Add mock for BackHandler

Reviewed By: fkgozali

Differential Revision: D4861901

fbshipit-source-id: b1b74e63731328d2235bf86794aa481a8275ed8c

Libraries/Utilities/__mocks__/BackHandler.js

@@ -0,0 +1,52 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+'use strict';
+
+const _backPressSubscriptions = new Set();
+
+const BackHandler = {
+  exitApp: jest.fn(),
+
+  addEventListener: function (
+    eventName: BackPressEventName,
+    handler: Function
+  ): {remove: () => void} {
+    _backPressSubscriptions.add(handler);
+    return {
+      remove: () => BackHandler.removeEventListener(eventName, handler),
+    };
+  },
+
+  removeEventListener: function(
+    eventName: BackPressEventName,
+    handler: Function
+  ): void {
+    _backPressSubscriptions.delete(handler);
+  },
+
+
+  mockPressBack: function() {
+    let invokeDefault = true;
+    const subscriptions = [..._backPressSubscriptions].reverse();
+    for (let i = 0; i < subscriptions.length; ++i) {
+      if (subscriptions[i]()) {
+        invokeDefault = false;
+        break;
+      }
+    }
+
+    if (invokeDefault) {
+      BackHandler.exitApp();
+    }
+  },
+};
+
+module.exports = BackHandler;