feat(common): introduce StateTransferInitializerModule (#916)

Author: Fabian Wiles

modules/common/public-api.ts

@@ -6,3 +6,4 @@
  * found in the LICENSE file at https://angular.io/license
  */
 export { TransferHttpCacheModule } from './src/transfer_http';
+export { StateTransferInitializerModule } from './src/state-transfer-initilizer/module';

modules/common/src/state-transfer-initilizer/README.md

@@ -0,0 +1,13 @@
+# State Transfer Initializer
+Delays the app bootstrap process to ensure that the DOM content is loaded before state transfer initilization
+
+Simply import the module into your project and you will no longer need to wrap your component bootstrap function in an `DOMContentLoaded` callback
+
+```
+import { StateTransferInitializerModule } from '@nguniversal/common'
+
+@NgModule({
+  imports: [ StateTransferInitializerModule ]
+})
+export class AppModule{}
+```

modules/common/src/state-transfer-initilizer/module.ts

@@ -0,0 +1,28 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import { NgModule, APP_INITIALIZER } from '@angular/core';
+import { DOCUMENT } from '@angular/common';
+
+export function domContentLoadedFactory(doc: Document) {
+  return () => new Promise ((resolve, _reject) => {
+    const contentLoaded = () => {
+      doc.removeEventListener('DOMContentLoaded', contentLoaded);
+      resolve();
+    };
+    doc.addEventListener('DOMContentLoaded', contentLoaded);
+  });
+}
+
+
+@NgModule({
+  providers: [
+    {provide: APP_INITIALIZER, multi: true, useFactory: domContentLoadedFactory, deps: [DOCUMENT]},
+  ]
+})
+export class StateTransferInitializerModule {}