## State生命周期
State的生命周期為:
![Flutter中的widget，docker入門書籍推薦_Java](https://s7.51cto.com/images/20210919/1632022636862560.jpg)
State類除了 build之外還提供了很多方法能够讓我們重寫，這些方法會在不同的狀態下由Flutter調起執行，所以這些方法我們就稱之為生命周期方法。在這裏我們用statefulwidget點擊按鈕後移除子statefulwidget。
```java
import 'package:flutter/material.dart';
void main () => runApp(MyApp());
class MyApp extends StatefulWidget {
@override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
bool isShowChild;
///當Widget第一次插入到Widget樹時會被調用，對於每一個State對象，Flutter framework只會調用一次該回調
@override
void initState() {
super.initState();
isShowChild = true;
debugPrint("parent initState......");
}
///初始化時，在initState()之後立刻調用
///當依賴的InheritedWidget rebuild,會觸發此接口被調用
@override
void didChangeDependencies() {
super.didChangeDependencies();
debugPrint("parent didChangeDependencies......");
}
///繪制界面，當setState觸發的時候會再次被調用
@override
Widget build(BuildContext context) {
debugPrint("parent build......");
return MaterialApp(
home: Scaffold(
body: Center(
child: RaisedButton(
onPressed: () {
setState(() {
isShowChild = !isShowChild;
});
},
child: isShowChild ? Child() : Text("演示移除Child"),
)),
),
);
}
///狀態改變的時候會調用該方法,比如調用了setState
@override
void didUpdateWidget(MyApp oldWidget) {
super.didUpdateWidget(oldWidget);
debugPrint("parent didUpdateWidget......");
}
///當State對象從樹中被移除時，會調用此回調
@override
void deactivate() {
super.deactivate();
debugPrint('parent deactivate......');
}
///當State對象從樹中被永久移除時調用；通常在此回調中釋放資源
@override
void dispose() {
super.dispose();
debugPrint('parent dispose......');
}
}
class Child extends StatefulWidget {
@override
_ChildState createState() => _ChildState();}
class _ChildState extends State<Child> {
@override
Widget build(BuildContext context) {
debugPrint("child build......");
return Text('lifeCycle');
}
@override
void initState() {
super.initState();
debugPrint("child initState......");
}
///初始化時，在initState()之後立刻調用
///當依賴的InheritedWidget rebuild,會觸發此接口被調用
@override
void didChangeDependencies() {
super.didChangeDependencies();
debugPrint("child didChangeDependencies......");
}
///父widget狀態改變的時候會調用該方法,比如父節點調用了setState
@override
void didUpdateWidget(Child oldWidget) {
super.didUpdateWidget(oldWidget);
debugPrint("child didUpdateWidget......");
}
///當State對象從樹中被移除時，會調用此回調
@override
void deactivate() {
super.deactivate();
debugPrint('child deactivate......');
}
///當State對象從樹中被永久移除時調用；通常在此回調中釋放資源
@override
void dispose() {
super.dispose();
debugPrint('child dispose......');
}
}


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I/flutter (22218): parent initState......
I/flutter (22218): parent didChangeDependencies......
I/flutter (22218): parent build......
I/flutter (22218): child initState......
I/flutter (22218): child didChangeDependencies......
I/flutter (22218): child build......


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I/flutter (22218): parent build......
I/flutter (22218): child deactivate......
I/flutter (22218): child dispose......


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I/flutter (22765): parent build......
I/flutter (22765): child didUpdateWidget......
I/flutter (22765): child build......


