flutter · chunhtai · Sep 6, 2019 · Aug 22, 2019
diff --git a/packages/flutter/lib/src/widgets/binding.dart b/packages/flutter/lib/src/widgets/binding.dart
@@ -782,6 +782,17 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
   Element get renderViewElement => _renderViewElement;
   Element _renderViewElement;
 
+  /// Schedules a [Timer] for attaching the root widget.
+  ///
+  /// This is called by [runApp] to configure the widget tree. Consider using
+  /// [attachRootWidget] if you want to build the widget tree synchronously.
+  @protected
+  void scheduleAttachRootWidget(Widget rootWidget) {
+    Timer.run(() {
+      attachRootWidget(rootWidget);
+    });
+  }
+
   /// Takes a widget and attaches it to the [renderViewElement], creating it if
   /// necessary.
   ///
@@ -843,7 +854,7 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
 ///    ensure the widget, element, and render trees are all built.
 void runApp(Widget app) {
   WidgetsFlutterBinding.ensureInitialized()
-    ..attachRootWidget(app)
+    ..scheduleAttachRootWidget(app)
     ..scheduleWarmUpFrame();
 }
 

diff --git a/packages/flutter/lib/src/widgets/nested_scroll_view.dart b/packages/flutter/lib/src/widgets/nested_scroll_view.dart
@@ -498,13 +498,8 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   bool get hasScrolledBody {
     for (_NestedScrollPosition position in _innerPositions) {
-      // TODO(chunhtai): Replace null check with assert once
-      // https://github.com/flutter/flutter/issues/31195 is fixed.
-      if (
-        position.minScrollExtent != null &&
-        position.pixels != null &&
-        position.pixels > position.minScrollExtent
-      ) {
+      assert(position.minScrollExtent != null && position.pixels != null);
+      if (position.pixels > position.minScrollExtent) {
         return true;
       }
     }

diff --git a/packages/flutter/test/widgets/nested_scroll_view_async_test.dart b/packages/flutter/test/widgets/nested_scroll_view_async_test.dart
diff --git a/packages/flutter/test/widgets/run_app_async_test.dart b/packages/flutter/test/widgets/run_app_async_test.dart
@@ -0,0 +1,31 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:quiver/testing/async.dart';
+
+void main() {
+  setUp(() {
+    WidgetsFlutterBinding.ensureInitialized();
+    WidgetsBinding.instance.resetEpoch();
+  });
+
+  test('WidgetBinding build rendering tree and warm up frame back to back', () {
+    final FakeAsync fakeAsync = FakeAsync();
+    fakeAsync.run((FakeAsync async) {
+      runApp(
+        const MaterialApp(
+          home: Material(
+            child: Text('test'),
+          ),
+        ),
+      );
+      // Rendering tree is not built synchronously.
+      expect(WidgetsBinding.instance.renderViewElement, isNull);
+      fakeAsync.flushTimers();
+      expect(WidgetsBinding.instance.renderViewElement, isNotNull);
+    });
+  });
+}
diff --git a/packages/flutter_test/lib/src/binding.dart b/packages/flutter_test/lib/src/binding.dart
@@ -988,6 +988,14 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     _currentFakeAsync.flushMicrotasks();
   }
 
+  @override
+  void scheduleAttachRootWidget(Widget rootWidget) {
+    // We override the default version of this so that the application-startup widget tree
+    // build does not schedule timers which we might never get around to running.
+    attachRootWidget(rootWidget);
+    _currentFakeAsync.flushMicrotasks();
+  }
+
   @override
   Future<void> idle() {
     final Future<void> result = super.idle();