flutter - Dart-如何让类初始化函数与构造函数一起使用？

Question

如果我通过构造函数创建实例，则忽略初始化函数。如何使构造函数也能在构造函数上工作？
这是我叫课的方式

User fireBaseUser = new User("12345","Test Name"); // shortenedName exists
var snap = {"uid" : "12345", "displayName" : "Test Name"};
User fireBaseUser = User.fromSnapshot(snap); // shortenedName wont exist

class User {
  final String uid;
  final String fireBaseDisplayName;
  String shortenedName;

  User.fromSnapshot( DocumentSnapshot document)
      : uid = snapshot.documentID,
        fireBaseDisplayName = snapshot['displayName'];

  User(
      {this.uid,
      this.fireBaseDisplayName,
      this.shortenedName,
     }) {
    shortenName(fireBaseDisplayName);
  }

  shortenName(fireBaseDisplayName) {
    shortenedName =
        fireBaseDisplayName.substring(0, fireBaseDisplayName.indexOf(' '));
  }

构造函数似乎只有在我像这样复制初始化函数时才起作用

class User {
  final String uid;
  final String fireBaseDisplayName;
  String shortenedName;

  User.fromSnapshot( DocumentSnapshot document)
      : uid = snapshot.documentID,
        fireBaseDisplayName = snapshot['displayName'];
        shortenedName = snapshot['displayName'].substring(0, snapshot['displayName'].indexOf(' '));

  User(
      {this.uid,
      this.fireBaseDisplayName,
      this.shortenedName,
     }) {
    shortenName(fireBaseDisplayName);
  }

  shortenName(fireBaseDisplayName) {
    shortenedName =
        fireBaseDisplayName.substring(0, fireBaseDisplayName.indexOf(' '));
  }

相关 How to initialize a class' fields with a function in dart?

Answer 1

这是一个generative constructor:

User(
      {this.uid,
      this.fireBaseDisplayName,
      this.shortenedName,
     }) {
    shortenName(fireBaseDisplayName);
  }

这是一个 named constructor:

User.fromSnapshot( DocumentSnapshot document)
      : uid = snapshot.documentID,
        fireBaseDisplayName = snapshot['displayName'];

在对象初始化 User fireBaseUser = User.fromSnapshot(snap);中，您正在调用 named constructor，因此您必须在正在调用的 shortenName(fireBaseDisplayName);中调用有趣的 named constructor，如下所示:

User.fromSnapshot( DocumentSnapshot document)
      : uid = snapshot.documentID, // snapshot or document?    :)
        fireBaseDisplayName = snapshot['displayName'] {
          shortenName(fireBaseDisplayName);
      }