無法覆寫Dart/Flutter值-在按下按鈕時正確設定今天的日期

為一個簡單的問題道歉，但我剛開始我的 Dart/Flutter 愛好并遇到了以下問題：我想要一個簡單的應用程式，它有一個定義的結束日期（“dt1”）并根據今天的日期計算剩余的天數。我讓它對開始日期（“dt2”）進行硬編碼，但是當我想添加一個按鈕來獲取今天的日期以覆寫預設的開始日期（“dt2”）時，它只是不承認它 - 相反它堅持我之前設定的 dt2 日期。

如何修改我的代碼，以便通過按下按鈕覆寫 dt2 并將 dt2 中的值替換為今天的日期？

任何幫助深表感謝！

import 'package:flutter/material.dart';

void main() {
runApp(MaterialApp(home: Home()));
}

class Home extends StatefulWidget {
@override
State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
@override
Widget build(BuildContext context) {
DateTime dt1 = DateTime.parse("2022-06-01");
DateTime dt2 = DateTime.parse("2022-01-01");
Duration diff = dt1.difference(dt2);

return Scaffold(
    appBar: AppBar(
      title: const Text("Time left at work"),
      centerTitle: true,
      backgroundColor: const Color.fromARGB(255, 131, 48, 75),
    ),
    body: Container(
      alignment: Alignment.center,
      color: Colors.black,
      padding: const EdgeInsets.all(20),
      child: Column(children: [
        TextButton(
          onPressed: () {
            DateTime dt2 = DateTime.now();
            Duration diff = dt1.difference(dt2);
            print(dt2);
          },
          child: const Text(
            "Refresh current date",
            style: TextStyle(
              color: Colors.deepOrange,
              fontWeight: FontWeight.bold,
              fontSize: 10.0,
            ),
          ),
        ),
        Text(
          "Days left: "   diff.inDays.toString(),
          style: const TextStyle(
            color: Colors.white,
            fontWeight: FontWeight.bold,
            fontSize: 40.0,
          ),
        ),
      ]),
    ));
  }
}

uj5u.com熱心網友回復：

當你想改變一個 var 的值時，你必須使用“setState()”。

因為，您想在“運行”模式下更改值。（idk怎么說）因為這個原因你需要使用它。

setState(() {
    var_you_want_change = new_value_of_the_var;
  });

在“OnPressed”中試試這個！

    onPressed: () {
  DateTime dt2 = DateTime.now();
  Duration diff = dt1.difference(dt2);
  print(dt2);
  setState(() {
      diff = dt1.difference(dt2);
      //try ToString if this doesn't work
  });
},

uj5u.com熱心網友回復：

這是當前修改后的代碼：

import 'package:flutter/material.dart';

void main() {
runApp(const MaterialApp(home: Home()));
}

class Home extends StatefulWidget {
const Home({Key? key}) : super(key: key);

@override
State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  DateTime dt1 = DateTime.parse("2022-06-01");
DateTime dt2 = DateTime.parse("2022-01-01");

Duration get diff => dt1.difference(dt2);

@override
Widget build(BuildContext context) {
return Scaffold(
    appBar: AppBar(
      title: const Text("Work Terminator"),
      centerTitle: true,
      backgroundColor: Colors.grey[850],
    ),
    body: Container(
      alignment: Alignment.center,
      color: Colors.black,
      padding: const EdgeInsets.all(20),
      child: Column(children: [
        TextButton(
          onPressed: () {
            print(dt2);
            dt2 = DateTime.now();
          },
          child: const Text(
            "Refresh current date",
            style: TextStyle(
              color: Colors.deepOrange,
              fontWeight: FontWeight.bold,
              fontSize: 10.0,
            ),
          ),
        ),
        Text(
          "Days left: "   diff.inDays.toString(),
          style: const TextStyle(
            color: Colors.white,
            fontWeight: FontWeight.bold,
            fontSize: 40.0,
          ),
        ),
      ]),
    ));
 }
}

uj5u.com熱心網友回復：

build從您的和onPressed函式中洗掉此代碼

DateTime dt1 = DateTime.parse("2022-06-01");
DateTime dt2 = DateTime.parse("2022-01-01");
Duration diff = dt1.difference(dt2);

宣告dt1和dt2作為你的_HomeState 領域

class _HomeState extends State<Home> {
DateTime dt1 = DateTime.parse("2022-06-01");
DateTime dt2 = DateTime.parse("2022-01-01");

diff在你的名字中宣告 getter_HomeState

 class _HomeState extends State<Home> {
    DateTime dt1 = DateTime.parse("2022-06-01");
    DateTime dt2 = DateTime.parse("2022-01-01");
   
    Duration get diff => dt1.difference(dt2);

最后在onPressed（重要setState的是為了更新 UI 進行包裝）：

 onPressed: () {
        setState((){
           dt2 = DateTime.now()
        });
      },

標籤：扑 日期 镖 变量

上一篇：使用globbing和額外的單詞擴展bash陣列

下一篇：如何在Python中創建帶有變數的變數字串？