Question

我正在听PageController来获取位置，然后将其与ListView同步。当操纵PageView时，将同时操纵ListView。

示例：https://github.com/Ramotion/cardslider-android

但是，在v0.6.0之后，我收到一个断言错误，即我的ScrollController没有附加到任何视图。每当有流事件触发.jumpTo()方法时，就会触发此事件。仍然可以，但是断言错误使我发疯。

[VERBOSE-2:shell.cc(181)] Dart Error: Unhandled exception:
'package:flutter/src/widgets/scroll_controller.dart': Failed assertion: line 169 pos 12: '_positions.isNotEmpty': ScrollController not attached to any scroll views.
#0      _AssertionError._doThrowNew (dart:core/runtime/liberrors_patch.dart:40:39)
#1      _AssertionError._throwNew (dart:core/runtime/liberrors_patch.dart:36:5)
#2      ScrollController.jumpTo (package:flutter/src/widgets/scroll_controller.dart:169:12)
#3      MyTitle.build.<anonymous closure> (file:///Users/lukepighetti/code/when-coin/when_coin_2/lib/screens/rate/widgets/title.dart:19:19)
#4      _RootZone.runUnaryGuarded (dart:async/zone.dart:1314:10)
#5      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#6      _DelayedData.perform (dart:async/stream_impl.dart:584:14)
#7      _StreamImplEvents.handleNext (dart:async/stream_impl.dart:700:11)
#8      _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:660:7)
#9      _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#10     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)

如何使用ScrollController.jumpTo()而不遇到此异常？

class MyTitle extends StatelessWidget {
  final List<Category> categories;
  MyTitle({this.categories});

  @override
  Widget build(BuildContext context) {
    final _controller = ScrollController();
    double height = 36.0;
    // double width = MediaQuery.of(context).size.width * 0.8;

    BlocProvider.of(context).page.listen((page) {
      _controller.jumpTo(height * page);
    });

    return Container(
      height: height,
      // width: width,
      child: ListView(
        controller: _controller,
        scrollDirection: Axis.vertical,
        physics: NeverScrollableScrollPhysics(),
        children: categories
            .map((c) => _Title(
                  title: c.title,
                  index: categories.indexOf(c),
                ))
            .toList(),
      ),
    );
  }
}

Answer 1

检查控制器具有客户端蚂蚁，然后延迟跳转：

if (_scrollController.hasClients) {
      Future.delayed(Duration(milliseconds: 50), () {
        _scrollController?.jumpTo(_scrollController.position.maxScrollExtent);
      });
    }

Answer 2

问题是您正在重新创建const MongoClient = require('mongodb').MongoClient; // Connection URL const url = 'mongodb://localhost:27017'; // Database Name const dbName = 'mytestingdb'; const retrieveCustomers = (db, callback)=>{ // Get the customers collection const collection = db.collection('customers'); // Find some customers collection.find({}).toArray((err, customers) =>{ if(err) throw err; console.log("Found the following records"); console.log(customers) callback(customers); }); } const retrieveCustomer = (db, callback)=>{ // Get the customers collection const collection = db.collection('customers'); // Find some customers collection.find({'name': 'mahendra'}).toArray((err, customers) =>{ if(err) throw err; console.log("Found the following records"); console.log(customers) callback(customers); }); } const insertCustomers = (db, callback)=> { // Get the customers collection const collection = db.collection('customers'); const dataArray = [{name : 'mahendra'}, {name :'divit'}, {name : 'aryan'} ]; // Insert some customers collection.insertMany(dataArray, (err, result)=> { if(err) throw err; console.log("Inserted 3 customers into the collection"); callback(result); }); } // Use connect method to connect to the server MongoClient.connect(url,{ useUnifiedTopology: true }, (err, client) => { console.log("Connected successfully to server"); const db = client.db(dbName); insertCustomers(db, ()=> { retrieveCustomers(db, ()=> { retrieveCustomer(db, ()=> { client.close(); }); }); }); });并在每个_controller上订阅它。必须将控制器作为最终的类作用域属性创建一次。此外，您应该使用build到StatefulWidget控制器并订阅以dispose方法流式传输。

initState

Answer 3

作为我上面回答的答案，您正在使用尚未附加到ListView或其他ScrollView的ScrollController。您可以使用hasClients属性进行检查。

if (_scrollController.hasClients) {
  await _scrollController.animateTo(
    0.0,
    curve: Curves.easeOut,
    duration: const Duration(milliseconds: 300),
  );
}

Answer 4

您正在尝试使用scrollController进行跳转，然后再将scrollController添加到ScrollView（列表视图）中。添加到控制器后，我们必须跳转。请参考下面的代码。

 // task1
 Future.delayed(Duration.zero, () => 
 { // task2
  BlocProvider.of(context).page.listen((page) { _controller.jumpTo(height * page); }); 
 });
// task3

这与DispatchQueue.main.async非常相似，因为持续时间为零。执行顺序为task1，task3，task2

ScrollController.jumpTo（）“ ScrollController未附加到任何滚动视图”

4 个答案: