Dart 튜토리얼 36편: Dart Cheatsheet(핵심 문법 빠르게 복습)

1
var count = 0; // 타입 추론(type inference)[^type-inference]
2
count = 1;
3

4
final now = DateTime.now(); // 실행 시점에 값이 결정됨
5
// now = DateTime.now(); // 다시 대입 불가
6

7
const pi = 3.141592; // 컴파일 타임 상수
8

9
late final String token;
10
token = 'abc123'; // 늦게 초기화(한 번만 대입)

1
bool ok = true;
2

3
int a = 1;
4
double b = 1.5;
5
num c = a + b; // num은 int/double을 모두 표현
6

7
String s = 'hello';

1
Object any = 'text'; // 어떤 객체든 담을 수 있음
2

3
dynamic x = 1; // 정적 타입 검사에서 더 느슨한 타입
4
x = 'now string';
5

6
void logIt(Object v) {
7
  print(v);
8
}
9

10
Never fail(String msg) {
11
  throw StateError(msg);
12
}

1
// 한 줄 주석
2

3
/*
4
여러 줄 주석
5
*/
6

7
/// 문서 주석: 이 함수가 무엇을 하는지 설명합니다.
8
int add(int a, int b) => a + b;

1
class Base {
2
  void run() {}
3
}
4

5
class Child extends Base {
6
  @override
7
  void run() {}
8

9
  @Deprecated('대신 newRun을 사용하세요.')
10
  void oldRun() {}
11

12
  void newRun() {}
13
}

1
import 'dart:math' as math;
2

3
double area(double r) => math.pi * r * r;

1
import 'package:my_pkg/my_pkg.dart' show ApiClient;
2
// import 'package:my_pkg/my_pkg.dart' hide InternalHelper;

1
// 산술
2
final a = 7 + 3;
3
final b = 7 - 3;
4
final c = 7 * 3;
5
final d = 7 / 3; // double
6
final e = 7 ~/ 3; // 정수 나눗셈(몫)
7
final f = 7 % 3; // 나머지

1
// 비교/동등
2
final gt = 3 > 2;
3
final eq = 3 == 3;
4
final ne = 3 != 4;

1
// 논리
2
final ok = (1 < 2) && (2 < 3);
3
final nope = (1 < 2) || (2 > 3);
4
final neg = !(1 < 2);

1
// 비트/시프트
2
final and = 0b1100 & 0b1010;
3
final or = 0b1100 | 0b1010;
4
final xor = 0b1100 ^ 0b1010;
5
final shl = 1 << 3;
6
final shr = 8 >> 2;

1
// 복합 대입
2
var x = 1;
3
x += 2;
4
x *= 3;

1
String stringify(int x, int y) {
2
  return '$x $y';
3
}

1
final rawPath = r'C:\\tmp\\file.txt';

1
final text = '''
2
첫 줄
3
둘째 줄
4
''';

1
int? a; // 초기값은 null
2
String? name = 'Jane';
3
String? address; // 초기값은 null

1
int? a; // = null
2
a ??= 3;
3
print(a); // 3
4

5
a ??= 5;
6
print(a); // 3 (이미 값이 있으니 유지)

1
String? foo = 'a string';
2
String? bar; // = null
3

4
String? baz = foo ?? bar; // foo가 null이면 bar

1
String? upperCaseIt(String? str) {
2
  return str?.toUpperCase();
3
}

1
int lengthOfName(String? name) {
2
  // name이 null이면 runtime error가 납니다.
3
  return name!.length;
4
}

1
String describe(Object x) {
2
  if (x is int) {
3
    return 'int: $x';
4
  }
5
  if (x is String) {
6
    return 'String: $x';
7
  }
8
  return 'other';
9
}

1
void useAs(Object x) {
2
  final s = x as String; // String이 아니면 runtime error
3
  print(s.toUpperCase());
4
}

1
int firstCharCode(String s) {
2
  assert(s.isNotEmpty);
3
  return s.codeUnitAt(0);
4
}

1
class NonNegativePoint {
2
  final int x;
3
  final int y;
4

5
  NonNegativePoint(this.x, this.y) : assert(x >= 0), assert(y >= 0);
6
}

1
String grade(int score) {
2
  if (score >= 90) {
3
    return 'A';
4
  }
5
  if (score >= 80) {
6
    return 'B';
7
  }
8
  return 'C';
9
}

