到目前为止，当我需要在小部件中使用条件语句时，我已经做了以下工作(使用中心和容器作为简化的虚拟示例):
new Center(
child: condition == true ? new Container() : new Container()
)
虽然当我尝试使用if/else语句时，它会导致一个死亡代码警告:
new Center(
child:
if(condition == true){
new Container();
}else{
new Container();
}
)
有趣的是，我尝试了一个switch case语句，它给了我同样的警告，因此我不能运行代码。我做错了什么，或者它是这样的，不能使用if/else或开关语句而不颤振认为有死代码?
当前回答
如果你想在Text()小部件中使用If语句，你可以像这样使用匿名函数:
class ConditionalStatmentExample extends StatelessWidget {
Widget build(BuildContext context) {
return Text(
(() {
if(true){
return "return a string";
}
return "any other string when the condition is not met";
})(),
textAlign: TextAlign.center, );
}
}
其他回答
Flutter Widget可以在不破坏代码树的情况下有条件地用父元素包装子树
import 'package:flutter/widgets.dart';
/// Conditionally wrap a subtree with a parent widget without breaking the code tree.
///
/// [condition]: the condition depending on which the subtree [child] is wrapped with the parent.
/// [child]: The subtree that should always be build.
/// [conditionalBuilder]: builds the parent with the subtree [child].
///
/// ___________
/// Usage:
/// ```dart
/// return ConditionalParentWidget(
/// condition: shouldIncludeParent,
/// child: Widget1(
/// child: Widget2(
/// child: Widget3(),
/// ),
/// ),
/// conditionalBuilder: (Widget child) => SomeParentWidget(child: child),
///);
/// ```
///
/// ___________
/// Instead of:
/// ```dart
/// Widget child = Widget1(
/// child: Widget2(
/// child: Widget3(),
/// ),
/// );
///
/// return shouldIncludeParent ? SomeParentWidget(child: child) : child;
/// ```
///
class ConditionalParentWidget extends StatelessWidget {
const ConditionalParentWidget({
Key key,
@required this.condition,
@required this.child,
@required this.conditionalBuilder,
}) : super(key: key);
final Widget child;
final bool condition;
final Widget Function(Widget child) conditionalBuilder;
@override
Widget build(BuildContext context) {
return condition ? this.conditionalBuilder(this.child) : this.child;
}
}
我不知道这是否是一个好的做法，但我正在使用:
class StatmentExample extends StatelessWidget {
Widget build(BuildContext context) {
return pageValue==1 ? Page1():pageValue== 2? Page2():pageValue==3 ? Page3():Page4();
}
}
更好的方法
Column(
children: [
if (firstCondition == true) ...[
DayScreen(),
] else if(secondCondition == true)...[
StatsScreen(),
], else...[
StatsScreen(),
],
],
),
我个人使用if/else语句在子语句中使用这种block语句。它只支持Dart 2.3.0以上版本。
If / else
Column(
children: [
if (_selectedIndex == 0) ...[
DayScreen(),
] else ...[
StatsScreen(),
],
],
),
If / else If
Column(
children: [
if (_selectedIndex == 0) ...[
DayScreen(),
] else if(_selectedIndex == 1)...[
StatsScreen(),
],
],
),
多部件示例
Column(
children: [
if (_selectedIndex == 0) ...[
DayScreen(),
AboutScreen(),
InfoScreen(),
] else if(_selectedIndex == 1)...[
HomeScreen(),
StatsScreen(),
],
],
),
有一个按钮
bool _paused = false;
CupertinoButton(
child: _paused ? Text('Play') : Text('Pause'),
color: Colors.blue,
onPressed: () {
setState(() {
_paused = !_paused;
});
},
),
