当我在 flutter 2.2.2 中使用地图时出现错误。（无法将参数类型“对象？”分配给参数类型“字符串”。）

Question

I was going with a course in Udemy , the instructer was using flutter 1.5.4 , i did exactly what he did but i end up with an error , the instructer got his error removed as he added ['questionText']

 Widget build(BuildContext context) {
   var questions = [
     {
       'questionText': 'What\'s your favorite color from these?',
       'answers ': ['Red', 'Blue', 'Green', 'Yellow'],
     },
     {
       'questionText': 'What\'s your favorite animal from these?',
       'answers ': ['Dogs', 'Cats', 'Birds', 'Rabbits'],
     },
     {
       'questionText': 'What\'s your favorite gaming device?',
       'answers ': ['Pc', 'Ps', 'Xbox', 'Nintendo Switch'],
     },
   ];
   return MaterialApp(
     home: Scaffold(
       appBar: AppBar(
         backgroundColor: Colors.red.shade400,
         title: Text('main page'),
       ),
       body: Column(
         children: [
           Question(
             questions[_questionIndex]['questionText'],

**The argument type 'Object?' can't be assigned to the parameter type 'String'.**
           

),
           Answer(_answerQuestion),
           Answer(_answerQuestion),
           Answer(_answerQuestion),
         ],
       ),
     ),
   );
 }
}

Answer 1

You need to do like this

Question((questions[_questionIndex]['questionText'] as String?) ?? ''),

If your are sure about your value will be String from map then you can use

Question((questions[_questionIndex]['questionText'] as String)),

Answer 2

If you are following a teacher, better use the same version of Flutter SDK he is using. A lot has changed between these versions. Null safety is one. If you plan to follow this course, use the below quick fix for every dart file you make. Add:

// @dart=2.9

at the top of your dart files to disable null safety.

I definitely think you should look for an updated course, as null safety is one of the important concepts in Dart/Flutter. And you should train with null safety enabled.

For that matter, try this:

questions[_questionIndex]['questionText']?.toString() ?? ''

The logic is this:

variable?.toString() ?? '' means dart will check if variable exists and is not null. If it is not, then the expression on the left side of ?? is performed: here it returns the desired value as String type. If it was null, the expression on the right will be performed; here it will return an empty string '' .

Without null safety, there could be a situation where questions[_questionIndex]['questionText'] is not present or created (hence null). If the code were to be run in this situation, it would crash.