1
String label(bool isOk) => isOk ? 'OK' : 'NO';

1
String toMessage(String command) {
2
  switch (command) {
3
    case 'OPEN':
4
      return '열기';
5
    case 'CLOSE':
6
      return '닫기';
7
    default:
8
      return '알 수 없음';
9
  }
10
}

1
String describeNum(int n) {
2
  // switch expression은 값을 바로 만들 때 쓸 수 있습니다.
3
  return switch (n) {
4
    0 => 'zero',
5
    1 => 'one',
6
    _ => 'many',
7
  };
8
}

1
int sumTo(int n) {
2
  var sum = 0;
3
  for (var i = 1; i <= n; i += 1) {
4
    sum += i;
5
  }
6
  return sum;
7
}

1
String joinLines(List<String> lines) {
2
  var out = '';
3
  for (final line in lines) {
4
    if (line.isEmpty) continue; // 빈 줄은 건너뛰기
5
    out = out.isEmpty ? line : '$out\\n$line';
6
  }
7
  return out;
8
}

1
int firstPositive(List<int> nums) {
2
  var i = 0;
3
  while (i < nums.length) {
4
    if (nums[i] > 0) return nums[i];
5
    i += 1;
6
  }
7
  return 0;
8
}

1
int tries(int target) {
2
  var n = 0;
3
  do {
4
    n += 1;
5
    if (n == target) break; // 조건을 만족하면 종료
6
  } while (true);
7
  return n;
8
}

1
int findFirst(List<List<int>> matrix, int target) {
2
  outer:
3
  for (final row in matrix) {
4
    for (final v in row) {
5
      if (v == target) return v;
6
      if (v < 0) continue outer; // 음수를 만나면 다음 row로
7
    }
8
  }
9
  return -1;
10
}

1
final aListOfStrings = ['one', 'two', 'three'];
2
final aSetOfStrings = {'one', 'two', 'three'};
3
final aMapOfStringsToInts = {'one': 1, 'two': 2, 'three': 3};

1
final aListOfInts = <int>[];
2
final aSetOfInts = <int>{};
3
final aMapOfIntToDouble = <int, double>{};

1
List<String> buildMenu(bool isAdmin) {
2
  return [
3
    'home',
4
    if (isAdmin) 'admin',
5
    'settings',
6
  ];
7
}

1
List<int> squaresUpTo(int n) {
2
  return [
3
    for (var i = 1; i <= n; i += 1) i * i,
4
  ];
5
}

1
Map<String, int> indexOf(List<String> items) {
2
  return {
3
    for (var i = 0; i < items.length; i += 1) items[i]: i,
4
  };
5
}

1
List<int> merge(List<int> a, List<int> b) {
2
  return [
3
    ...a,
4
    ...b,
5
  ];
6
}

1
List<String> safeMerge(List<String>? maybe, List<String> base) {
2
  return [
3
    ...base,
4
    ...?maybe,
5
  ];
6
}

1
bool hasEmpty = ['one', '', 'three'].any((s) => s.isEmpty);

1
String joinWithCommas(List<String> strings) => strings.join(',');

1
// required positional
2
int add(int a, int b) => a + b;
3

4
// optional positional
5
int sumUpToFive(int a, [int b = 2, int c = 3, int d = 4, int e = 5]) {
6
  return a + b + c + d + e;
7
}
8

9
// named (기본값/nullable/required 조합)
10
String greet({required String name, String? title, String suffix = '님'}) {
11
  final t = title == null ? '' : '$title ';
12
  return '$t$name$suffix';
13
}

1
int apply(int x, int Function(int) f) => f(x);
2

3
void main() {
4
  final doubled = apply(10, (n) => n * 2);
5
  print(doubled); // 20
6
}

1
Function makeCounter() {
2
  var count = 0;
3
  return () {
4
    count += 1;
5
    return count;
6
  };
7
}
8

9
void main() {
10
  final c = makeCounter();
11
  print(c()); // 1
12
  print(c()); // 2
13
}

1
Iterable<int> oddsUpTo(int n) sync* {
2
  for (var i = 1; i <= n; i += 2) {
3
    yield i;
4
  }
5
}

1
Iterable<int> concat(Iterable<int> a, Iterable<int> b) sync* {
2
  yield* a;
3
  yield* b;
4
}

1
typedef IntMapper = int Function(int);
2

3
int apply(IntMapper f, int x) => f(x);

1
typedef Json = Map<String, Object?>;

1
class BigObject {
2
  int anInt = 0;
3
  String aString = '';
4
  List<double> aList = [];
5
  bool _done = false;
6

