Dart 치트시트

// Hello World in Dart
void main() {
  print('Hello, World!');
}

/* Multi-line
   block comment */

/// Documentation comment
/// Supports Markdown
void greet(String name) {
  print('Hi, $name');
}

var name = 'Alice';        // type inferred
String city = 'NYC';        // explicit type
final age = 30;             // runtime constant
const PI = 3.14;            // compile-time constant
final now = DateTime.now(); // OK: runtime value
// const time = DateTime.now(); // ERROR: not compile-time

const list = [1, 2, 3];     // const list (immutable)
final list2 = [4, 5, 6];    // final ref, mutable content

int value = 42;              // non-nullable
int? nullableValue;          // nullable (default null)
print(nullableValue);        // null
print(nullableValue ?? 0);   // 0 (null coalescing)
nullableValue ??= 10;        // assign if null
print(nullableValue);        // 10

// int x = null;            // ERROR: non-nullable
String name = 'Alice';
print(name.length);          // safe, no null check

import 'dart:io';

void main() {
  print('Hello');                    // stdout with newline
  stdout.write('no newline');        // no trailing newline
  stderr.writeln('an error');        // stderr with newline

  String? input = stdin.readLineSync(); // read a line
  int? n = int.tryParse(input ?? '');   // safe parse
  print('You entered: $n');
}

var x = 10;          // inferred as int
var y = 3.14;        // inferred as double
var s = 'hi';        // inferred as String

dynamic d = 10;      // type can change
d = 'now string';    // OK
// d.foo();          // compiles, may fail at runtime

Object o = 'hello';  // supertype of all non-null types
// o.length;         // ERROR: Object has no length
print((o as String).length);  // 5 (cast)

int a = 42;
double b = 3.14;
num c = 10;     // num is supertype of int & double
num d = 2.71;
print(a.bitLength);          // 6
print(b.toStringAsFixed(2)); // '3.14'
print(10 ~/ 3);             // 3 (integer division)
print(10.remainder(3));     // 1
print(0xFF);                // 255 (hex)
print(1.5e3);               // 1500.0 (scientific)

var s1 = 'single';
var s2 = "double";
var s3 = '''multi
line string''';
var name = 'Alice';
print('Hi, $name');               // interpolation
print('Length: ${name.length}'); // expression interpolation

var raw = r'No escape: \n';      // raw string (literal)
var escaped = 'It\'s ok';        // escaped quote
print(s1[0]);                     // 's' (index access)

bool isTrue = true;
bool isFalse = false;
print(!isTrue);           // false
print(isTrue && isFalse); // false
print(isTrue || isFalse); // true

// Only bool is allowed in conditions; no truthy/falsy
// if (1) {} // ERROR: must be bool
if ('text'.isNotEmpty) {
  print('non-empty');
}

var list = [1, 2, 3];
var typed = <String>['a', 'b'];
var constList = const [1, 2, 3];
list.add(4);
list.addAll([5, 6]);
print(list.length);          // 6
print(list[0]);              // 1
print(list.sublist(1, 3));   // [2, 3]
var spread = [...list, 7];   // spread
// constList.add(0);         // ERROR: immutable

var map = {
  'name': 'Alice',
  'age': 30,
};
var typed = <String, int>{'a': 1, 'b': 2};
map['city'] = 'NYC';              // add entry
print(map['name']);               // Alice
print(map.length);                // 3
print(map.containsKey('age'));    // true
map.forEach((k, v) => print('$k: $v'));
var keys = map.keys.toList();     // [name, age, city]

var set = {1, 2, 3};
set.add(2);              // no duplicate added
set.add(4);
print(set);              // {1, 2, 3, 4}
print(set.contains(2));  // true
print(set.intersection({2, 3, 5})); // {2, 3}

// Runes (Unicode code points)
var heart = '♥';        // ♥
print(heart);                // ♥
print('A'.codeUnitAt(0));    // 65

print(5 + 3);   // 8
print(5 - 3);   // 2
print(5 * 3);   // 15
print(5 / 3);   // 1.6666... (double)
print(5 ~/ 3);  // 1 (integer division)
print(5 % 3);   // 2 (modulo)
print(-(5));    // -5 (unary minus)
print(2.toString()); // '2'

var i = 5;
print(i++);  // 5 (postfix: use, then add)
print(i);    // 6
print(++i);  // 7 (prefix: add, then use)
print(i--);  // 7
print(--i);  // 5

print(3 == 3);   // true
print(3 != 4);   // true
print(3 < 4);    // true
print(3 > 4);    // false
print(3 <= 3);   // true
print(3 >= 4);   // false
print('a' == 'a'); // true (content equality)
var l1 = [1, 2];
var l2 = [1, 2];
print(l1 == l2); // false (reference equality)

bool a = true, b = false;
print(a && b);  // false
print(a || b);  // true
print(!a);      // false

