Dart 用默认值初始化成员的最优雅的方法

对于常量来说，Chris Buckett's answer是很棒的. 显然，有一个三元运算符可以在Member中工作 初始化器.因此，如果一个字段的初始化代价很高(比方说 需要函数调用和/或创建对象)，此方法似乎有效:

• 不必费心初始化类中的成员-更喜欢 构造函数.否则，可能会白费力气.
• 跳过NICE this.member参数语法.而是使用成员名称和 使分配中具有this.个成员的成员获得资格.
• Make use of the ?parm with ternary operator in member initializer. Here is an example in which creating the default values for the member is assumed expensive.

class Formats {
  static Map<String, dynamic> defaultFormats() {
    print("Expensive call - avoid if possible");
    return {"th": 'default th', "td": 'default td'};
  }

  Map<String, dynamic> leftTbl;
  Map<String, dynamic> rightTbl;

  Formats(
      {Map<String, dynamic>? leftTbl,
      Map<String, dynamic>? rightTbl})
      : leftTbl = leftTbl ?? defaultFormats(),
        rightTbl = rightTbl ?? defaultFormats();

  @override
  String toString() {
    return """
l => $leftTbl,
r => $rightTbl
""";
  }
}

示例用法:

print(new Formats());
print(new Formats(leftTbl: {"th":'solid #089', "td":'solid #089' }));
print(new Formats(leftTbl: {"th":'solid #189', "td":'solid #189'},
      rightTbl: {"th":'solid #189', "td":'solid #189'}));

输出:

Expensive call - avoid if possible
Expensive call - avoid if possible
l => {th: default th, td: default td},
r => {th: default th, td: default td}

Expensive call - avoid if possible
l => {th: solid #089, td: solid #089},
r => {th: default th, td: default td}

l => {th: solid #189, td: solid #189},
r => {th: solid #189, td: solid #189}