飞镖的构造函数无效

Question

请告诉我。为什么默认情况下pr2 null而不是str1？

void main() {
  var house = Home(pr1: 4, pr3: 5);
  house.printDate();
}

class Home {
  int pr1=1;
  String? pr2 = 'str1';
  int pr3=3;
  Home({required this.pr1, this.pr2, required this.pr3});

  void printDate() {
    print('The pr1: $pr1');
    print('The pr2: $pr2');
    print('The pr3: $pr3');
  }
}

pr1：4 pr2：null pr3：5

Answer 1

因为默认情况下，可选参数是null，除非在构造函数定义中另有说明。如果要声明另一个默认值，则需要使用此语法：

void main() {
  var house = Home(pr1: 4, pr3: 5);
  house.printDate();
  // The pr1: 4
  // The pr2: str1
  // The pr3: 5
}

class Home {
  int pr1=1;
  String? pr2;
  int pr3=3;
  Home({required this.pr1, this.pr2 = 'str1', required this.pr3});

  void printDate() {
    print('The pr1: $pr1');
    print('The pr2: $pr2');
    print('The pr3: $pr3');
  }
}

Answer 2

我对您的代码进行了重新分配以确保可读性，并将使用上述IF-ELSE语句删除第一个解决方案

class Home {
  //initialize the variables using the final keyword
  final int? pr1;
  final String? pr2;
  final int? pr3;
  
  //set the constructors
  Home({required this.pr1, this.pr2, required this.pr3});
  
void printDate(){
  //using the pr1 value
  print('The pr1: $pr1');
  //perfoming a runtime assertation for dev
  //assert(pr2 == null);//if the value for pr2 is null; it run smoothly
  //using if-else statement, I can tell wht is shows as outcome
  if (pr2 == null){
    print('The pr2: str1');
  }else{print('The pr2: $pr2');}//I would be rewriting this as a tenary operator


  //using the pr3 value
  print('The pr3: $pr3');
}
}
//the app starts ro run from here  
void main(){
  var house = Home(pr1:5,pr3:6,);
  house.printDate();
}

，您可以看到，当Pr2的条件为null时，它将显示文本“ str1”，如果不是null，则显示PR2的值。
希望这有帮助。

Answer 3

第二种方法是使用终止操作员检查null

class Home {
  //initialize the variables using the final keyword
  final int? pr1;
  final String? pr2;
  final int? pr3;

  //set the constructors
  Home({required this.pr1, this.pr2, required this.pr3});

  void printDate() {
    //using the pr1 value
    print('The pr1: $pr1');
//pr2 check for null safety using tenary
    pr2 == null ? print('The Pr2: str1'):print('The pr2:$pr2');
    //using the pr3 value
    print('The pr3: $pr3');
  }
}

//the app starts ro run from here
void main() {
  var house = Home(
    pr1: 5,
    pr3: 6,
  );
  house.printDate();
}

是否含义是什么意思？简而言之，将其视为逻辑条件，一种if-else的形式，但以一种更简单的方式表达出来的方法是：

 condition?expression1:expression2;

如果条件为真，则表达式1运行，如果条件为false，则表达式2运行。
为了回答您的问题，从Dart 2.12开始，它变得无效，这不是变量到构造函数的安全做法。我很高兴进一步解释是否需要。