为什么新值没有显示在屏幕上

Question

我正在更改Screen2类中Text小部件的值，它是tex变量，但它不会在屏幕上更新。我在RaisedButton中打印tex值，它为我打印我分配给文本的新值，但它不会在屏幕上更新。

这些是我的代码：

import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Screen1(),
);
}
}
class Screen1 extends StatefulWidget {
@override
_Screen1State createState() => _Screen1State();
}
class _Screen1State extends State<Screen1> {
var tec1 = TextEditingController(text: "");
var tec2 = TextEditingController(text: "");
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Screen 1'),
),
body: Center(
child: Column(
children: [
TextField(
controller: tec1,
),
TextField(
controller: tec2,
),
RaisedButton(
child: Text("calculate"),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(
builder: (context) =>
Screen2(Operation(tec1.text, tec2.text)),
),
);
},
),
],
),
),
);
}
}
class Operation {
String n1;
String n2;
Operation(this.n1, this.n2);
}
class Screen2 extends StatefulWidget {
final Operation par;
Screen2(this.par);
@override
_Screen2State createState() => _Screen2State();
}
class _Screen2State extends State<Screen2> {
String tex;
Widget build(BuildContext context) {
var n1 = double.parse(widget.par.n1);
var n2 = double.parse(widget.par.n2);
var plus = n1 + n2;
var min = n1 - n2;
var div = n1 / n2;
var mul = n1 * n2;
tex = '${widget.par.n1} + ${widget.par.n2} = $plus';
return Scaffold(
appBar: AppBar(
title: Text('Screen 2'),
),
body: Center(
child: Column(
children: [
Text(
tex,
style: TextStyle(fontSize: 20),
),
Center(
child: Row(
children: [
Padding(
padding: const EdgeInsets.all(8.0),
child: RaisedButton(
child: Icon(Icons.add),
onPressed: () {
setState(() {
tex = '${widget.par.n1} + ${widget.par.n2} = $plus';
});
},
),
),
RaisedButton(
child: Text("-"),
onPressed: () {
setState(() {
tex = '${widget.par.n1} - ${widget.par.n2} = $min';
print(tex); 
});
},
),
SizedBox(
width: 10,
),
RaisedButton(
child: Text("*"),
onPressed: () {
setState(() {
tex = '${widget.par.n1} * ${widget.par.n2} = $mul';
});
},
),
SizedBox(
width: 10,
),
RaisedButton(
child: Text("/"),
onPressed: () {
setState(
() {
tex = '${widget.par.n1} / ${widget.par.n2} = $div';
},
);
},
)
],
),
),
],
),
),
);
}
}

Answer 1

由于每次按下按钮(使用setState时(UI重新生成时调用tex = '${widget.par.n1} + ${widget.par.n2} = $plus';，因此会发生此错误。

该代码适用于

import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Screen1(),
);
}
}
class Screen1 extends StatefulWidget {
@override
_Screen1State createState() => _Screen1State();
}
class _Screen1State extends State<Screen1> {
var tec1 = TextEditingController(text: "");
var tec2 = TextEditingController(text: "");
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Screen 1'),
),
body: Center(
child: Column(
children: [
TextField(
controller: tec1,
),
TextField(
controller: tec2,
),
RaisedButton(
child: Text("calculate"),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(
builder: (context) =>
Screen2(Operation(tec1.text, tec2.text)),
),
);
},
),
],
),
),
);
}
}
class Operation {
String n1;
String n2;
Operation(this.n1, this.n2);
}
class Screen2 extends StatefulWidget {
final Operation par;
Screen2(this.par);
@override
_Screen2State createState() => _Screen2State();
}
class _Screen2State extends State<Screen2> {
String tex;
var n1;
var n2;
var plus;
var min;
var div;
var mul;
void initState() {
super.initState();
n1 = double.parse(widget.par.n1);
n2 = double.parse(widget.par.n2);
plus = n1 + n2;
min = n1 - n2;
div = n1 / n2;
mul = n1 * n2;
tex = '${widget.par.n1} + ${widget.par.n2} = $plus';
}
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Screen 2'),
),
body: Center(
child: Column(
children: [
Text(
tex ?? '',
style: TextStyle(fontSize: 20),
),
Center(
child: Row(
children: [
Padding(
padding: const EdgeInsets.all(8.0),
child: RaisedButton(
child: Icon(Icons.add),
onPressed: () {
setState(() {
tex = '${widget.par.n1} + ${widget.par.n2} = $plus';
});
},
),
),
RaisedButton(
child: Text("-"),
onPressed: () {
setState(() {
tex = '${widget.par.n1} - ${widget.par.n2} = $min';
print(tex);
});
},
),
SizedBox(
width: 10,
),
RaisedButton(
child: Text("*"),
onPressed: () {
setState(() {
tex = '${widget.par.n1} * ${widget.par.n2} = $mul';
});
},
),
SizedBox(
width: 10,
),
RaisedButton(
child: Text("/"),
onPressed: () {
setState(
() {
tex = '${widget.par.n1} / ${widget.par.n2} = $div';
},
);
},
)
],
),
),
],
),
),
);
}
}