flutter - 命名为 ":"的构造函数

Question

这个问题在这里已经有了答案:





Colon : in Dart constructor syntax

(2 个回答)


1年前关闭。




我正在通过 Firebase 查找 flutter 文档并找到了这一行，我不知道为什么使用“:”这个符号，我是 dart 的新手

 Record.fromMap(Map<String, dynamic> map, {this.reference})
     : assert(map['name'] != null),
       assert(map['votes'] != null),
       name = map['name'],
       votes = map['votes'];

为什么我不能像这样初始化

Record.fromMap(Map<String, dynamic> map, {this.reference}) {
  name = map['name'],
  votes = map['votes'];
}

代码实验室链接 https://codelabs.developers.google.com/codelabs/flutter-firebase/#4

断言函数有什么作用？

Answer 1

它被称为初始化列表，它是调用构造函数时调用的第一件事。

来自 docs

Initializer list

Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs. Separate initializers with commas.

// Initializer list sets instance variables before
// the constructor body runs.
Point.fromJson(Map<String, num> json)
    : x = json['x'],
      y = json['y'] {
  print('In Point.fromJson(): ($x, $y)');
}

Warning: The right-hand side of an initializer does not have access to this.

During development, you can validate inputs by using assert in the initializer list.

Point.withAssert(this.x, this.y) : assert(x >= 0) {
  print('In Point.withAssert(): ($x, $y)');
}

Initializer lists are handy when setting up final fields. The following example initializes three final fields in an initializer list. Click Run to execute the code.

class Point {
  final num x;
  final num y;
  final num distanceFromOrigin;

  Point(x, y)
      : x = x,
        y = y,
        distanceFromOrigin = sqrt(x * x + y * y);
}

Redirecting constructors

Sometimes a constructor’s only purpose is to redirect to another constructor in the same class. A redirecting constructor’s body is empty, with the constructor call appearing after a colon (:).

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);
}