dart學習（對象操作符）

1）條件運算符 ?.

void main() {
  Person person = new Person();
  person?.work();
}

class Person {
  String name;
  int age;

  void work() {
    print("Work...");
  }
}

運行結果：
Work...

2）類型轉換：as

void main() {
  var person;
  person = "";
  person = new Person();
  (person as Person).work();
}

class Person {
  String name;
  int age;

  void work() {
    print("Work...");
  }
}

運行結果：
Work...

3）是否指定類型：is   、  is!

if (person is Person) {
    person?.work();
  }

4）級聯操作：..

  Person person = new Person();
  person
    ..name = "Tom"
    ..age = 10
    ..work();