flutter - `Class._()..property` 是什么意思？

Question

._().. 是什么意思在下面的代码中是什么意思？

class CounterState {
  int counter;

  CounterState._();

  factory CounterState.init() {
    return CounterState._()..counter = 0;
  }
}

更准确地说 - 两个点是什么 ..平均过去 ._() ?

Answer 1

级联符号 (..)

在 Dart Languague tour ，您应该真正检查一下，因为它包含很多有用的信息，您还可以找到有关您提到的级联符号的信息:

Cascades (..) allow you to make a sequence of operations on the same object. In addition to function calls, you can also access fields on that same object. This often saves you the step of creating a temporary variable and allows you to write more fluid code.

例如，如果你想更新渲染对象的多个字段，你可以简单地使用级联符号来保存一些字符:

renderObject..color = Colors.blue
    ..position = Offset(x, y)
    ..text = greetingText
    ..listenable = animation;

// The above is the same as the following:
renderObject.color = Colors.blue;
renderObject.position = Offset(x, y);
renderObject.text = greetingText;
renderObject.listenable = animation;

当您想要 时，它也会有所帮助返回 与赋值或调用函数在同一行中的对象:

canvas.drawPaint(Paint()..color = Colors.amberAccent);

命名构造函数
._()是 named private constructor .如果一个类没有指定另一个非私有(private)的构造函数(默认的或命名的)，该类 不能从库外部实例化。

class Foo {
  Foo._(); // Cannot be called from outside of this file.

  // Foo(); <- If this was added, the class could be instantiated, even if the private named constructor exists.
}

Learn more关于私有(private)构造函数。