// short-circuit evaluation
bool check() { print('called'); return true; }
false && check();  // check() NOT called
true || check();   // check() NOT called

Object x = 'hello';
print(x is String);   // true
print(x is! int);     // true
if (x is String) {
  print(x.length);    // smart-cast to String
}
Object y = 42;
print((y as int) + 1); // 43 (cast)
// (y as String);      // runtime TypeError

class Builder {
  String? name;
  int? size;
  Builder setName(String n) { name = n; return this; }
  Builder setSize(int s) { size = s; return this; }
}

var b = Builder()
  ..setName('widget')
  ..setSize(10);
print(b.name); // widget

String? path;
print(path?.length);     // null (safe access)
print(path?.length ?? 0); // 0

int score = 85;
if (score >= 90) {
  print('A');
} else if (score >= 80) {
  print('B');
} else {
  print('C');
}

// ternary expression
var grade = score >= 60 ? 'pass' : 'fail';
print(grade); // pass

// if (score) {} // ERROR: condition must be bool

for (var i = 0; i < 3; i++) {
  print(i);
}

var list = ['a', 'b', 'c'];
for (var item in list) {
  print(item);
}

// for-in with Map entries
var map = {'x': 1, 'y': 2};
for (var entry in map.entries) {
  print('${entry.key}: ${entry.value}');
}

var i = 0;
while (i < 3) {
  print('while $i');
  i++;
}

var j = 0;
do {
  print('do $j');
  j++;
} while (j < 3);

// Classic switch statement
var color = 'red';
switch (color) {
  case 'red':
    print('stop');
    break;
  case 'green':
    print('go');
    break;
  default:
    print('unknown');
}

// Dart 3 switch expression
String describe(int n) => switch (n) {
  0 => 'zero',
  1 || 2 => 'small',
  >= 3 && <= 10 => 'medium',
  _ => 'large',
};
print(describe(5)); // medium

for (var i = 0; i < 5; i++) {
  if (i == 2) continue; // skip 2
  if (i == 4) break;    // stop at 4
  print(i);  // 0, 1, 3
}

// labels for nested loops
outer:
for (var i = 0; i < 3; i++) {
  for (var j = 0; j < 3; j++) {
    if (i + j > 2) break outer;
    print('$i,$j');
  }
}

void setAge(int age) {
  assert(age >= 0, 'age must be non-negative');
  // ...
}

assert(1 == 1);  // passes in debug
// assert(1 == 2, 'math is broken'); // fails in debug

// Assertions are stripped in release/production builds

// named function with return type
int add(int a, int b) {
  return a + b;
}

// functions are first-class objects
int Function(int) makeAdder(int n) {
  return (int x) => x + n;
}

var add5 = makeAdder(5);
print(add5(3)); // 8

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

// single-expression function with =>
int square(int x) => x * x;
String greet(String name) => 'Hi, $name';

// arrow with nullable
String? firstChar(String? s) => s?.isEmpty ?? true ? null : s[0];

print(square(4));      // 16
print(greet('Al'));    // Hi, Al
print(firstChar('hi')); // h

// optional positional params wrapped in []
String greet(String name, [String? title]) {
  if (title != null) {
    return 'Hello, $title $name';
  }
  return 'Hello, $name';
}

print(greet('Alice'));        // Hello, Alice
print(greet('Bob', 'Dr.'));   // Hello, Dr. Bob

// named params wrapped in {}, required marks mandatory
void createUser({
  required String name,
  int age = 0,
  String? email,
}) {
  print('$name, $age, $email');
}

createUser(name: 'Alice', age: 30);
createUser(name: 'Bob', email: '[email protected]');

// named params are passed by name, order-independent

// default values for optional positional
double calc(double a, [double b = 1.0, double c = 0.0]) {
  return a * b + c;
}

// default values for named
void config({String host = 'localhost', int port = 8080}) {
  print('$host:$port');
}

print(calc(5));       // 5.0
print(calc(5, 2));    // 10.0
config(port: 3000);   // localhost:3000

