Flow strict Slider (#22127)

Summary:
Related to #22100

Turn on Flow strict mode for Slider.
Enhanced event type and props callbacks type defs for Slider.

- All flow tests succeed.

[GENERAL] [ENHANCEMENT] [Slider.js] - Flow strict mode
Pull Request resolved: https://github.com/facebook/react-native/pull/22127

Differential Revision: D12946817

Pulled By: TheSavior

fbshipit-source-id: 631391f70c04fddf0bfa6fec92f5cb769a555547

Libraries/Components/Slider/Slider.js

@@ -5,7 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
- * @flow
+ * @flow strict-local
  */
 
 'use strict';
@@ -21,10 +21,19 @@ import type {ImageSource} from 'ImageSource';
 import type {ViewStyleProp} from 'StyleSheet';
 import type {ColorValue} from 'StyleSheetTypes';
 import type {ViewProps} from 'ViewPropTypes';
+import type {SyntheticEvent} from 'CoreEventTypes';
 
 const RCTSlider = requireNativeComponent('RCTSlider');
 
-type Event = Object;
+type Event = SyntheticEvent<
+  $ReadOnly<{|
+    value: number,
+    /**
+     * Android Only.
+     */
+    fromUser?: boolean,
+  |}>,
+>;
 
 type IOSProps = $ReadOnly<{|
   /**
@@ -118,14 +127,14 @@ type Props = $ReadOnly<{|
   /**
    * Callback continuously called while the user is dragging the slider.
    */
-  onValueChange?: ?Function,
+  onValueChange?: ?(value: number) => void,
 
   /**
    * Callback that is called when the user releases the slider,
    * regardless if the value has changed. The current value is passed
    * as an argument to the callback handler.
    */
-  onSlidingComplete?: ?Function,
+  onSlidingComplete?: ?(value: number) => void,
 
   /**
    * Used to locate this view in UI automation tests.
@@ -209,7 +218,8 @@ const Slider = (
       if (Platform.OS === 'android') {
         // On Android there's a special flag telling us the user is
         // dragging the slider.
-        userEvent = event.nativeEvent.fromUser;
+        userEvent =
+          event.nativeEvent.fromUser != null && event.nativeEvent.fromUser;
       }
       props.onValueChange &&
         userEvent &&