方法"jumpToDay"在null上被调用. [扑] [英] The method 'jumpToDay' was called on null. [Flutter]

问题描述

在此应用程序中，用户将能够添加任务并将其显示在PageView.builder中.用户也可以通过向左或向右滑动来滚动PageView.builder.浏览量映射到日期(例如:当前页面映射到今天的日期，下一页映射到明天的日期，等等).我还想实现一个跳转功能，该功能允许用户通过在showDatepicker小部件上指定日期来移动到新页面.

In this application, the user will be able to add tasks and display it in a PageView.builder. The user will also be able to scroll through the PageView.builder by swiping left or right. The pageview is mapped to dates (ex: current page maps to today's date, next page maps to tomorrow's date, etc.). I also wanted to implement a jump feature which lets the user move to a new page by specifying a date on a showDatepicker widget.

PageView.builder是通过使用此项目实现的.该项目还具有跳转到页面的功能: https://github.com/ZedTheLed/calendar_views

The PageView.builder was implemented by using this project. this project also has the jump to page feature : https://github.com/ZedTheLed/calendar_views

showDatepicker通过以下方法实现.通过点击Raisedbutton来调用它:

the showDatepicker is implemented in the method below. it is called by clicking on a Raisedbutton:

  _selectDate() async {
    final DateTime picker =await showDatePicker(
        context: context,
        initialDate: DateTime.now(),
        firstDate: DateTime(2010),
        lastDate: DateTime(2040)
    );

    if (picker != null) {
      print(picker);
      return _daysPageController.jumpToDay(picker); // this method jumps the user to a selected date
//        Navigator.pop(context);

//        print(selectedDate);
//       await runJump(selectedDate);
//       return selectedDate;
    }
  }

当用户单击日期时，变量DateTime picker成功返回用户在打印语句中选择的日期.但是，当我将此值传递给jumptopage方法时，它给出了他的错误:The method 'jumpToDay' was called on null.

when the user clicks on a date, the variable DateTime picker successfully returns the user-selected date in the print statement. But when i pass this value to the jumptopage method, it gives his error : The method 'jumpToDay' was called on null.

PageView.builder在以下代码中实现:

  final List<DateTime> days;


Scaffold(
      floatingActionButton:    new RaisedButton(
        child: new Text("Jump To Today"),
        onPressed: () {
          _selectDate();
        },
      ),
      body: new Column(
        children: widget.days.map(((day) => Container(
              constraints: new BoxConstraints(
                maxHeight: MediaQuery.of(context).size.height * 1.00,
                maxWidth: MediaQuery.of(context).size.width * 1.00

              ),
                  child: Starting_screen(_makeTextString(day)),  //this screen displays the tasks added by the user
                )
            ),
          ).toList()
      ),
    );

方法_makeTextString只是修改了DateTime值，并以理想的格式返回了它.

the method _makeTextString just modifies a DateTime value and returns it in a desirable format.

我可以对如何处理此错误提出建议吗?

Could i get a suggestion on how to handle this error?

完整的项目在这里可用: https://bitbucket.org/NotAnurag/todolist_restarted/src/master/

full project is available here : https://bitbucket.org/NotAnurag/todolist_restarted/src/master/

推荐答案

当用户单击日期时，变量DateTime选择器将在打印语句中成功返回用户选择的日期.但是当我将此值传递给jumptopage方法时，它给出了他的错误:方法jumpToDay被调用为空"

When the user clicks on a date, the variable DateTime picker successfully returns the user-selected date in the print statement. But when I pass this value to the jumptopage method, it gives his error: "The method jumpToDay was called on null"

我在代码中看到的问题是_daysPageController未初始化. IDE有助于建议您可以在DaysPageController上调用的各种方法，但事实是声明了它，但是没有为其赋值(意味着它是null):

The problem that I see in the code is that _daysPageController is not initialized. IDE helpfully suggests various methods that you can call on DaysPageController, but the fact remains that it is declared, but no value is assigned to it (meaning that it is null):

DaysPageController _daysPageController;  // remains null unless assigned a value

因此，错误试图告诉您的是在null上调用了jumpToDay(DateTime)(显然没有这种方法).如果您查看堆栈跟踪，那里会隐含一些更明确的提示:

Therefore, what the error is attempting to tell is that jumpToDay(DateTime) is called on null (which obviously does not have such a method). If you look at the stack trace a bit more explicit hint is buried in there:

[ERROR:flutter/lib/ui/ui_dart_state.cc(148)]未处理的异常:NoSuchMethodError:方法'jumpToDay'在null上调用.

[ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: NoSuchMethodError: The method 'jumpToDay' was called on null.

尝试初始化控制器，看看它是否有助于解决问题.

Try to initialize the controller and see if it helps to resolve the issue.

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