// anonymous function assigned to a variable
var multiply = (int a, int b) => a * b;
print(multiply(3, 4)); // 12

// closure capturing a variable
Function counter() {
  int count = 0;
  return () => ++count;
}

var c = counter();
print(c()); // 1
print(c()); // 2

// used as callbacks
[1, 2, 3].forEach((n) => print(n));

class Person {
  String name;
  int age;

  // constructor
  Person(this.name, this.age);

  void greet() {
    print('Hi, I am $name');
  }
}

var p = Person('Alice', 30);
p.greet(); // Hi, I am Alice
print(p.name); // Alice

class Point {
  double x;
  double y;

  Point(this.x, this.y);

  double distanceTo(Point other) {
    return ((x - other.x) * (x - other.x) +
            (y - other.y) * (y - other.y));
  }

  void moveBy(double dx, double dy) {
    this.x += dx;   // 'this' is optional when unambiguous
    y += dy;
  }
}

class Rectangle {
  double width, height;
  Rectangle(this.width, this.height);

  // computed getter
  double get area => width * height;
  set size(double v) {
    width = v;
    height = v;
  }
}

var r = Rectangle(3, 4);
print(r.area);  // 12 (accessed like a field)
r.size = 10;
print(r.area);  // 100

class MathUtils {
  static const double PI = 3.14159;
  static double circleArea(double r) => PI * r * r;
}

// accessed via the class, not an instance
print(MathUtils.PI);             // 3.14159
print(MathUtils.circleArea(2));  // 12.566

// static members belong to the class, not instances

class Logger {
  final String name;
  static final Map<String, Logger> _cache = {};

  // factory may return a cached instance
  factory Logger(String name) {
    return _cache.putIfAbsent(name, () => Logger._internal(name));
  }

  Logger._internal(this.name);
}

var a = Logger('app');
var b = Logger('app');
print(identical(a, b)); // true (same cached instance)

class Book {
  String title;
  Book(this.title);

  @override
  String toString() => 'Book($title)';

  @override
  bool operator ==(Object other) =>
      other is Book && other.title == title;

  @override
  int get hashCode => title.hashCode;
}

print(Book('Dart')); // Book(Dart)
print(Book('A') == Book('A')); // true

class Animal {
  String species;

  // generative constructor
  Animal(this.species);

  // if no constructor is written, Dart provides:
  // Animal() : species = 'unknown';
}

var a = Animal('cat');
print(a.species); // cat

class Point {
  double x, y;
  Point(this.x, this.y);

  // named constructor
  Point.origin() : x = 0, y = 0;
  Point.fromList(List<double> l) : x = l[0], y = l[1];
}

var p1 = Point.origin();
var p2 = Point.fromList([3, 4]);
print('${p1.x},${p1.y}'); // 0.0,0.0
print('${p2.x},${p2.y}'); // 3.0,4.0

class Temperature {
  final double celsius;

  // initializer list runs before body
  Temperature(double c) : celsius = c;
  Temperature.fromFahrenheit(double f)
      : celsius = (f - 32) * 5 / 9;

  // assert in initializer list
  Temperature.clamped(double c)
      : assert(c >= -273.15),
        celsius = c < -273.15 ? -273.15 : c;
}

print(Temperature.fromFahrenheit(32).celsius); // 0.0

class Point {
  double x, y;
  Point(this.x, this.y);

  // redirect to another constructor with 'this'
  Point.alongX(double x) : this(x, 0);
  Point.origin() : this(0, 0);
  Point.fromDouble(double n) : this.alongX(n);
}

print(Point.alongX(5).y); // 0.0
print(Point.origin().x);  // 0.0

class ImmutablePoint {
  final double x;
  final double y;

  // const constructor: creates compile-time constant instances
  const ImmutablePoint(this.x, this.y);

  static const origin = ImmutablePoint(0, 0);
}

const p = ImmutablePoint(1, 2);
const o = ImmutablePoint.origin;
print(identical(o, ImmutablePoint(0, 0))); // true

class Shape {
  final String type;
  const Shape._(this.type);

  factory Shape(String kind) {
    return switch (kind) {
      'circle' => const Shape._('circle'),
      'square' => const Shape._('square'),
      _ => const Shape._('unknown'),
    };
  }
}

print(Shape('circle').type); // circle
print(identical(Shape('circle'), Shape('circle'))); // true

class Animal {
  String name;
  Animal(this.name);

  void speak() => print('$name makes a sound');
}

class Dog extends Animal {
  Dog(String name) : super(name);

