flutter 生命周期viewWillAppear，viewW

关键字 MaterialApp navigatorObservers
Flutter的StatefulWidget StatelessWidget生命周期中没有组件出现或者消失的回调，主要是要靠路由的监听
1.创建一个路由监听

final RouteObserver<ModalRoute<void>> _routeObserver =
      RouteObserver<ModalRoute<void>>();

2.这里封装了一个单例，方便在各个页面里获取这个路由监听

import 'package:flutter/material.dart';

class AppRouteObserver {
  //这是实际上的路由监听器
  static final RouteObserver<ModalRoute<void>> _routeObserver =
      RouteObserver<ModalRoute<void>>();
  //这是个单例
  static final AppRouteObserver _appRouteObserver =
      AppRouteObserver._internal();

  AppRouteObserver._internal() {}
  //通过单例的get方法轻松获取路由监听器
  RouteObserver<ModalRoute<void>> get routeObserver {
    return _routeObserver;
  }

  factory AppRouteObserver() {
    return _appRouteObserver;
  }
}

3.添加到MaterialApp的navigatorObservers中

MaterialApp(
.....

navigatorObservers: [
                    AppRouteObserver().routeObserver
                  ],

...)

4.需要监听的页面进行订阅 with RouteAware

class _xxxPageState extends State<xxx> with RouteAware {
...

  @override
  void didChangeDependencies() {
    // TODO: implement didChangeDependencies
    super.didChangeDependencies();
  /// 路由订阅
    AppRouteObserver().routeObserver.subscribe(this, ModalRoute.of(context)!);
  }

  @override
  void dispose() {
    // TODO: implement dispose
  /// 取消路由订阅
    AppRouteObserver().routeObserver.unsubscribe(this);
    super.dispose();
  }


  /// 当前的页面被push显示到用户面前 viewWillAppear.
  @override
  void didPush() { }


  /// 当前的页面被pop viewWillDisappear.
  @override
  void didPop() { }


  /// 上面的页面被pop后当前页面被显示时 viewWillAppear.
  @override
  void didPopNext() { }

  
  /// 从当前页面push到另一个页面 viewWillDisappear.
  @override
  void didPushNext() { }