Added missing files

Author: maneesht

packages/data-connect/CHANGELOG.md

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+# Unreleased
+* Added app check support 

packages/data-connect/src/core/AppCheckTokenProvider.ts

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+/**
+ * @license
+ * Copyright 2021 Google LLC
+ *
+ * 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.
+ */
+
+import {
+  AppCheckInternalComponentName,
+  AppCheckTokenListener,
+  AppCheckTokenResult,
+  FirebaseAppCheckInternal
+} from '@firebase/app-check-interop-types';
+import { Provider } from '@firebase/component';
+
+
+/**
+ * Abstraction around AppCheck's token fetching capabilities.
+ */
+export class AppCheckTokenProvider {
+  private appCheck?: FirebaseAppCheckInternal;
+  constructor(
+    private appName_: string,
+    private appCheckProvider?: Provider<AppCheckInternalComponentName>
+  ) {
+    this.appCheck = appCheckProvider?.getImmediate({ optional: true });
+    if (!this.appCheck) {
+      appCheckProvider?.get().then(appCheck => (this.appCheck = appCheck));
+    }
+  }
+
+  getToken(forceRefresh?: boolean): Promise<AppCheckTokenResult> {
+    if (!this.appCheck) {
+      return new Promise<AppCheckTokenResult>((resolve, reject) => {
+        // Support delayed initialization of FirebaseAppCheck. This allows our
+        // customers to initialize the RTDB SDK before initializing Firebase
+        // AppCheck and ensures that all requests are authenticated if a token
+        // becomes available before the timoeout below expires.
+        setTimeout(() => {
+          if (this.appCheck) {
+            this.getToken(forceRefresh).then(resolve, reject);
+          } else {
+            resolve(null);
+          }
+        }, 0);
+      });
+    }
+    return this.appCheck.getToken(forceRefresh);
+  }
+
+  addTokenChangeListener(listener: AppCheckTokenListener) {
+    this.appCheckProvider
+      ?.get()
+      .then(appCheck => appCheck.addTokenListener(listener));
+  }
+
+  // Not currently used at the moment. Will update if needed.
+  // notifyForInvalidToken(): void {
+  //   warn(
+  //     `Provided AppCheck credentials for the app named "${this.appName_}" ` +
+  //       'are invalid. This usually indicates your app was not initialized correctly.'
+  //   );
+  // }
+}