variables & operator

Author: yinhuasheng

basic/libraries.dart

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+void main() {
+  var tom = Person("Tom", 12);
+  tom.sayHello();
+  tom.sayHi();
+}
+
+class Person {
+  String name;
+  int age;
+  Person(this.name, this.age);
+
+  @Deprecated("use sayHi")
+  void sayHello() {
+    print("hello $name");
+  }
+
+  @MyMeta("test", "test my meta date")
+  void sayHi() {
+    print("Hi $name");
+  }
+}
+
+//dart的注解很简洁 一个class就可以作为注解
+class MyMeta {
+  final String who;
+  final String what;
+
+  const MyMeta(this.who, this.what);
+}

basic/metadata.dart

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+void main() {
+  const a = 10;
+  const b = 3;
+  // dart的/是除，但会保留小数，返回类型是double 这与大部分编程语言不同
+  const c = a / b; //3.333333
+  print(c);
+  // ~/对应的是整除，只保留整数部分
+  const d = a ~/ b; //3
+  print(d);
+
+  var point1 = Point(1, 3);
+  var point2 = Point(1, 3);
+  print(point1 == point2); //true
+
+  // ??=运算符 a ??= b等价于 if(a == null){a == b} else{ do nothing}
+  var a1 = 12;
+  int? b1 = null;
+  b1 ??= a1;
+  var c1 = 13;
+  c1 ??= a1;
+  print(b1);
+  print(c1);
+
+  //??=运算符其实就是??加上赋值运算符=
+  //下面的expr1 ?? expr2  如果 expr1 非空，则返回其值；否则，计算并返回 expr2 的值。
+  //我们可以将dart的??理解为if null表达式
+  var result1 = point1.x ?? point1.y;
+
+  //级联运算符 这要用于函数式编程的写法，如:
+  var point3 = Point(11, 12)
+    ..x = 1
+    ..printIt()
+    ..y = 2
+    ..printIt();
+
+  //spread  展开运算符 主要用于集合，将一个几个展开插入到另一个集合中
+}
+
+//dart支持运算符重载 这个功能很棒
+class Point {
+  int? x;
+  int y;
+
+  Point(this.x, this.y);
+
+  @override
+  bool operator ==(Object other) {
+    //类型检测/转换 as 是将对象转为某一类型，is 和is!是类型判断
+    return other is Point && this.x == other.x && this.y == other.y;
+  }
+
+  //重写 == 一定也要重写hash 避免用到hash的场景
+  @override
+  int get hashCode => Object.hash(this.x, this.y);
+
+  void printIt() {
+    print('x:$x,y:$y');
+  }
+}

basic/operator.dart

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+import '../start/main.dart';
+
+void main() {
+  var person1 = Person("Tom", 12);
+  var person2 = Person("Lee", null);
+  print(person1);
+  print(person2);
+
+  var even1 = person1.age?.isEven;
+  print(even1); //true
+  var even2 = person2.age?.isEven;
+  print(even2); //null
+
+  var student = new Student();
+  student.setName("Jerry");
+  student.sayHello();
+}
+
+class Person {
+  String name;
+  int? age;
+
+  Person(this.name, this.age);
+
+  @override
+  String toString() {
+    return 'name:$name,age:$age';
+  }
+}
+
+class Student {
+  late String name;
+  void setName(String name) {
+    this.name = name;
+  }
+
+  void sayHello() {
+    print("Hello $name");
+  }
+}