学习目标：

• 1.Dart 入门知识

学习内容：

1.Dart 入门知识

1. initialized value 3 种写法？

class TestData {
      
	//一.Add a question mark after the property？
	int? sum = 0;  
	//一.late关键字
	late int total= 0;  
	int data= 0;  
	//一.Constructor parameter assignment such as：this.data
	TestData(this.data);
}

2. 继承 StatefulWidget How to implement stateful later？

	
import 'package:flutter/material.dart';

void main() {
    
  runApp(AlertView());
}

class AlertView extends StatefulWidget {
    
  @override
  _AlertViewState createState() => _AlertViewState();
}

class _AlertViewState extends State<AlertView> {
    
 //放在buildinside will be reset,只有放在 build Only outside the method can be stateful
  var newDate = DateTime.now();
  
  @override
  Widget build(BuildContext context) {
    
    return Container(
        child: Row(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      children: [
        Text('$newDate'),
        TextButton(
        	//After clicking on the date component
            onPressed: () async {
    
              final currentDate = DateTime.now();
              final selectedDate = await showDatePicker(
                  context: context,
                  initialDate: currentDate,
                  firstDate: currentDate,
                  lastDate: DateTime(2333));
                  //执行 setState The above will be re-executed build 方法,如果newDate放在buildinside will be reset
              setState(() {
    
                newDate = selectedDate!;
              });
            },
            child: Text('日期选择'))
      ],
    ));
  }
}