Flutter 测试颤振应用程序时，黑色背景前出现奇怪的蓝色加载圈

我目前正在通过Udacity教程学习颤振开发。在向我的应用程序添加新资产后进行测试时，出现了一个奇怪的黑色屏幕，屏幕上有一个永不结束的蓝色加载圈

这里是一个截图：

这个屏幕什么时候出现？或者为什么？有没有办法让这一切消失

我已尝试重置android emulator并删除对代码的更新

我在GitHub上有代码：

谢谢你的帮助

更新：

我能够解决这个问题，但我不知道为什么会出现这个问题

这是导致问题的文件：

    // Copyright 2018 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 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';

import 'backdrop.dart';
import 'category.dart';
import 'category_tile.dart';
import 'unit.dart';
import 'unit_converter.dart';

/// Loads in unit conversion data, and displays the data.
///
/// This is the main screen to our app. It retrieves conversion data from a
/// JSON asset and from an API. It displays the [Categories] in the back panel
/// of a [Backdrop] widget and shows the [UnitConverter] in the front panel.
///
/// While it is named CategoryRoute, a more apt name would be CategoryScreen,
/// because it is responsible for the UI at the route's destination.
class CategoryRoute extends StatefulWidget {
  const CategoryRoute();

  @override
  _CategoryRouteState createState() => _CategoryRouteState();
}

class _CategoryRouteState extends State<CategoryRoute> {
  Category _defaultCategory;
  Category _currentCategory;
  // Widgets are supposed to be deeply immutable objects. We can update and edit
  // _categories as we build our app, and when we pass it into a widget's
  // `children` property, we call .toList() on it.
  // For more details, see https://github.com/dart-lang/sdk/issues/27755
  final _categories = <Category>[];
  // TODO: Remove _categoryNames as they will be retrieved from the JSON asset
  static const _categoryNames = <String>[
    'Length',
    'Area',
    'Volume',
    'Mass',
    'Time',
    'Digital Storage',
    'Energy',
    'Currency',
  ];
  static const _baseColors = <ColorSwatch>[
    ColorSwatch(0xFF6AB7A8, {
      'highlight': Color(0xFF6AB7A8),
      'splash': Color(0xFF0ABC9B),
    }),
    ColorSwatch(0xFFFFD28E, {
      'highlight': Color(0xFFFFD28E),
      'splash': Color(0xFFFFA41C),
    }),
    ColorSwatch(0xFFFFB7DE, {
      'highlight': Color(0xFFFFB7DE),
      'splash': Color(0xFFF94CBF),
    }),
    ColorSwatch(0xFF8899A8, {
      'highlight': Color(0xFF8899A8),
      'splash': Color(0xFFA9CAE8),
    }),
    ColorSwatch(0xFFEAD37E, {
      'highlight': Color(0xFFEAD37E),
      'splash': Color(0xFFFFE070),
    }),
    ColorSwatch(0xFF81A56F, {
      'highlight': Color(0xFF81A56F),
      'splash': Color(0xFF7CC159),
    }),
    ColorSwatch(0xFFD7C0E2, {
      'highlight': Color(0xFFD7C0E2),
      'splash': Color(0xFFCA90E5),
    }),
    ColorSwatch(0xFFCE9A9A, {
      'highlight': Color(0xFFCE9A9A),
      'splash': Color(0xFFF94D56),
      'error': Color(0xFF912D2D),
    }),
  ];

  //  Remove the overriding of initState(). Instead, we use
  // didChangeDependencies()
//  @override
//  void initState() {
//    super.initState();
//    for (var i = 0; i < _categoryNames.length; i++) {
//      var category = Category(
//        name: _categoryNames[i],
//        color: _baseColors[i],
//        iconLocation: Icons.cake,
//        units: _retrieveUnitList(_categoryNames[i]),
//      );
//      if (i == 0) {
//        _defaultCategory = category;
//      }
//      _categories.add(category);
//    }
//  }

  // Uncomment this out. We use didChangeDependencies() so that we can
  // wait for our JSON asset to be loaded in (async).
    @override
    Future<void> didChangeDependencies() async {
      super.didChangeDependencies();
      // We have static unit conversions located in our
      // assets/data/regular_units.json
      if (_categories.isEmpty) {
        await _retrieveLocalCategories();
      }
    }

  /// Retrieves a list of [Categories] and their [Unit]s
  Future<void> _retrieveLocalCategories() async {
    // Consider omitting the types for local variables. For more details on Effective
    // Dart Usage, see https://www.dartlang.org/guides/language/effective-dart/usage
    final json = DefaultAssetBundle
        .of(context)
        .loadString('assets/data/regular_units.json');
    final data = JsonDecoder().convert(await json);
    if (data is! Map) {
      throw ('Data retrieved from API is not a Map');
    }
    // TODO: Create Categories and their list of Units, from the JSON asset
  }

