函数定义

Function 类

  • 函数也是对象.
  1. bool isNoble(int atomicNumber) {
  2.   return _nobleGases[atomicNumber] != null;
  3. }
  5. // 函数签名可以忽略返回值
  6. isNoble(atomicNumber) {
  7.   return _nobleGases[atomicNumber] != null;
  8. }

函数体只有一行可以简写:

  1. bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

参数

可选参数

可选参数可以是 named 或者 positional, 但不能都是.

Named 参数

  • {} 
  1. // 定义
  2. /// Sets the [bold] and [hidden] flags ...
  3. void enableFlags({bool bold, bool hidden}) {...}
  5. // 调用
  6. // 也就是说 named 参数在传递时可以时随意位置
  7. enableFlags(bold: true, hidden: false);
  • 强制某个参数必须赋值
  1. // 使用 required 注解依赖 meta 包
  2. import package:meta/meta.dart
  4. const Scrollbar({Key key, @required Widget child})

Positional 参数

  • [] 
  1. // 定义
  2. String say(String from, String msg, [String device]) {
  3.   var result = '$from says $msg';
  4.   if (device != null) {
  5.     result = '$result with a $device';
  6.   }
  7.   return result;
  8. }
  10. // 调用
  11. assert(say('Bob', 'Howdy') == 'Bob says Howdy');
  12. // 也就是说 positional 参数在传递时必须按照顺序
  13. assert(say('Bob', 'Howdy', 'smoke signal') ==
  14.     'Bob says Howdy with a smoke signal');

默认参数值

  • 参数值必须是编译时常量

为 named 参数指定默认值:

  1. /// Sets the [bold] and [hidden] flags ...
  2. void enableFlags({bool bold = false, bool hidden = false}) {...}
  4. // bold will be true; hidden will be false.
  5. enableFlags(bold: true);

为 positional 参数指定默认值:

  1. String say(String from, String msg,
  2.     [String device = 'carrier pigeon', String mood]) {
  3.   var result = '$from says $msg';
  4.   if (device != null) {
  5.     result = '$result with a $device';
  6.   }
  7.   if (mood != null) {
  8.     result = '$result (in a $mood mood)';
  9.   }
  10.   return result;
  11. }
  13. assert(say('Bob', 'Howdy') ==
  14.     'Bob says Howdy with a carrier pigeon');

参数使用 list 或 map 默认值:

  1. void doStuff(
  2.     {List<int> list = const [1, 2, 3],
  3.     Map<String, String> gifts = const {
  4.       'first': 'paper',
  5.       'second': 'cotton',
  6.       'third': 'leather'
  7.     }}) {
  8.   print('list:  $list');
  9.   print('gifts: $gifts');
  10. }

main() 函数

  • main() 函数必须有
  • .. 被称为 cascade (级联)
  1. void main() {
  2.   querySelector('#sample_text_id')
  3.     ..text = 'Click me!'
  4.     ..onClick.listen(reverseText);
  5. }
  • main() 函数可以有参数, 用于接收命令行参数
  1. // Run the app like this: dart args.dart 1 test
  2. void main(List<String> arguments) {
  3.   print(arguments);
  5.   assert(arguments.length == 2);
  6.   assert(int.parse(arguments[0]) == 1);
  7.   assert(arguments[1] == 'test');
  8. }

函数是一等对象

  • 函数可以作为参数传递给其它函数
  1. void printElement(int element) {
  2.   print(element);
  3. }
  5. var list = [1, 2, 3];
  7. // Pass printElement as a parameter.
  8. list.forEach(printElement);
  • 给变量分配函数值
  1. var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
  2. assert(loudify('hello') == '!!! HELLO !!!');

匿名函数

  • 定义匿名函数时, type 可以不写
  1. // 语法
  2. ([[Type] param1[, …]]) {
  3.   codeBlock;
  4. };
  6. var list = ['apples', 'bananas', 'oranges'];
  7. list.forEach((item) {
  8.   print('${list.indexOf(item)}: $item');
  9. });

词法范围

变量的范围是静态确定的.

  1. bool topLevel = true;
  3. void main() {
  4.   var insideMain = true;
  6.   void myFunction() {
  7.     var insideFunction = true;
  9.     void nestedFunction() {
  10.       var insideNestedFunction = true;
  12.       assert(topLevel);
  13.       assert(insideMain);
  14.       assert(insideFunction);
  15.       assert(insideNestedFunction);
  16.     }
  17.   }
  18. }

词法闭包

  1. /// Returns a function that adds [addBy] to the
  2. /// function's argument.
  3. Function makeAdder(int addBy) {
  4.   return (int i) => addBy + i;
  5. }
  7. void main() {
  8.   // Create a function that adds 2.
  9.   var add2 = makeAdder(2);
  11.   // Create a function that adds 4.
  12.   var add4 = makeAdder(4);
  14.   assert(add2(3) == 5);
  15.   assert(add4(3) == 7);
  16. }

测试函数是否相等

  1. void foo() {} // A top-level function
  3. class A {
  4.   static void bar() {} // A static method
  5.   void baz() {} // An instance method
  6. }
  8. void main() {
  9.   var x;
  11.   // Comparing top-level functions.
  12.   x = foo;
  13.   assert(foo == x);
  15.   // Comparing static methods.
  16.   x = A.bar;
  17.   assert(A.bar == x);
  19.   // Comparing instance methods.
  20.   var v = A(); // Instance #1 of A
  21.   var w = A(); // Instance #2 of A
  22.   var y = w;
  23.   x = w.baz;
  25.   // These closures refer to the same instance (#2),
  26.   // so they're equal.
  27.   assert(y.baz == x);
  29.   // These closures refer to different instances,
  30.   // so they're unequal.
  31.   assert(v.baz != w.baz);
  32. }

返回值

所有函数都有返回值, 如果在代码中没写 return, 那么 dart 自动在函数体最后加上 return null; .

  1. foo() {}
  3. assert(foo() == null);