Dart 的构造函数总结

基础

class Point {
  num x, y;

  Point(num x, num y) {
    // There's a better way to do this, stay tuned.
    this.x = x;
    this.y = y;
  }
}

语法糖

class Point {
  num x, y;

  // Syntactic sugar for setting x and y
  // before the constructor body runs.
  Point(this.x, this.y);
}

命名构造函数

class Point {
  num x, y;

  Point(this.x, this.y);

  // Named constructor
  Point.origin() {
    x = 0;
    y = 0;
  }
}

调用父类构造函数

class Person {
  String firstName;

  Person.fromJson(Map data) {
    print('in Person');
  }
}

class Employee extends Person {
  // Person does not have a default constructor;
  // you must call super.fromJson(data).
  Employee.fromJson(Map data) : super.fromJson(data) {
    print('in Employee');
  }
}

main() {
  var emp = new Employee.fromJson({});

  // Prints:
  // in Person
  // in Employee
  if (emp is Person) {
    // Type check
    emp.firstName = 'Bob';
  }
  (emp as Person).firstName = 'Bob';
}

可重定向的构造函数

class Point {
  num x, y;

  // The main constructor for this class.
  Point(this.x, this.y);

  // Delegates to the main constructor.
  Point.alongXAxis(num x) : this(x, 0);
}

工厂构造函数

class Logger {
  final String name;
  bool mute = false;

  // _cache is library-private, thanks to
  // the _ in front of its name.
  static final Map<String, Logger> _cache =
      <String, Logger>{};

  factory Logger(String name) {
    if (_cache.containsKey(name)) {
      return _cache[name];
    } else {
      final logger = Logger._internal(name);
      _cache[name] = logger;
      return logger;
    }
  }

  Logger._internal(this.name);

  void log(String msg) {
    if (!mute) print(msg);
  }
}

构造函数默认值

• 不需要 required
• 命名参数，可以将函数的参数包括在花括号{}内,传递参数时需要提供命名名称

class ButtonWidget extends StatefulWidget {
  // callback for the button
  final VoidCallback onPressed;
  final String label;
  final Color bgColor;

  const ButtonWidget({
    Key? key,
    required this.onPressed,
    required this.label,
    this.bgColor = Colors.grey,
  }) : super(key: key);

  @override
  State<ButtonWidget> createState() => _ButtonWidgetState();
}