
+
+type PageScrollStateChangedEvent = SyntheticEvent<
+  $ReadOnly<{|
+    pageScrollState: PageScrollState,
+  |}>,
+>;
+
+type PageSelectedEvent = SyntheticEvent<
+  $ReadOnly<{|
+    position: number,
+  |}>,
+>;
 
 export type ViewPagerScrollState = $Enum<{
   idle: string,
@@ -47,7 +67,7 @@ type Props = $ReadOnly<{|
    *    Value x means that (1 - x) fraction of the page at "position" index is
    *    visible, and x fraction of the next page is visible.
    */
-  onPageScroll?: ?Function,
+  onPageScroll?: ?(e: PageScrollEvent) => void,
 
   /**
    * Function called when the page scrolling state has changed.
@@ -57,15 +77,15 @@ type Props = $ReadOnly<{|
    * - settling, meaning that there was an interaction with the page scroller, and the
    *   page scroller is now finishing it's closing or opening animation
    */
-  onPageScrollStateChanged?: ?Function,
+  onPageScrollStateChanged?: ?(e: PageScrollState) => void,
 
   /**
    * This callback will be called once ViewPager finish navigating to selected page
    * (when user swipes between pages). The `event.nativeEvent` object passed to this
    * callback will have following fields:
    *  - position - index of page that has been selected
    */
-  onPageSelected?: ?Function,
+  onPageSelected?: ?(e: PageSelectedEvent) => void,
 
   /**
    * Blank space to show between pages. This is only visible while scrolling, pages are still
@@ -194,7 +214,7 @@ class ViewPagerAndroid extends React.Component<Props> {
     });
   };
 
-  _onPageScroll = (e: Event) => {
+  _onPageScroll = (e: PageScrollEvent) => {
     if (this.props.onPageScroll) {
       this.props.onPageScroll(e);
     }
@@ -203,13 +223,13 @@ class ViewPagerAndroid extends React.Component<Props> {
     }
   };
 
-  _onPageScrollStateChanged = (e: Event) => {
+  _onPageScrollStateChanged = (e: PageScrollStateChangedEvent) => {
     if (this.props.onPageScrollStateChanged) {
       this.props.onPageScrollStateChanged(e.nativeEvent.pageScrollState);
     }
   };
 
-  _onPageSelected = (e: Event) => {
+  _onPageSelected = (e: PageSelectedEvent) => {
     if (this.props.onPageSelected) {
       this.props.onPageSelected(e);
     }