  /// Function to call when a [Category] is tapped.
  void _onCategoryTap(Category category) {
    setState(() {
      _currentCategory = category;
    });
  }

  /// Makes the correct number of rows for the list view, based on whether the
  /// device is portrait or landscape.
  ///
  /// For portrait, we use a [ListView]. For landscape, we use a [GridView].
  Widget _buildCategoryWidgets(Orientation deviceOrientation) {
    if (deviceOrientation == Orientation.portrait) {
      return ListView.builder(
        itemBuilder: (BuildContext context, int index) {
          return CategoryTile(
            category: _categories[index],
            onTap: _onCategoryTap,
          );
        },
        itemCount: _categories.length,
      );
    } else {
      return GridView.count(
        crossAxisCount: 2,
        childAspectRatio: 3.0,
        children: _categories.map((Category c) {
          return CategoryTile(
            category: c,
            onTap: _onCategoryTap,
          );
        }).toList(),
      );
    }
  }

  // TODO: Delete this function; instead, read in the units from the JSON asset
  // inside _retrieveLocalCategories()
  /// Returns a list of mock [Unit]s.
  List<Unit> _retrieveUnitList(String categoryName) {
    // when the app first starts up
    return List.generate(10, (int i) {
      i += 1;
      return Unit(
        name: '$categoryName Unit $i',
        conversion: i.toDouble(),
      );
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_categories.isEmpty) {
      return Center(
        child: Container(
          height: 180.0,
          width: 180.0,
          child: CircularProgressIndicator(),
        ),
      );
    }

    // Based on the device size, figure out how to best lay out the list
    // You can also use MediaQuery.of(context).size to calculate the orientation
    assert(debugCheckHasMediaQuery(context));
    final listView = Padding(
      padding: EdgeInsets.only(
        left: 8.0,
        right: 8.0,
        bottom: 48.0,
      ),
      child: _buildCategoryWidgets(MediaQuery.of(context).orientation),
    );
    return Backdrop(
      currentCategory:
      _currentCategory == null ? _defaultCategory : _currentCategory,
      frontPanel: _currentCategory == null
          ? UnitConverter(category: _defaultCategory)
          : UnitConverter(category: _currentCategory),
      backPanel: listView,
      frontTitle: Text('Unit Converter'),
      backTitle: Text('Select a Category'),
    );
  }
}

// Copyright 2018 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 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';

import 'backdrop.dart';
import 'category.dart';
import 'category_tile.dart';
import 'unit.dart';
import 'unit_converter.dart';

/// Loads in unit conversion data, and displays the data.
///
/// This is the main screen to our app. It retrieves conversion data from a
/// JSON asset and from an API. It displays the [Categories] in the back panel
/// of a [Backdrop] widget and shows the [UnitConverter] in the front panel.
///
/// While it is named CategoryRoute, a more apt name would be CategoryScreen,
/// because it is responsible for the UI at the route's destination.
class CategoryRoute extends StatefulWidget {
  const CategoryRoute();

  @override
  _CategoryRouteState createState() => _CategoryRouteState();
}

class _CategoryRouteState extends State<CategoryRoute> {
  Category _defaultCategory;
  Category _currentCategory;
  // Widgets are supposed to be deeply immutable objects. We can update and edit
  // _categories as we build our app, and when we pass it into a widget's
  // `children` property, we call .toList() on it.
  // For more details, see https://github.com/dart-lang/sdk/issues/27755
  final _categories = <Category>[];
  // TODO: Remove _categoryNames as they will be retrieved from the JSON asset
  static const _categoryNames = <String>[
    'Length',
    'Area',
    'Volume',
    'Mass',
    'Time',
    'Digital Storage',
    'Energy',
    'Currency',
  ];
  static const _baseColors = <ColorSwatch>[
    ColorSwatch(0xFF6AB7A8, {
      'highlight': Color(0xFF6AB7A8),
      'splash': Color(0xFF0ABC9B),
    }),
    ColorSwatch(0xFFFFD28E, {
      'highlight': Color(0xFFFFD28E),
      'splash': Color(0xFFFFA41C),
    }),
    ColorSwatch(0xFFFFB7DE, {
      'highlight': Color(0xFFFFB7DE),
      'splash': Color(0xFFF94CBF),
    }),
    ColorSwatch(0xFF8899A8, {
      'highlight': Color(0xFF8899A8),
      'splash': Color(0xFFA9CAE8),
    }),
    ColorSwatch(0xFFEAD37E, {
      'highlight': Color(0xFFEAD37E),
      'splash': Color(0xFFFFE070),
    }),
    ColorSwatch(0xFF81A56F, {
      'highlight': Color(0xFF81A56F),
      'splash': Color(0xFF7CC159),
    }),
    ColorSwatch(0xFFD7C0E2, {
      'highlight': Color(0xFFD7C0E2),
      'splash': Color(0xFFCA90E5),
    }),
    ColorSwatch(0xFFCE9A9A, {
      'highlight': Color(0xFFCE9A9A),
      'splash': Color(0xFFF94D56),
      'error': Color(0xFF912D2D),
    }),
  ];

