函数必须有一个明确的参数列表

Question

我创建了这个类。但我得到了这个错误

函数必须有一个明确的参数列表。尝试添加参数列表

    class DatabaseHelper{

     static late  DatabaseHelper? _databaseHelper;
     factory DatabaseHelper{
    if(_databaseHelper==null  ){
      _databaseHelper=DatabaseHelper.internal();
      return _databaseHelper!;
    }else{
      return _databaseHelper!;

    }
    }
    DatabaseHelper.internal();
   }

我试过的

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    [√] Flutter (Channel beta, 2.5.0-5.2.pre, on Microsoft Windows [Version 10.0.19042.1110], locale tr-TR)
        • Flutter version 2.5.0-5.2.pre at C:\src\flutter
        • Upstream repository https://github.com/flutter/flutter.git
        • Framework revision 19c61fed0d (4 days ago), 2021-08-18 17:10:31 -0700
        • Engine revision 7a4c4505f6
        • Dart version 2.14.0 (build 2.14.0-377.7.beta)
    
    [√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
        • Android SDK at C:\Users\alige\AppData\Local\Android\sdk
        • Platform android-31, build-tools 31.0.0
        • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
        • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
        • All Android licenses accepted.
    
    [√] Chrome - develop for the web
        • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe
    
    [√] Android Studio (version 2020.3)
        • Android Studio at C:\Program Files\Android\Android Studio
        • Flutter plugin can be installed from:
           https://plugins.jetbrains.com/plugin/9212-flutter
        • Dart plugin can be installed from:
           https://plugins.jetbrains.com/plugin/6351-dart
        • Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
    
    [√] VS Code (version 1.59.1)
        • VS Code at C:\Users\alige\AppData\Local\Programs\Microsoft VS Code
        • Flutter extension version 3.25.0
    
    [√] Connected device (3 available)
        • SM G532F (mobile) • 4200c1b8c4d94413 • android-arm    • Android 6.0.1 (API 23)
        • Chrome (web)      • chrome           • web-javascript • Google Chrome 92.0.4515.159
        • Edge (web)        • edge             • web-javascript • Microsoft Edge 92.0.902.78
    
    • No issues found!

Answer 1

无需更新IDE或Flutter。

您正在定义工厂构造函数，但它缺少结尾的括号。它被定义为一个类，而不是。只需像这样添加缺少的括号，它就可以作为一个函数运行，错误就会消失：

class DatabaseHelper {
  static late DatabaseHelper? _databaseHelper;

  factory DatabaseHelper() {
    if (_databaseHelper == null) {
      _databaseHelper = DatabaseHelper.internal();
      return _databaseHelper!;
    } else {
      return _databaseHelper!;
    }
  }

  DatabaseHelper.internal();
}