创建一个示例来学习Flutter
中的回调。这是一个简单的程序来增加GestureDetetor
的计数器 onTap 但是,回调方法不起作用。热重载时计数增加但未点击。以下是带注释的代码。
class BoxState extends State<ChangeBoxState>{
int _counter = 0;
//Callback method changes the state onTap of GestureDetector widget. It is not calling onTap.
increaseCount(){
setState(() {
++_counter;
print(_counter);
});
}
@override
Widget build(BuildContext context) {
// Passing the callback method,"increaseCount()" to stateless class where GestureDetector is defined.
return BoxWidget(onPressed: increaseCount(), counter: _counter,);
}
}
无国籍班级:
class BoxWidget extends StatelessWidget{
BoxWidget({this.onPressed, this.counter});
final VoidCallback onPressed;
final int counter;
@override
Widget build(BuildContext context) {
// TODO: implement build
return Container(
decoration: BoxDecoration(color: Colors.blue[500]),
child: Column(
children: <Widget>[Center(child: Text('Hello, world!')),
GestureDetector(
onTap: onPressed, //Passing onPressed to onTap.
child: Container(
margin: const EdgeInsets.only(left:0.0, top:200.0, right:0.0, bottom:0.0),
height: 200.0,
width: 200.0,
decoration: BoxDecoration(
color: Colors.teal[200],
border: Border.all(color: Colors.yellow, width: 10.0, style: BorderStyle.solid),
borderRadius: BorderRadius.all(Radius.circular(20.0)),
),
child: Center(child: Text(counter.toString())),
),
),
],
),
);
}
}
2 回答
删除
increaseCount()
中的括号,因为使用您正在创建VoidCallback
实例的括号,这将只运行一次,所以试试这个您应该为
onPressed
回调提供increaseCount
的引用。在这里,您将
increaseCount()
(检查大括号)分配给首先调用increaseCount()
函数的回调,并将其返回值分配给onPressed
。这就是为什么它只在热重载上增加一次。