Android开发Dart Constructors构造函数使用技巧整理

final ehe = MyClass(); // Creates an instanceclass MyClass {
  MyClass(); // Fires immediately when created (this guy is cons.)
}

// Default Constructor
// 默认什么都不做
class User {
  String name = 'ehe';
  User();
}
///
// Constructor with parameters
// 构造时初始变量
class User {
  String name;
  User(this.name);
}
///
// Constructor with the initial method
// 构造函数内写入你的逻辑
class User {
  String name;
  User(this.name) {
    // do some magic
  }
}
/
// Constructor with assertion
// 使用 Asserts 去检查你的规则
class User {
  String name;
  User(this.name) : assert(name.length > 3);
}
// Constructor with initializer
// 初始化你的变量
class User {
  static String uppercase(String e) => e.toUpperCase();
  String name;
  User(name) : name = yell(name);
  static String yell(String e) => e.toUpperCase();
}
/
// Constructor with super()
// override 变量
class Person {
  String id;
  Person(this.id);
}
class User extends Person {
  String name;
  User(this.name, String id) : super(id);
}
/
// Constructor with this()
// 命名构造函数
class User {
  String name;
  int salary;
  User(this.name, this.salary);
  User.worker(String name) : this(name, 10);
  User.boss(String name) : this(name, 9999999);
}

class Print {
  static void log(String message) => print(message);
}
Print.log('ehe');
// 你想写一个像这样的util，但有一个问题，因为你也可以创建一个我们不想要的实例。
Print(); // 在这种情况下，这是绝对不必要的
// 如何防止这种情况？答案是私有构造函数！
class Print {
  Print._(); // 这将阻止创建实例
  static void log(String message) => print(message);
}
Print(); // 这将给出现在的编译时错误
Your instance is safe now!

class User {
  String name;
  int salary;
  User.worker(this.name) : salary = 10;
  User.boss(this.name) : salary = 99999999;
}

class User {
  String name;
  int salary;
  User.worker(this.name) : salary = 10;
  User.boss(this.name) : salary = 99999999;
  User._mafia(this.name) : salary = 9999999999999;
}

class User {
  User._privateConstructor();
  static final User instance = User._privateConstructor();
}

const user = User('ehe');
class User {
  final String name;
  const User(this.name);
}

class User {
  final String name;
  User(this.name);
  factory User.fromJson(Map<String, dynamic> json) {
    return User(json["name"]);
  }
}
// Singleton Example
class User {
  User._internal();
  static final User _singleton = Singleton._internal();
  factory User() => _singleton;
}