Functions | Phaisarn Sutheebanjard

Dart is a true object-oriented language, so even functions are objects and have a type, Function.

main() {
  print(isOld(50));
  print(isNotOld(50));
  print(isFifty(50));
  print(isSixty(60));
}

bool isOld(int old) {
  return old > 50;
}

isNotOld(old) {
  return old <= 50;
}

bool isFifty(int old) => old == 50;

bool isSixty(int old) => old == 60 ? true : false;

main() {

print(isOld(50));

print(isNotOld(50));

print(isFifty(50));

print(isSixty(60));

}

bool isOld(int old) {

return old > 50;

}

isNotOld(old) {

return old <= 50;

}

bool isFifty(int old) => old == 50;

bool isSixty(int old) => old == 60 ? true : false;

บรรทัดที่ 8 : การประกาศฟังก์ชัน
บรรทัดที่ 12 : การประกาศฟังก์ชัน แบบไม่ระบุชนิดของตัวแปร และชนิดของ returnt type
บรรทัดที่ 16 : การประกาศฟังก์ชันแบบ shorthand syntax
บรรทัดที่ 18 : การประกาศฟังก์ชันแบบ shorthand syntax ร่วมกับการใช้ conditional expression

Optional positional parameters
ใส่พารามิเตอร์ใน [] ทำให้กลายเป็น optional parameters

main() {
  assert(say('Bob', 'Howdy') == 'Bob says Howdy');

  assert(say('Bob', 'Howdy', 'smoke signal') ==
      'Bob says Howdy with a smoke signal');
}

String say(String from, String msg, [String device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

main() {

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

assert(say('Bob', 'Howdy', 'smoke signal') ==

'Bob says Howdy with a smoke signal');

}

String say(String from, String msg, [String device]) {

var result = '$from says $msg';

if (device != null) {

result = '$result with a $device';

}

return result;

}

Default parameter values

main() {
  assert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');
  assert(say('Bob', 'Howdy', 'iPhone') == 'Bob says Howdy with a iPhone');
}

String say(String from, String msg,
    [String device = 'carrier pigeon', String mood]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  if (mood != null) {
    result = '$result (in a $mood mood)';
  }
  return result;
}

main() {

assert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');

assert(say('Bob', 'Howdy', 'iPhone') == 'Bob says Howdy with a iPhone');

}

String say(String from, String msg,

[String device = 'carrier pigeon', String mood]) {

var result = '$from says $msg';

if (device != null) {

result = '$result with a $device';

}

if (mood != null) {

result = '$result (in a $mood mood)';

}

return result;

}

กำหนด List และ Map เป็น default value

main() {
  doStuff();
}

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

main() {

doStuff();

}

void doStuff(

{List<int> list = const [1, 2, 3],

Map<String, String> gifts = const {

'first': 'paper',

'second': 'cotton',

'third': 'leather'

}}) {

print('list: $list');

print('gifts: $gifts');

}

จะได้

list:  [1, 2, 3]
gifts: {first: paper, second: cotton, third: leather}

1 2	list: [1, 2, 3] gifts: {first: paper, second: cotton, third: leather}

The main() function

void main(List<String> arguments) {
  print(arguments);

  assert(arguments.length == 2);
  assert(int.parse(arguments[0]) == 1);
  assert(arguments[1] == 'test');
}

void main(List<String> arguments) {

print(arguments);

assert(arguments.length == 2);

assert(int.parse(arguments[0]) == 1);

assert(arguments[1] == 'test');

}

รันทาง command line จะไม่แสดง error ของ assert

D:\Project\dart\my_dart\bin>dart my_dart.dart
[]

1 2	D:\Project\dart\my_dart\bin>dart my_dart.dart []

ต้องใส่ --enable-asserts ด้วย ก็จะแสดง error ของ assert

D:\Project\dart\my_dart\bin>dart --enable-asserts my_dart.dart
[]
Unhandled exception:
'file:///D:/Project/dart/my_dart/bin/my_dart.dart': Failed assertion: line 5 pos 10: 'arguments.length == 2': is not tru
e.
#0      _AssertionError._doThrowNew (dart:core/runtime/liberrors_patch.dart:40:39)
#1      _AssertionError._throwNew (dart:core/runtime/liberrors_patch.dart:36:5)
#2      main (file:///D:/Project/dart/my_dart/bin/my_dart.dart:5:10)
#3      _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:287:32)
#4      _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)

D:\Project\dart\my_dart\bin>dart --enable-asserts my_dart.dart

[]

Unhandled exception:

'file:///D:/Project/dart/my_dart/bin/my_dart.dart': Failed assertion: line 5 pos 10: 'arguments.length == 2': is not tru

#0 _AssertionError._doThrowNew (dart:core/runtime/liberrors_patch.dart:40:39)

#1 _AssertionError._throwNew (dart:core/runtime/liberrors_patch.dart:36:5)

#2 main (file:///D:/Project/dart/my_dart/bin/my_dart.dart:5:10)

#3 _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:287:32)

#4 _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)

