如何解决异常 -
未处理的异常:'package:flutter/src/widgets/page_view.dart':失败的断言:第 179 行位置 7:'positions.isNotEmpty':在使用它构建 PageView 之前无法访问 PageController.page。
注意:- 我在两个屏幕中使用它,当我在屏幕之间切换时,它显示上述异常。
@override
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((_) => _animateSlider());
}
void _animateSlider() {
Future.delayed(Duration(seconds: 2)).then(
(_) {
int nextPage = _controller.page.round() + 1;
if (nextPage == widget.slide.length) {
nextPage = 0;
}
_controller
.animateToPage(nextPage,
duration: Duration(milliseconds: 300), curve: Curves.linear)
.then(
(_) => _animateSlider(),
);
},
);
}
最佳答案
我没有足够的信息来准确了解您的问题出在哪里,但我刚刚遇到了一个类似的问题,我想在同一个小部件中对 PageView 和标签进行分组,并且我想将当前幻灯片和标签标记为事件状态,所以我需要访问controler.page
为了做到这一点。这是我的修复:
修复 PageView
之前访问页面索引的问题小部件是使用 FutureBuilder
构建的小部件
class Carousel extends StatelessWidget {
final PageController controller;
Carousel({this.controller});
/// Used to trigger an event when the widget has been built
Future<bool> initializeController() {
Completer<bool> completer = new Completer<bool>();
/// Callback called after widget has been fully built
WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
completer.complete(true);
});
return completer.future;
} // /initializeController()
Widget build(BuildContext context) {
return Stack(
children: <Widget>[
// **** FIX **** //
FutureBuilder(
future: initializeController(),
builder: (BuildContext context, AsyncSnapshot<void> snap) {
if (!snap.hasData) {
// Just return a placeholder widget, here it's nothing but you have to return something to avoid errors
return SizedBox();
}
// Then, if the PageView is built, we return the labels buttons
return Column(
children: <Widget>[
CustomLabelButton(
child: Text('Label 1'),
isActive: controller.page.round() == 0,
onPressed: () {},
),
CustomLabelButton(
child: Text('Label 2'),
isActive: controller.page.round() == 1,
onPressed: () {},
),
CustomLabelButton(
child: Text('Label 3'),
isActive: controller.page.round() == 2,
onPressed: () {},
),
],
);
},
),
// **** /FIX **** //
PageView(
physics: BouncingScrollPhysics(),
controller: controller,
children: <Widget>[
CustomPage(),
CustomPage(),
CustomPage(),
],
),
],
);
}
}
修复是否直接在 PageView 子项中需要索引
您可以改用有状态小部件:
class Carousel extends StatefulWidget {
Carousel();
@override
_HomeHorizontalCarouselState createState() => _CarouselState();
}
class _CarouselState extends State<Carousel> {
final PageController controller = PageController();
int currentIndex = 0;
@override
void initState() {
super.initState();
/// Attach a listener which will update the state and refresh the page index
controller.addListener(() {
if (controller.page.round() != currentIndex) {
setState(() {
currentIndex = controller.page.round();
});
}
});
}
@override
void dispose() {
controller.dispose();
super.dispose();
}
Widget build(BuildContext context) {
return Stack(
children: <Widget>[
Column(
children: <Widget>[
CustomLabelButton(
child: Text('Label 1'),
isActive: currentIndex == 0,
onPressed: () {},
),
CustomLabelButton(
child: Text('Label 2'),
isActive: currentIndex == 1,
onPressed: () {},
),
CustomLabelButton(
child: Text('Label 3'),
isActive: currentIndex == 2,
onPressed: () {},
),
]
),
PageView(
physics: BouncingScrollPhysics(),
controller: controller,
children: <Widget>[
CustomPage(isActive: currentIndex == 0),
CustomPage(isActive: currentIndex == 1),
CustomPage(isActive: currentIndex == 2),
],
),
],
);
}
}
关于Flutter : PageController. 页面在使用它构建 PageView 之前无法访问,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/61058420/