Flutter: Functional Widget Summary

1. WillPopScope intercepts page exit events:

WillPopScope(//对页面根布局包装一层
  onWillPop: () async {
    if (条件) {//对退出页面加判断条件
      return false;  //false拦截事件，不退出当前页
    }
    return true;  //true放行事件，退出当前页
  },
  child: 页面根布局
)

2. Sharing data across Widgets:

(1) InheritedWidget implements the widget tree to share data from top to bottom:

<1> Implement InheritedWidget subclass sharing data:

class 自定义InheritedWidget<T> extends InheritedWidget {
  final T 共享变量;    //需要共享的数据
  自定义InheritedWidget({
    Key? key,
    required this.共享变量,
    required Widget child,
  }) : super(key: key, child: child);
  static 自定义InheritedWidget? of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<自定义InheritedWidget>();  //供外部调用获取共享数据
  }
  @override
  bool updateShouldNotify(自定义InheritedWidget<T> old) {
    return old.共享变量 != 共享变量; //共享变量数据发生变化时，通知使用共享变量的Widget重新build
  }
}

<2> Use the InheritedWidget subclass as a sub-layout and set shared data:

class _SetDataState extends State<SetDataWidget> {
  @override
  Widget build(BuildContext context) {
    return DataWidget(
        共享变量: 设置的共享数据,
        child: 获取共享数据Widget()  //在此子Widget中获取共享数据
    );
  }
}

<3> Obtain shared data in sub-Widget:

class 获取共享数据Widget extends StatefulWidget {
  const 获取共享数据Widget({super.key});
  @override
  State<获取共享数据Widget> createState() => _GetDatatState();
}
class _GetDatatState extends State<获取共享数据Widget> {
  @override
  Widget build(BuildContext context) {
    Object data = 自定义InheritedWidget.of(context)!.共享变量;  //获取共享数据
    return ...;
  }
}

(2) ValueListenableBuilder implements data sharing between Widgets:

final ValueNotifier<User> shardData = ValueNotifier<User>(User("张xx")); //需要共享的原数据使用ValueNotifier包装，当数字变化时通知ValueListenableBuilder
...
ValueListenableBuilder<User>(
  builder: (BuildContext context, User user, Widget? child) {  // builder方法只会在shardData变化时被调用
    return Text("使用共享数据shardData: ${user.name}"); //user为共享的数据
  },
  valueListenable: shardData, //将共享的数据传递给valueListenable
  child: const Text('不依赖共享数据shardData的其他Widget'),  //指定 child 属性来缓存不使用共享数据的子Widget
)

3. Asynchronous UI update:

(1) FutureBuilder implements asynchronous update UI (receives a single asynchronous result, such as interface data):

FutureBuilder<User>( //FutureBuilder也是Widget
  future: getNetData(),   //异步耗时任务
  initialData: User(...), //初始默认
  builder: (BuildContext context, AsyncSnapshot snapshot) { //执行的各阶段被触发
    if (snapshot.connectionState != ConnectionState.done) {//done为任务完成、waiting为等待状态
      return CircularProgressIndicator();  //任务执行中，显示进度条
    }
    if (!snapshot.hasError) { //成功
      //获取异步返回的数据
      User user = snapshot.data;
      return ...; //显示数据UI
    }
    return ...; //显示失败UI
  },
)
...
Future<User> getNetData() async { //异步耗时任务
  ...  //省略耗时任务
  return ...;
}

(2) StreamBuilder implements asynchronous update UI (receiving single or multiple asynchronous results, such as streams when files are downloaded):

StreamBuilder<int>( //StreamBuilder也是Widget
  stream: downloadFile(),  //异步耗时任务
  builder: (BuildContext context, AsyncSnapshot<int> snapshot) {   //执行的各阶段被触发
    if (snapshot.hasError) return ...; //显示网络请求失败UI
    if (snapshot.connectionState == ConnectionState.none) return ...; //显示获取文件失败UI
    if (snapshot.connectionState == ConnectionState.waiting) return CircularProgressIndicator(); //等待中，显示圆形加载中
    if(snapshot.connectionState == ConnectionState.done){ //下载完成
      //...处理下载完成逻辑
      return ...;  //显示文件下载完成UI
    }
    //...处理下载中逻辑
    return ...; //显示下载进度条UI
  },
)
...
Stream<int> downloadFile() {
  ...  //省略耗时任务
  return ...;
}