wrap channel passed to verifier to nofity request

so request is notified when verifier sends an event

src/crypto/verification/request/RequestCallbackChannel.js

@@ -0,0 +1,59 @@
+/*
+Copyright 2019 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/** a key verification channel that wraps over an actual channel to pass it to a verifier,
+ * to notify the VerificationRequest when the verifier tries to send anything over the channel.
+ * This way, the VerificationRequest can update its state based on events sent by the verifier.
+ * Anything that is not sending is just routing through to the wrapped channel.
+ */
+export default class RequestCallbackChannel {
+    constructor(request, channel) {
+        this._request = request;
+        this._channel = channel;
+    }
+
+    get transactionId() {
+        return this._channel.transactionId;
+    }
+
+    get needsDoneMessage() {
+        return this._channel.needsDoneMessage;
+    }
+
+    handleEvent(event, request) {
+        return this._channel.handleEvent(event, request);
+    }
+
+    completedContentFromEvent(event) {
+        return this._channel.completedContentFromEvent(event);
+    }
+
+    completeContent(type, content) {
+        return this._channel.completeContent(type, content);
+    }
+
+    async send(type, uncompletedContent) {
+        this._request.handleVerifierSend(type, uncompletedContent);
+        const result = await this._channel.send(type, uncompletedContent);
+        return result;
+    }
+
+    async sendCompleted(type, content) {
+        this._request.handleVerifierSend(type, content);
+        const result = await this._channel.sendCompleted(type, content);
+        return result;
+    }
+}