[RN] Promised-base RTCPeerConnection API

Recent changes in lib-jitsi-meet probably led to (1) our
RTCPeerConnection customizations on react-native not being used which is
a problem because we need them for at least NAT64 on iOS in order to
pass the review in Apple's App Store and (2) unexpected exceptions
inside react-native-webrtc.

The Promise-based WebRTC API should be merged from react-native-webrtc's
upstream but I don't want to do it right now because last time we got
multiple bugs in addition.

react/features/base/lib-jitsi-meet/native/RTCPeerConnection.js

+// @flow
+
 import { NativeModules } from 'react-native';
 import { RTCPeerConnection, RTCSessionDescription } from 'react-native-webrtc';
 
  *
  * @class
  */
-export default function _RTCPeerConnection(...args) {
+export default function _RTCPeerConnection(...args: any[]) {
 
     /* eslint-disable indent, no-invalid-this */
 
     // _RTCPeerConnection's prototype may (or may not, I don't know) work but I
     // don't want to try because the following approach appears to work and I
     // understand it.
+
+    // $FlowFixMe
     Object.defineProperty(this, 'onaddstream', {
         configurable: true,
         enumerable: true,
 _RTCPeerConnection.prototype = Object.create(RTCPeerConnection.prototype);
 _RTCPeerConnection.prototype.constructor = _RTCPeerConnection;
 
+_RTCPeerConnection.prototype.addIceCandidate
+    = _makePromiseAware(RTCPeerConnection.prototype.addIceCandidate, 1, 0);
+
+_RTCPeerConnection.prototype.createAnswer
+    = _makePromiseAware(RTCPeerConnection.prototype.createAnswer, 0, 1);
+_RTCPeerConnection.prototype.createOffer
+    = _makePromiseAware(RTCPeerConnection.prototype.createOffer, 0, 1);
+
 _RTCPeerConnection.prototype._invokeOnaddstream = function(...args) {
     const onaddstream = this._onaddstream;
 
     this._onaddstreamQueue.push(Array.from(args));
 };
 
+_RTCPeerConnection.prototype.setLocalDescription
+  = _makePromiseAware(RTCPeerConnection.prototype.setLocalDescription, 1, 0);
+
 _RTCPeerConnection.prototype.setRemoteDescription = function(
         sessionDescription,
         successCallback,
     console && console.error && console.error(...args);
 }
 
+/**
+ * Makes a {@code Promise}-returning function out of a specific void function
+ * with {@code successCallback} and {@code failureCallback}.
+ *
+ * @param {Function} f - The (void) function with {@code successCallback} and
+ * {@code failureCallback}.
+ * @param {number} beforeCallbacks - The number of arguments before
+ * {@code successCallback} and {@code failureCallback}.
+ * @param {number} afterCallbacks - The number of arguments after
+ * {@code successCallback} and {@code failureCallback}.
+ * @returns {Promise}
+ */
+function _makePromiseAware(
+        f: Function,
+        beforeCallbacks: number,
+        afterCallbacks: number) {
+    return function(...args) {
+        return new Promise((resolve, reject) => {
+
+            if (args.length <= beforeCallbacks + afterCallbacks) {
+                args.splice(beforeCallbacks, 0, resolve, reject);
+            }
+
+            let fPromise;
+
+            try {
+                // eslint-disable-next-line no-invalid-this
+                fPromise = f.apply(this, args);
+            } catch (e) {
+                reject(e);
+            }
+
+            // If the super implementation returns a Promise from the deprecated
+            // invocation by any chance, try to make sense of it.
+            if (fPromise) {
+                const { then } = fPromise;
+
+                typeof then === 'function'
+                    && then.call(fPromise, resolve, reject);
+            }
+        });
+    };
+}
+
 /**
  * Adapts react-native-webrtc's {@link RTCPeerConnection#setRemoteDescription}
  * implementation which uses the deprecated, callback-based version to the

react/features/base/lib-jitsi-meet/native/polyfills-webrtc.js

     if (typeof global.MediaStreamTrack === 'undefined') {
         global.MediaStreamTrack = MediaStreamTrack;
     }
+    if (typeof global.RTCIceCandidate === 'undefined') {
+        global.RTCIceCandidate = RTCIceCandidate;
+    }
+    if (typeof global.RTCPeerConnection === 'undefined') {
+        global.RTCPeerConnection = RTCPeerConnection;
+    }
     if (typeof global.webkitRTCPeerConnection === 'undefined') {
         global.webkitRTCPeerConnection = RTCPeerConnection;
     }
     if (typeof global.RTCSessionDescription === 'undefined') {
         global.RTCSessionDescription = RTCSessionDescription;
     }
-    if (typeof global.RTCIceCandidate === 'undefined') {
-        global.RTCIceCandidate = RTCIceCandidate;
-    }
 
     const navigator = global.navigator;