7
  void allDone() => _done = true;
8
}
9

10
BigObject fillBigObject(BigObject obj) {
11
  return obj
12
    ..anInt = 1
13
    ..aString = 'String!'
14
    ..aList.add(3)
15
    ..allDone();
16
}

1
class ShoppingCart {
2
  List<double> _prices = [];
3

4
  double get total => _prices.fold(0, (e, t) => e + t);
5

6
  set prices(List<double> value) {
7
    if (value.any((p) => p < 0)) {
8
      throw InvalidPriceException();
9
    }
10
    _prices = value;
11
  }
12
}
13

14
class InvalidPriceException {}

1
String joinWithCommas(int a, [int? b, int? c, int? d, int? e]) {
2
  var total = '$a';
3
  if (b != null) total = '$total,$b';
4
  if (c != null) total = '$total,$c';
5
  if (d != null) total = '$total,$d';
6
  if (e != null) total = '$total,$e';
7
  return total;
8
}

1
void printName(String firstName, String lastName, {String? middleName}) {
2
  print('$firstName ${middleName ?? ''} $lastName');
3
}

1
class MyDataObject {
2
  final int anInt;
3
  final String aString;
4
  final double aDouble;
5

6
  MyDataObject({this.anInt = 1, this.aString = 'Old!', this.aDouble = 2.0});
7

8
  MyDataObject copyWith({int? newInt, String? newString, double? newDouble}) {
9
    return MyDataObject(
10
      anInt: newInt ?? anInt,
11
      aString: newString ?? aString,
12
      aDouble: newDouble ?? aDouble,
13
    );
14
  }
15
}

1
(int, int) swap((int, int) pair) {
2
  final (a, b) = pair;
3
  return (b, a);
4
}

1
({int x, int y}) moveRight(({int x, int y}) point) {
2
  return (x: point.x + 1, y: point.y);
3
}

1
class Box<T> {
2
  final T value;
3
  const Box(this.value);
4
}
5

6
Box<int> a = const Box(1);
7
Box<String> b = const Box('ok');

1
T first<T>(List<T> items) => items.first;

1
abstract class Animal {
2
  void speak();
3
}
4

5
mixin Walker {
6
  void walk() {
7
    print('걷는다');
8
  }
9
}
10

11
class Dog extends Animal with Walker {
12
  @override
13
  void speak() {
14
    print('멍멍');
15
  }
16
}
17

18
class RobotDog implements Animal {
19
  @override
20
  void speak() {
21
    print('삐빅');
22
  }
23
}

1
class Point {
2
  final int x;
3
  final int y;
4

5
  // default constructor + this. 파라미터
6
  const Point(this.x, this.y);
7

8
  // named constructor + initializer list
9
  Point.origin() : x = 0, y = 0;
10

11
  // redirecting constructor
12
  const Point.square(int v) : this(v, v);
13

14
  // factory constructor
15
  factory Point.fromJson(Map<String, int> json) {
16
    return Point(json['x']!, json['y']!);
17
  }
18
}

1
class Counter {
2
  static int created = 0;
3
  int value = 0;
4

5
  Counter() {
6
    created += 1;
7
  }
8
}
9

10
void main() {
11
  Counter();
12
  Counter();
13
  print(Counter.created); // 2
14
}

1
class BankAccount {
2
  int _balance = 0;
3

4
  void deposit(int amount) {
5
    _balance += amount;
6
  }
7
}

1
class Base {
2
  String greet() => 'hi';
3
}
4

5
class Child extends Base {
6
  @override
7
  String greet() => '${super.greet()}!';
8
}

1
class Vec2 {
2
  final int x;
3
  final int y;
4
  const Vec2(this.x, this.y);
5

6
  Vec2 operator +(Vec2 other) => Vec2(x + other.x, y + other.y);
7
}

1
class Adder {
2
  final int delta;
3
  const Adder(this.delta);
4

5
  int call(int x) => x + delta;
6
}
7

8
void main() {
9
  final add10 = Adder(10);
10
  print(add10(5)); // 15
11
}

1
mixin LoggerMixin {
2
  void log(String msg) {
3
    print('LOG: $msg');
4
  }
5
}
6

7
class Service with LoggerMixin {
8
  void run() {
9
    log('start');
10
  }
11
}

1
enum Status { ready, running, done }
2

3
String label(Status s) {
4
  return switch (s) {
5
    Status.ready => '준비',
6
    Status.running => '진행',
7
    Status.done => '완료',
8
  };
9
}

1
extension StringX on String {
2
  bool get isBlank => trim().isEmpty;
3
}
4

5
bool ok(String s) => !s.isBlank;