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import 'package:flutter/material.dart';
/// main方法 調用runApp傳遞Widget，這個Widget成為widget樹的根
void main() => runApp(TextApp());
/// 1、單一文本Text
//創建一個無狀態的Widget
class TextApp extends
StatelessWidget {
@override
Widget build(BuildContext context) {
//封裝了應用程序實現Material Design所需要的一些widget
return MaterialApp(
title: "Text演示",
//標題,顯示在recent時候的標題 
//主頁面 
//Scaffold : Material Design布局結構的基本實現。
home: Scaffold(
//ToolBar/ActionBar
appBar: AppBar(title: Text("Text")),
body: Text("Hello,Flutter"
),
),
);
}
}


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Widget _TextBody() {
return Text(
"Hello,Flutter",
style: TextStyle(
//顏色
color: Colors.red,
//字號 默認14
fontSize: 18,
//粗細
fontWeight: FontWeight.w800,
//斜體
fontStyle: FontStyle.italic,
//underline：下劃線，overline：上劃線，lineThrough：删除線
decoration: TextDecoration.lineThrough,
decorationColor: Colors.black,
//solid：實線，double：雙線，dotted：點虛線，dashed：橫虛線，wavy：波浪線
decorationStyle:
TextDecorationStyle.wavy),
);
}
class TextApp extends StatelessWidget {
@override
Widget
build(BuildContext context)
{
return MaterialApp(
title: "Text演示",
home: Scaffold(
appBar: AppBar(title: Text("Text")),
body: _TextBody(),
),
);
}
}


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Widget _RichTextBody() {
var textSpan = TextSpan(
text: "Hello",
style: TextStyle(color: Colors.red),
children: [
TextSpan(text: "Flu", style: TextStyle(color: Colors.blue)),
TextSpan(text: "uter", style: TextStyle(color: Colors.yellow)),
],
);
//Text.rich(textSpan);
return RichText(text: textSpan);
}


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Widget _DefaultStyle(){
DefaultTextStyle(
//設置文本默認樣式
style: TextStyle(
color:Colors.red,
fontSize: 20.0,
),
textAlign: TextAlign.start,
child: Column(
crossAxisAlignment:
CrossAxisAlignment.start,
children: <Widget>[
Text("Hello Flutter!"),
Text("Hello Flutter!"),
Text("Hello Flutter!",
style: TextStyle(
inherit: false,
//不繼承默認樣式
color: Colors.grey
),
),
],
),
);
}


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Widget flutterLogo() {
return FlutterLogo(
//大小
size: 100,
//logo顏色 默認為 Colors.blue
colors: Colors.red,
//markOnly：只顯示logo，horizontal：logo右邊顯示flutter文字，stacked：logo下面顯示文字
style: FlutterLogoStyle.stacked,
//logo上文字顏色
textColor: Colors.blue,
);
}


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Widget icon() {
return Icon(
//使用預定義Material icons 
// https://docs.flutter.io/flutter/material/Icons-class.html
Icons.add,
size: 100,
color: Colors.red);
}


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return MaterialApp(
title: "Image演示",
home: Scaffold(
appBar: AppBar(title: Text("Image")),
body: Image.asset("assets/banner.jpeg"),
),
);


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class ImageState extends State<ImageApp> {
Image image;
@override
void initState() {
super.initState();
getExternalStorageDirectory().then((path) {
setState(() {
image = Image.file(File("${path.path}${Platform.pathSeparator}banner.jpeg"
));
});
});
}
@override
Widget build(BuildContext context) {
return MaterialApp(
title: "Image演示",
home: Scaffold(
appBar: AppBar(title: Text("Image")),
body: image,
),
);
}
}


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Future<List<int>> _imageByte() async {
String path = (await getExternalStorageDirectory()).path;
return await File("$path${Platform.pathSeparator}banner.jpeg").readAsBytes();
}
class ImageState extends State<ImageApp> {
Image image;
@override
void initState() {
super.initState();
_imageByte().then((bytes) {
setState(() {
image = Image.memory(bytes);
});
});
}
@override
Widget build(BuildContext context) {
return MaterialApp(
title: "Image演示",
home: Scaffold(
appBar: AppBar(title: Text("Image")),
# 驚喜
最後還准備了一套上面資料對應的面試題（有答案哦）和面試時的高頻面試算法題（如果面試准備時間不够，那麼集中把這些算法題做完即可，命中率高達85%+）
![Flutter中的widget，docker入門書籍推薦_程序員_09](https://s3.51cto.com/images/20210919/1632022639377226.jpg)
![Flutter中的widget，docker入門書籍推薦_Java_10](https://s4.51cto.com/images/20210919/1632022639398198.jpg)
**[CodeChina開源項目：【一線大廠Java面試題解析+核心總結學習筆記+最新講解視頻】](https://ali1024.coding.net/public/P7/Java/git)**

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