  //  Remove the overriding of initState(). Instead, we use
  // didChangeDependencies()
//  @override
//  void initState() {
//    super.initState();
//    for (var i = 0; i < _categoryNames.length; i++) {
//      var category = Category(
//        name: _categoryNames[i],
//        color: _baseColors[i],
//        iconLocation: Icons.cake,
//        units: _retrieveUnitList(_categoryNames[i]),
//      );
//      if (i == 0) {
//        _defaultCategory = category;
//      }
//      _categories.add(category);
//    }
//  }

  // Uncomment this out. We use didChangeDependencies() so that we can
  // wait for our JSON asset to be loaded in (async).
    @override
    Future<void> didChangeDependencies() async {
      super.didChangeDependencies();
      // We have static unit conversions located in our
      // assets/data/regular_units.json
      if (_categories.isEmpty) {
        await _retrieveLocalCategories();
      }
    }

  /// Retrieves a list of [Categories] and their [Unit]s
  Future<void> _retrieveLocalCategories() async {
    // Consider omitting the types for local variables. For more details on Effective
    // Dart Usage, see https://www.dartlang.org/guides/language/effective-dart/usage
    final json = DefaultAssetBundle
        .of(context)
        .loadString('assets/data/regular_units.json');
    final data = JsonDecoder().convert(await json);
    if (data is! Map) {
      throw ('Data retrieved from API is not a Map');
    }
    // TODO: Create Categories and their list of Units, from the JSON asset
  }

  /// Function to call when a [Category] is tapped.
  void _onCategoryTap(Category category) {
    setState(() {
      _currentCategory = category;
    });
  }

  /// Makes the correct number of rows for the list view, based on whether the
  /// device is portrait or landscape.
  ///
  /// For portrait, we use a [ListView]. For landscape, we use a [GridView].
  Widget _buildCategoryWidgets(Orientation deviceOrientation) {
    if (deviceOrientation == Orientation.portrait) {
      return ListView.builder(
        itemBuilder: (BuildContext context, int index) {
          return CategoryTile(
            category: _categories[index],
            onTap: _onCategoryTap,
          );
        },
        itemCount: _categories.length,
      );
    } else {
      return GridView.count(
        crossAxisCount: 2,
        childAspectRatio: 3.0,
        children: _categories.map((Category c) {
          return CategoryTile(
            category: c,
            onTap: _onCategoryTap,
          );
        }).toList(),
      );
    }
  }

  // TODO: Delete this function; instead, read in the units from the JSON asset
  // inside _retrieveLocalCategories()
  /// Returns a list of mock [Unit]s.
  List<Unit> _retrieveUnitList(String categoryName) {
    // when the app first starts up
    return List.generate(10, (int i) {
      i += 1;
      return Unit(
        name: '$categoryName Unit $i',
        conversion: i.toDouble(),
      );
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_categories.isEmpty) {
      return Center(
        child: Container(
          height: 180.0,
          width: 180.0,
          child: CircularProgressIndicator(),
        ),
      );
    }

    // Based on the device size, figure out how to best lay out the list
    // You can also use MediaQuery.of(context).size to calculate the orientation
    assert(debugCheckHasMediaQuery(context));
    final listView = Padding(
      padding: EdgeInsets.only(
        left: 8.0,
        right: 8.0,
        bottom: 48.0,
      ),
      child: _buildCategoryWidgets(MediaQuery.of(context).orientation),
    );
    return Backdrop(
      currentCategory:
      _currentCategory == null ? _defaultCategory : _currentCategory,
      frontPanel: _currentCategory == null
          ? UnitConverter(category: _defaultCategory)
          : UnitConverter(category: _currentCategory),
      backPanel: listView,
      frontTitle: Text('Unit Converter'),
      backTitle: Text('Select a Category'),
    );
  }
}

