如何解决Flutter中的“断言失败：布尔表达式不能为空”异常

问题描述

我正在开发Flutter应用程序，并在logcat中不断收到此错误字符串。

I was working on a Flutter application and kept receiving this error String in the logcat.

Failed assertion: boolean expression must not be null

这是有问题的代码：

@override
Widget build(BuildContext context) {
    return Center(
        child: ListView(
            children: <Widget>[
                TextField(
                    controller: controller,
                    decoration: InputDecoration(
                        hintText: "Type in something..."
                    ),
                ),
                RaisedButton(
                    child: Text("Submit"),
                    onPressed: () => addString(),
                ),
                Flex(
                    direction: Axis.vertical,
                    children: (list_two = null) ? [] :
                    list_two.map((String s) => Text(s)).toList() 
                )
            ],
        ),
    );
}

是什么引起了问题？

推荐答案

解决方案很简单，此行在这里：

The solution was a simple one, this line here:

            Flex(
                ...
                children: (list_two = null) ? [] :
                ...
            )

需要让孩子比较为布尔值，需要2个等号。

Needed to have the children comparison be a boolean, which requires 2 equals signs.

            Flex(
                ...
                children: (list_two == null) ? [] :
                ...
            )

使用Android Studio并用Java编写时，通常会引发编译器错误并且无法运行，但是在使用Flutter插件（1.0截至今天，2018-06-26）进行dart编写时，未显示任何编译器错误，而我们看到了运行时错误。

While using Android studio and writing in Java, this would normally throw a compiler error and not run, but while writing in dart with the Flutter plugin (1.0 as of today, 2018-06-26) no compiler error is shown and we instead see a runtime error.

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