1
extension type EmailAddress(String value) {
2
  bool get isValid => value.contains('@');
3
}
4

5
void main() {
6
  final email = EmailAddress('[email protected]');
7
  print(email.isValid);
8
}

1
// 예시: 같은 패키지 안에서만 상속 가능(개념 예시)
2
base class BaseApi {}
3

4
// 예시: 구현은 가능하지만 상속은 제한(개념 예시)
5
interface class ApiShape {}
6

7
// 예시: 상속/구현을 막아서 확장 포인트를 닫음(개념 예시)
8
final class Closed {}
9

10
// 예시: 같은 라이브러리에서만 서브타입을 허용(개념 예시)
11
sealed class Result {}

1
String describePair((int, int) p) {
2
  return switch (p) {
3
    (0, 0) => 'origin',
4
    (final x, 0) => 'x=$x on x-axis',
5
    (0, final y) => 'y=$y on y-axis',
6
    _ => 'other',
7
  };
8
}

1
void ifCase(Object value) {
2
  if (value case (int x, int y)) {
3
    print('record: x=$x y=$y');
4
  }
5
}

1
try {
2
  breedMoreLlamas();
3
} on OutOfLlamasException {
4
  buyMoreLlamas();
5
} on Exception catch (e) {
6
  print('알 수 없는 예외: $e');
7
} catch (e) {
8
  print('정말 알 수 없는 값: $e');
9
}

1
try {
2
  breedMoreLlamas();
3
} catch (e) {
4
  print('예외가 발생했지만, 여기서는 로그만 남긴다.');
5
  rethrow;
6
} finally {
7
  cleanLlamaStalls();
8
}

1
Future<String> loadUserName() async {
2
  // 예시를 단순화하기 위해 즉시 완료되는 Future를 사용합니다.
3
  final name = await Future.value('Jane');
4
  return 'user=$name';
5
}

1
Stream<int> countUpTo(int n) async* {
2
  for (var i = 1; i <= n; i += 1) {
3
    yield i;
4
  }
5
}

1
Future<int> sumStream(Stream<int> s) async {
2
  var sum = 0;
3
  await for (final v in s) {
4
    sum += v;
5
  }
6
  return sum;
7
}

1
class MyColor {
2
  int red;
3
  int green;
4
  int blue;
5

6
  MyColor(this.red, this.green, this.blue);
7
}

1
class MyColor {
2
  int red;
3
  int green;
4
  int blue;
5

6
  MyColor({required this.red, required this.green, required this.blue});
7
}

1
class MyColor {
2
  int red;
3
  int green;
4
  int blue;
5

6
  MyColor([this.red = 0, this.green = 0, this.blue = 0]);
7
}

1
class FirstTwoLetters {
2
  final String letterOne;
3
  final String letterTwo;
4

5
  FirstTwoLetters(String word)
6
      : assert(word.length >= 2),
7
        letterOne = word[0],
8
        letterTwo = word[1];
9
}

1
class Point {
2
  double x, y;
3

4
  Point(this.x, this.y);
5

6
  Point.origin() : x = 0, y = 0;
7
}
8

9
final p = Point.origin();

1
class Square extends Shape {}
2
class Circle extends Shape {}
3

4
class Shape {
5
  Shape();
6

7
  factory Shape.fromTypeName(String typeName) {
8
    if (typeName == 'square') return Square();
9
    if (typeName == 'circle') return Circle();
10
    throw ArgumentError('Unrecognized $typeName');
11
  }
12
}

1
class Automobile {
2
  String make;
3
  String model;
4
  int mpg;
5

6
  Automobile(this.make, this.model, this.mpg);
7

8
  Automobile.hybrid(String make, String model) : this(make, model, 60);
9

10
  Automobile.fancyHybrid() : this.hybrid('Futurecar', 'Mark 2');
11
}

1
class Color {
2
  int red;
3
  int green;
4
  int blue;
5

6
  Color(this.red, this.green, this.blue);
7

8
  Color.black() : this(0, 0, 0);
9
}

1
class ImmutablePoint {
2
  static const ImmutablePoint origin = ImmutablePoint(0, 0);
3

4
  final int x;
5
  final int y;
6

7
  const ImmutablePoint(this.x, this.y);
8
}

1
class Recipe {
2
  final List<String> ingredients;
3
  final int calories;
4
  final double milligramsOfSodium;
5

6
  const Recipe(this.ingredients, this.calories, this.milligramsOfSodium);
7
}