//作者版权所有。版权所有。
//此源代码的使用受BSD样式许可证的约束，该许可证可以
//在许可证文件中找到。
导入“dart:async”；
导入“dart:convert”；
进口“包装：颤振/材料.省道”；
导入“background.dart”；
导入“category.dart”；
导入“类别_tile.dart”；
输入“单位省道”；
输入“单位单位换算器.省道”；
///加载单位转换数据，并显示数据。
///
///这是我们应用程序的主屏幕。它从数据库中检索转换数据
///JSON资产和来自API的数据。它在后面板中显示[类别]
///显示[Background]小部件，并在前面板中显示[UnitConverter]。
///
///虽然它被命名为CategoryRoute，但更合适的名称是CategoryScreen，
///因为它负责路由目的地的UI。
类CategoryRoute扩展StatefulWidget{
常量范畴（）；
@凌驾
_CategoryRouteState createState（）=>U CategoryRouteState（）；
}
类_categoryroutstate扩展状态{
类别_defaultCategory；
类别_currentCategory；
//小部件应该是高度不可变的对象。我们可以更新和编辑
//_当我们构建应用程序时，以及当我们将其传递到小部件的
//“children”属性，我们称之为.toList（）。
//有关详细信息，请参阅https://github.com/dart-lang/sdk/issues/27755
最终_类别=[]；
//TODO:删除_categoryNames，因为它们将从JSON资产中检索
静态常数_categoryNames=[
“长度”，
"地区",，
"卷",，
"弥撒",，
“时间”，
“数字存储”，
“能源”，
"货币",，
];
静态常数_基色=[
色样（0xFF6AB7A8{
“高光”：颜色（0xFF6AB7A8），
“飞溅”：颜色（0xFF0ABC9B），
}),
色样（0xFFFFD28E{
“高光”：颜色（0xFFFFD28E），
“飞溅”：颜色（0xFFFF41C），
}),
色样（0xFFFFB7DE{
“高光”：颜色（0xFFFFB7DE），
“飞溅”：颜色（0xFFF94CBF），
}),
色样（0xFF8899A8{
“高光”：颜色（0xFF8899A8），
“飞溅”：颜色（0xFFA9CAE8），
}),
色样（0xFFEAD37E{
“高光”：颜色（0xFFEAD37E），
“飞溅”：颜色（0xFFFF070），
}),
色样（0xFF81A56F{
“高光”：颜色（0xFF81A56F），
“飞溅”：颜色（0xFF7CC159），
}),
色样（0xFFD7C0E2{
“高光”：颜色（0xFFD7C0E2），
“飞溅”：颜色（0xFFCA90E5），
}),
色样（0xFFCE9A9A{
“高光”：颜色（0xFFCE9A9A），
“飞溅”：颜色（0xFFF94D56），
“错误”：颜色（0xFF912D2D），
}),
];
//删除对initState（）的重写
//didChangeDependencies（）
//@覆盖
//void initState（）{
//super.initState（）；
//对于（变量i=0；i<_categoryNames.length；i++）{
//var类别=类别(
//名称：_categoryNames[i]，
//颜色：基色[i]，
//iconLocation:Icons.cake，
//单位：_retrieveUnitList（_categoryNames[i]），
//      );
//如果（i==0）{
//_defaultCategory=类别；
//      }
//_类别。添加（类别）；
//    }
//  }
//取消对此的注释。我们使用didChangeDependencies（）以便
//等待加载JSON资产（异步）。
@凌驾
Future didChangeDependencies（）异步{
super.didChangeDependencies（）；
//我们有静态单位转换位于我们的
//资产/数据/常规单位.json
如果（_categories.isEmpty）{
等待检索目录（）；
}
}
///检索[类别]及其[单位]的列表
未来_retrievelocategories（）异步{
/考虑省略局部变量的类型。
//省道使用，请参阅https://www.dartlang.org/guides/language/effective-dart/usage
最终json=DefaultAssetBundle
.of（上下文）
.loadString（'assets/data/regular_units.json'）；
final data=JsonDecoder（）.convert（等待json）；
if（数据为！地图）{
throw（'从API检索的数据不是映射'）；
}
//TODO：从JSON资产创建类别及其单位列表
}
///点击[Category]时调用的函数。
void _onCategoryTap（类别）{
设置状态（）{
_当前类别=类别；
});
}
///根据
///设备为纵向或横向。
///
///对于纵向，我们使用[ListView]。对于横向，我们使用[GridView]。
Widget\u buildCategoryWidgets（定向设备定向）{
if（deviceOrientation==方向.纵向）{
返回ListView.builder(
itemBuilder:（构建上下文，int索引）{
返回分类(
类别：_类别[索引]，
onTap:_onCategoryTap，
);
},
itemCount:_categories.length，
);
}否则{
返回GridView.count(
交叉轴计数：2，
childAspectRatio:3.0，
子类：_categories.map（（c类）{
返回分类(
类别：c，
onTap:_onCategoryTap，
);
}).toList（），
);
}
}
//TODO:删除此函数；而是从JSON资产中读入单位
//内部(检索VelocialCategories)
///返回
if (_categories.isEmpty) {
      return Center(
        child: Container(
          height: 180.0,
          width: 180.0,
          child: CircularProgressIndicator(),
        ),
      );
    }

if (_categories.isEmpty) {
      return Center(
        child: Container(
        ),
      );
    }