RCTPicker (facebook#22996)

Summary:
[iOS] [Changed] - As facebook#22990 said, move requireNativeComponent to a separate file.
I am not familiar with flow, I try to follow the https://pastebin.com/RFpdT76V example but I am not sure I have done it right.
Pull Request resolved: facebook#22996

Differential Revision: D13697082

Pulled By: cpojer

fbshipit-source-id: c0c87a8e1a7f0553da994aba230f69b496140200

Author: Chi-AnTai

Libraries/Components/Picker/PickerIOS.ios.js

@@ -18,7 +18,7 @@ const ReactNative = require('ReactNative');
 const StyleSheet = require('StyleSheet');
 const View = require('View');
 const processColor = require('processColor');
-const requireNativeComponent = require('requireNativeComponent');
+const RCTPickerNativeComponent = require('RCTPickerNativeComponent');
 
 import type {SyntheticEvent} from 'CoreEventTypes';
 import type {ColorValue} from 'StyleSheetTypes';
@@ -52,10 +52,6 @@ type RCTPickerIOSType = Class<
   >,
 >;
 
-const RCTPickerIOS: RCTPickerIOSType = (requireNativeComponent(
-  'RCTPicker',
-): any);
-
 type Label = Stringish | number;
 
 type Props = $ReadOnly<{|
@@ -111,7 +107,7 @@ class PickerIOS extends React.Component<Props, State> {
   render() {
     return (
       <View style={this.props.style}>
-        <RCTPickerIOS
+        <RCTPickerNativeComponent
           ref={picker => {
             this._picker = picker;
           }}

Libraries/Components/Picker/RCTPickerNativeComponent.js

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ * @format
+ */
+'use strict';
+
+const requireNativeComponent = require('requireNativeComponent');
+
+import type {SyntheticEvent} from 'CoreEventTypes';
+import type {TextStyleProp} from 'StyleSheet';
+import type {NativeComponent} from 'ReactNative';
+
+type PickerIOSChangeEvent = SyntheticEvent<
+  $ReadOnly<{|
+    newValue: number | string,
+    newIndex: number,
+  |}>,
+>;
+
+type RCTPickerIOSItemType = $ReadOnly<{|
+  label: ?Label,
+  value: ?(number | string),
+  textColor: ?number,
+|}>;
+
+type Label = Stringish | number;
+
+type RCTPickerIOSType = Class<
+  NativeComponent<
+    $ReadOnly<{|
+      items: $ReadOnlyArray<RCTPickerIOSItemType>,
+      onChange: (event: PickerIOSChangeEvent) => void,
+      onResponderTerminationRequest: () => boolean,
+      onStartShouldSetResponder: () => boolean,
+      selectedIndex: number,
+      style?: ?TextStyleProp,
+      testID?: ?string,
+    |}>,
+  >,
+>;
+
+module.exports = ((requireNativeComponent('RCTPicker'): any): RCTPickerIOSType);