// 声明返回值

int add(int a, int b) {

  return a + b;

}

// 不声明返回值

add2(int a, int b) {

  return a + b;

}

// =>是return语句的简写

add3(a, b) => a + b; 

//命名参数、位置参数、参数默认值

sayHello({String name}) {

  print("hello, my name is $name");

}

sayHello2({name: String}) {

  print("hello, my name is $name");

}

//位置参数

sayHello(String name, int age, [String hobby]) { // 位置参数可以有多个，比如[String a, int b]

  StringBuffer sb = new StringBuffer();

  sb.write("hello, this is $name and I am $age years old");

  if (hobby != null) {

    sb.write(", my hobby is $hobby");

  }

  print(sb.toString());

}

// 命名参数的默认值

int add({int a, int b = 3}) { // 不能写成：int add({a: int, b: int = 3})

  return a + b;

}

// 位置参数的默认值

int sum(int a, int b, [int c = 3]) {

  return a + b + c;

}

//函数作为一类对象

printNum(int a) {

  print("$a");

}

//将一个函数赋值给某个变量

printNum(int a) {

  print("$a");

}

//匿名函数

test(Function callback) {

  callback("hello");

}

//

void main() {

  print("hello world!");

  var a = 1;

  int b = 10;

  String s = "hello";

  dynamic c = 0.5;

  var count = 10;

  final Num = count;  // final 只能赋值一次

  const Num1 = 10; // const赋值必须是编译时常量

  // numbers

  var a = 0;

  int b = 1;

  double c = 0.1;

  // strings

  var s1 = 'hello';

  String s2 = "world";

  // booleans

  var real = true;

  bool isReal = false;

  // lists

  var arr = [1, 2, 3, 4, 5];

  List<String> arr2 = ['hello', 'world', "123", "456"];

  List<dynamic> arr3 = [1, true, 'haha', 1.0];

  // maps

  var map = new Map();

  map['name'] = 'zhangsan';

  map['age'] = 10;

  Map m = new Map();

  m['a'] = 'a';

  //runes，Dart 中 使用runes 来获取UTF-32字符集的字符。String的 codeUnitAt and codeUnit属性可以获取UTF-16字符集的字符

  var clapping = '\u{1f44f}';

  print(clapping); // 打印的是拍手emoji的表情

  // symbols

  print(#s == new Symbol("s")); // true

  print(add(1, 2)); // 3

  print(add2(2, 3)); // 5

  print(add3(1, 2)); // 3

  // hello, this is zhangsan and I am 20 years old

  sayHello("zhangsan", 20);

  // hello, this is zhangsan and I am 20 years old, my hobby is play football

  sayHello("zhangsan", 20, "play football");

  //  依次打印：

  //  1

  //  2

  //  3

  var arr = [1, 2, 3];

  arr.forEach(printNum);

  var f1 = printNum;

  Function f2 = printNum;

  var f3 = (int a) => print("a = $a");

  f1(1);

  f2(2);

  f3(6);

  test((param) {

    // 打印hello

    print(param);

  });

}