[add] Linking: event listeners

Fix ##1650
Close #1863

packages/docs/src/apis/Linking/Linking.stories.mdx

@@ -9,6 +9,14 @@ Linking gives you a general interface for securely opening external URLs from Ja
 
 ## Methods
 
+### addEventListener(event, callback)
+
+Adds a event listener for the specified event. The callback will be called when the said event is dispatched.
+
+### removeEventListener(event, callback)
+
+Removes a previously added event listener for the specified event. The callback must be the same object as the one passed to `addEventListener`.
+
 ### canOpenURL(url)
 
 Returns a `Promise` that resolves to a boolean indicating whether the app can open the URL.
@@ -23,9 +31,23 @@ Try to open the given url in a secure fashion. The method returns a `Promise`
 object. If the url opens, the promise is resolved. If not, the promise is
 rejected.
 
+> On opening URL successfully, dispatches the `onOpen` event.
+
+## Events
+
+### `onOpen`
+
+Dispatched when a call to `Linking.openURL` succeeds. Type signature of the callback is:
+
+```js
+(url: string) => void
+```
+
 ## Example
 
-<Preview withSource='none'>
+> Listener added to the`onOpen` event. Check the console for the event log after a URL is opened.
+
+<Preview withSource="none">
   <Story name="openURL">
     <Stories.openURL />
   </Story>

packages/docs/src/apis/Linking/examples/OpenURL.js

@@ -1,15 +1,31 @@
+/**
+ * @flow
+ */
+
 import { Linking, StyleSheet, Text, View } from 'react-native';
 import React, { PureComponent } from 'react';
 
 const url = 'https://mathiasbynens.github.io/rel-noopener/malicious.html';
 
 class OpenURLExample extends PureComponent {
+  componentDidMount() {
+    Linking.addEventListener('onOpen', this.onOpenURL);
+  }
+
+  componentWillUnmount() {
+    Linking.removeEventListener('onOpen', this.onOpenURL);
+  }
+
   handlePress() {
     Linking.canOpenURL(url).then(supported => {
       return Linking.openURL(url);
     });
   }
 
+  onOpenURL(url: string) {
+    console.log(`%c    opened the url: ${url}    `, 'background: #D32F2F; color: #FFFFFF');
+  }
+
   render() {
     return (
       <View>

packages/react-native-web/src/exports/Linking/index.js

@@ -13,28 +13,77 @@ import invariant from 'fbjs/lib/invariant';
 
 const initialURL = canUseDOM ? window.location.href : '';
 
-const Linking = {
-  addEventListener() {},
-  removeEventListener() {},
+type Callback = (...args: any) => void;
+
+type OnOpenCallback = (event: 'onOpen', callback: (url: string) => void) => void;
+type GenericCallback = (event: string, callback: Callback) => void;
+
+class Linking {
+  /**
+   * An object mapping of event name
+   * and all the callbacks subscribing to it
+   */
+  _eventCallbacks: { [key: string]: Array<Callback> } = {};
+
+  _dispatchEvent(event: string, ...data: any) {
+    const listeners = this._eventCallbacks[event];
+    if (listeners != null && Array.isArray(listeners)) {
+      listeners.map(listener => {
+        listener(...data);
+      });
+    }
+  }
+
+  /**
+   * Adds a event listener for the specified event. The callback will be called when the
+   * said event is dispatched.
+   */
+  addEventListener: OnOpenCallback | GenericCallback = (event: string, callback: Callback) => {
+    if (!this._eventCallbacks[event]) {
+      this._eventCallbacks[event] = [callback];
+      return;
+    }
+    this._eventCallbacks[event].push(callback);
+  };
+
+  /**
+   * Removes a previously added event listener for the specified event. The callback must
+   * be the same object as the one passed to `addEventListener`.
+   */
+  removeEventListener: OnOpenCallback | GenericCallback = (event: string, callback: Callback) => {
+    const callbacks = this._eventCallbacks[event];
+    const filteredCallbacks = callbacks.filter(c => c.toString() !== callback.toString());
+    this._eventCallbacks[event] = filteredCallbacks;
+  };
+
   canOpenURL(): Promise<boolean> {
     return Promise.resolve(true);
-  },
+  }
+
   getInitialURL(): Promise<string> {
     return Promise.resolve(initialURL);
-  },
+  }
+
+  /**
+   * Try to open the given url in a secure fashion. The method returns a Promise object.
+   * If the url opens, the promise is resolved. If not, the promise is rejected.
+   * Dispatches the `onOpen` event if `url` is opened successfully
+   */
   openURL(url: string): Promise<Object | void> {
     try {
       open(url);
+      this._dispatchEvent('onOpen', url);
       return Promise.resolve();
     } catch (e) {
       return Promise.reject(e);
     }
-  },
+  }
+
   _validateURL(url: string) {
     invariant(typeof url === 'string', 'Invalid URL: should be a string. Was: ' + url);
     invariant(url, 'Invalid URL: cannot be empty');
   }
-};
+}
 
 const open = url => {
   if (canUseDOM) {
@@ -42,4 +91,4 @@ const open = url => {
   }
 };
 
-export default Linking;
+export default (new Linking(): Linking);