ทีนี้ใส่พารามิเตอร์ไป 2 ตัวคือ 1 test ก็จะผ่านละ ไม่ error

D:\Project\dart\my_dart\bin>dart --enable-asserts my_dart.dart 1 test
[1, test]

1 2	D:\Project\dart\my_dart\bin>dart --enable-asserts my_dart.dart 1 test [1, test]

Pass a function as a parameter to another function

void main() {
  var list = [1, 2, 3];

  // Pass printElement as a parameter.
  list.forEach(printElement);
}

void printElement(int element) {
  print(element);
}

void main() {

var list = [1, 2, 3];

// Pass printElement as a parameter.

list.forEach(printElement);

}

void printElement(int element) {

print(element);

}

จะได้

1
2
3

ลองเขียนฟังก์ช้นด้วย shorthand syntax

void main() {
  var list = [1, 2, 3];
  list.forEach(printElement);
}

void printElement(int element) => print(element * 10);

void main() {

var list = [1, 2, 3];

list.forEach(printElement);

}

void printElement(int element) => print(element * 10);

ฟังก์ชันซ้อนอยู่ใน ฟังก์ชันได้

void main() {
  void printElement(int element) => print(element * 20);

  var list = [1, 2, 3];
  list.forEach(printElement);
}

void main() {

void printElement(int element) => print(element * 20);

var list = [1, 2, 3];

list.forEach(printElement);

}

Assign a function to a variable

void main() {
  var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
  assert(loudify('hello') == '!!! HELLO !!!');
}

void main() {

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';

assert(loudify('hello') == '!!! HELLO !!!');

}

Anonymous functions

void main() {
  var list = ['apples', 'bananas', 'oranges'];
  list.forEach((item) {
    print('${list.indexOf(item)}: $item');
  });
}

void main() {

var list = ['apples', 'bananas', 'oranges'];

list.forEach((item) {

print('${list.indexOf(item)}: $item');

});

}

หรือ

void main() {
  var list = ['apples', 'bananas', 'oranges'];
  list.forEach((item) => print('${list.indexOf(item)}: $item'));
}

void main() {

var list = ['apples', 'bananas', 'oranges'];

list.forEach((item) => print('${list.indexOf(item)}: $item'));

}

จะได้

0: apples
1: bananas
2: oranges

0: apples

1: bananas

2: oranges

ขอบเขตของตัวแปร (Lexical scope)

bool topLevel = true;

void main() {
  var insideMain = true;

  void myFunction() {
    var insideFunction = true;

    void nestedFunction() {
      var insideNestedFunction = true;

      assert(topLevel);
      assert(insideMain);
      assert(insideFunction);
      assert(insideNestedFunction);
    }
  }
}

bool topLevel = true;

void main() {

var insideMain = true;

void myFunction() {

var insideFunction = true;

void nestedFunction() {

var insideNestedFunction = true;

assert(topLevel);

assert(insideMain);

assert(insideFunction);

assert(insideNestedFunction);

}

Lexical closures

/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(num addBy) {
  return (num i) => addBy + i;
}

void main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);

  // Create a function that adds 4.
  var add4 = makeAdder(4);

  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

/// Returns a function that adds [addBy] to the

/// function's argument.

Function makeAdder(num addBy) {

return (num i) => addBy + i;

}

void main() {

// Create a function that adds 2.

var add2 = makeAdder(2);

// Create a function that adds 4.

var add4 = makeAdder(4);

assert(add2(3) == 5);

assert(add4(3) == 7);

}

Testing functions for equality

void foo() {} // A top-level function

class A {
  static void bar() {} // A static method
  void baz() {} // An instance method
}

void main() {
  var x;

  // Comparing top-level functions.
  x = foo;
  assert(foo == x);

  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);

  // Comparing instance methods.
  var v = A(); // Instance #1 of A
  var w = A(); // Instance #2 of A
  var y = w;
  x = w.baz;

  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);

  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

void foo() {} // A top-level function

class A {

static void bar() {} // A static method

void baz() {} // An instance method

}

void main() {

var x;

// Comparing top-level functions.

x = foo;

assert(foo == x);

// Comparing static methods.

x = A.bar;

assert(A.bar == x);

// Comparing instance methods.

var v = A(); // Instance #1 of A

var w = A(); // Instance #2 of A

var y = w;

x = w.baz;

// These closures refer to the same instance (#2),

// so they're equal.

assert(y.baz == x);

// These closures refer to different instances,

// so they're unequal.

assert(v.baz != w.baz);

}

Return values
ปกติฟังก์ชันจะคืนค่า ถ้าไม่กำหนด return ก็จะได้คืนค่าเป็น null
ยกเว้นเรากำหนด void ไว้หน้าฟังก์ชัน ฟังก์ชันก็จะไม่คืนค่า

main() {
  foo() {}
  assert(foo() == null);

  void bar() {}
  //assert(bar() == null); // Uncommenting this causes an error.
}

main() {

foo() {}

assert(foo() == null);

void bar() {}

//assert(bar() == null); // Uncommenting this causes an error.

}

Link

A Tour of the Dart Language – Functions