android – LinearProgressIndicator Flutter用法

概述我正在学习Flutter allthought我不知道它是否是正确的决定.任何方式我想使用Material Librery的LinearProgressIndicator组件,但我没有得到如何使用它我试过这段代码： import 'dart:async';import 'package:flutter/material.dart';void main() {runApp(new Mater 我正在学习Flutter allthought我不知道它是否是正确的决定.任何方式我想使用Material librery的linearProgressIndicator组件,但我没有得到如何使用它我试过这段代码：

import 'dart:async';import 'package:Flutter/material.dart';voID main() {runApp(new MaterialApp(deBUGShowCheckedModeBanner: false,home: new MyApp(),));}class MyApp extends StatefulWidget {@overrIDeMyAppState createState() => new MyAppState();}class MyAppState extends State<MyApp> {@overrIDeWidget build(BuildContext context) {return new Scaffold(  appbar: new Appbar(    Title: new Text('SlIDer Demo'),),body: new Container(    color: colors.blueAccent,padding: new EdgeInsets.all(32.0),child: new ProgressIndicatorDemo(),);}}class ProgressIndicatorDemo extends StatefulWidget {@overrIDe_ProgressIndicatorDemoState createState() =>  new _ProgressIndicatorDemoState();}class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo>with SingleTickerProvIDerStateMixin {AnimationController _controller;@overrIDevoID initState() {super.initState();_controller = new AnimationController(  duration: const Duration(milliseconds: 1500),vsync: this,);_playAnimation();} Future<Null> _playAnimation() async {  try {  await _controller.repeat().orCancel;  } on TickerCanceled {  // the animation got canceled,probably because we were disposed  }  }  @overrIDe  voID dispose() {  _controller.stop();  super.dispose();  }  @overrIDe  Widget build(BuildContext context) {  return _buildAnimation(context,null); } Widget _buildAnimation(BuildContext context,Widget child) { return new Center(    child: new Container(  color: colors.redAccent,child: new linearProgressIndicator(    value: _controller.value,backgroundcolor: colors.redAccent,));  } }

但我总是得到这个错误

I/Flutter ( 3819): ══╡ EXCEPTION CAUGHT BY WidgetS liBRARY ╞═══════════════════════════════════════════════════════════I/Flutter ( 3819): The following NoSuchMethodError was thrown building ProgressIndicatorDemo(dirty,state:I/Flutter ( 3819): _ProgressIndicatorDemoState#80400(ticker active)):I/Flutter ( 3819): The getter 'value' was called on null.I/Flutter ( 3819): Receiver: nullI/Flutter ( 3819): TrIEd calling: valueI/Flutter ( 3819):I/Flutter ( 3819): When the exception was thrown,this was the stack:I/Flutter ( 3819): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)I/Flutter ( 3819): #1      _ProgressIndicatorDemoState.build (file:///E:/Flutter_work/animation_test/lib/main.dart:67:26)I/Flutter ( 3819): #2      StatefulElement.build (package:Flutter/src/Widgets/framework.dart:3730:27)I/Flutter ( 3819): #3      ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3642:15)I/Flutter ( 3819): #4      Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #5      StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #6      Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #7      SingleChildRenderObjectElement.update (package:Flutter/src/Widgets/framework.dart:4661:14)I/Flutter ( 3819): #8      Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #9      SingleChildRenderObjectElement.update (package:Flutter/src/Widgets/framework.dart:4661:14)I/Flutter ( 3819): #10     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #11     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #12     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #13     StatelessElement.update (package:Flutter/src/Widgets/framework.dart:3702:5)I/Flutter ( 3819): #14     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #15     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #16     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #17     ProxyElement.update (package:Flutter/src/Widgets/framework.dart:3909:5)I/Flutter ( 3819): #18     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #19     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #20     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #21     ProxyElement.update (package:Flutter/src/Widgets/framework.dart:3909:5)I/Flutter ( 3819): #22     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #23     RenderObjectElement.updateChildren (package:Flutter/src/Widgets/framework.dart:4379:32)I/Flutter ( 3819): #24     MultiChildRenderObjectElement.update (package:Flutter/src/Widgets/framework.dart:4769:17)I/Flutter ( 3819): #25     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #26     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #27     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #28     StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #29     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #30     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #31     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #32     ProxyElement.update (package:Flutter/src/Widgets/framework.dart:3909:5)I/Flutter ( 3819): #33     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #34     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #35     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #36     StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #37     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #38     SingleChildRenderObjectElement.update (package:Flutter/src/Widgets/framework.dart:4661:14)I/Flutter ( 3819): #39     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #40     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #41     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #42     StatelessElement.update (package:Flutter/src/Widgets/framework.dart:3702:5)I/Flutter ( 3819): #43     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)   I/Flutter ( 3819): #44     SingleChildRenderObjectElement.update (package:Flutter/src/Widgets/framework.dart:4661:14)I/Flutter ( 3819): #45     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #46     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #47     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #48     StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #49     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #50     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #51     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #52     StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #53     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #54     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #55     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #56     ProxyElement.update (package:Flutter/src/Widgets/framework.dart:3909:5)I/Flutter ( 3819): #57     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #58     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #59     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #60     ProxyElement.update (package:Flutter/src/Widgets/framework.dart:3909:5)I/Flutter ( 3819): #61     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #62     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #63     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)I/Flutter ( 3819): #64     StatefulElement.update (package:Flutter/src/Widgets/framework.dart:3799:5)I/Flutter ( 3819): #65     Element.updateChild (package:Flutter/src/Widgets/framework.dart:2699:15)I/Flutter ( 3819): #66     ComponentElement.performRebuild (package:Flutter/src/Widgets/framework.dart:3653:16)I/Flutter ( 3819): #67     Element.rebuild (package:Flutter/src/Widgets/framework.dart:3495:5)  I/Flutter ( 3819): #68     BuildOwner.buildScope (package:Flutter/src/Widgets/framework.dart:2242:33) Reloaded 0 of 391 librarIEs in 829ms. Lost connection to device.

如何解决这个问题,我喜欢做这个演示Flutter Progress Indicator Demo ,但同样的结果任何人都可以帮我做一个关于那个的例子,或者可以告诉我我的代码中有什么问题？
问候

解决方法 您没有使用动画对象.

import 'dart:async';import 'package:Flutter/material.dart';voID main() {  runApp(new MaterialApp(    deBUGShowCheckedModeBanner: false,));}class MyApp extends StatefulWidget {  @overrIDe  MyAppState createState() => new MyAppState();}class MyAppState extends State<MyApp> {  @overrIDe  Widget build(BuildContext context) {    return new Scaffold(      appbar: new Appbar(        Title: new Text('SlIDer Demo'),body: new Container(        color: colors.blueAccent,);  }}class ProgressIndicatorDemo extends StatefulWidget {  @overrIDe  _ProgressIndicatorDemoState createState() =>      new _ProgressIndicatorDemoState();}class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo>    with SingleTickerProvIDerStateMixin {  AnimationController controller;  Animation<double> animation;  @overrIDe  voID initState() {    super.initState();    controller = AnimationController(        duration: const Duration(milliseconds: 2000),vsync: this);    animation = Tween(begin: 0.0,end: 1.0).animate(controller)      ..addListener(() {        setState(() {          // the state that has changed here is the animation object’s value        });      });    controller.repeat();  }  @overrIDe  voID dispose() {    controller.stop();    super.dispose();  }  @overrIDe  Widget build(BuildContext context) {    return new Center(        child: new Container(          child:  linearProgressIndicator( value:  animation.value,)    );  }}

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