  @override
  void speak() => print('$name barks');
}

var d = Dog('Rex');
d.speak(); // Rex barks

class Vehicle {
  int speed = 0;
  void accelerate(int by) => speed += by;
  void describe() => print('Vehicle at $speed');
}

class Car extends Vehicle {
  @override
  void accelerate(int by) {
    super.accelerate(by);  // call parent method
    print('Car now at $speed');
  }
}

Car().accelerate(10); // Car now at 10

class Base {
  String tag;
  Base(this.tag) {
    print('Base created: $tag');
  }
}

class Derived extends Base {
  Derived(String tag) : super(tag) {
    print('Derived created');
  }
}

// order: super initializer -> super body -> derived body
Derived('x');
// Base created: x
// Derived created

class Proxy implements Object {
  @override
  dynamic noSuchMethod(Invocation inv) {
    print('Called: ${inv.memberName}');
    return null;
  }
}

var p = Proxy();
p.someMissingMethod(); // Called: Symbol("someMissingMethod")

sealed class Shape {
  const Shape();
}

class Circle extends Shape {
  final double r;
  const Circle(this.r);
}
class Square extends Shape {
  final double s;
  const Square(this.s);
}

// switch is exhaustive over sealed subtypes
double area(Shape s) => switch (s) {
  Circle(:var r) => 3.14 * r * r,
  Square(:var s) => s * s,
};

print(area(const Circle(2))); // 12.56

abstract class Animal {
  // abstract method: no body, must be overridden
  void makeSound();

  // concrete method: inherited as-is
  void breathe() => print('breathing');
}

class Cat extends Animal {
  @override
  void makeSound() => print('meow');
}

var c = Cat();
c.makeSound(); // meow
// Animal();    // ERROR: cannot instantiate abstract class

class Television {
  void turnOn() => print('on');
  void turnOff() => print('off');
}

// every class implicitly defines an interface
class SmartTV implements Television {
  @override
  void turnOn() => print('smart on');
  @override
  void turnOff() => print('smart off');
}

SmartTV().turnOn(); // smart on

abstract class Flyer {
  void fly();
}
abstract class Swimmer {
  void swim();
}

// implement multiple interfaces
class Duck implements Flyer, Swimmer {
  @override
  void fly() => print('flying');
  @override
  void swim() => print('swimming');
}

Duck().fly();   // flying
Duck().swim();  // swimming

class Base {
  void greet() => print('hello');
  void wave() => print('waving');
}

// extends: reuse implementation, single parent
class A extends Base {
  @override
  void greet() => print('A says hi');
}

// implements: contract only, must override all
class B implements Base {
  @override
  void greet() => print('B says hi');
  @override
  void wave() => print('B waving');
}

// Abstract class: share code among related types
abstract class Repository<T> {
  T? find(int id);          // abstract
  void save(T item) => print('saved'); // shared
}

// Interface: define a capability
abstract class Comparable<T> {
  int compareTo(T other);
}

class Product extends Repository<Product>
    implements Comparable<Product> {
  @override
  Product? find(int id) => null;
  @override
  int compareTo(Product other) => 0;
}

mixin Greeter {
  String get name;

  void greet() => print('Hello, $name!');
}

class User with Greeter {
  @override
  String name;
  User(this.name);
}

User('Alice').greet(); // Hello, Alice!

mixin Walker {
  void walk() => print('walking');
}
mixin Talker {
  void talk() => print('talking');
}

// mix in multiple mixins
class Person extends Object with Walker, Talker {
  String name;
  Person(this.name);
}

var p = Person('Bob');
p.walk(); // walking
p.talk(); // talking

mixin Musician on Performer {
  void playNote() => print('playing note');
}

abstract class Performer {
  void perform();
}

class Singer extends Performer with Musician {
  @override
  void perform() => print('singing');
}

Singer().playNote(); // playing note
// Musician can only be mixed into Performer subtypes

// 'mixin class' can be both extended and mixed in
mixin class Counter {
  int _count = 0;
  int get count => _count;
  void increment() => _count++;
}

class App extends Counter {}
class Tool with Counter {}

print(App().count); // 0
App().increment();
Tool().increment();
print(App().count); // 0 (separate instance)

mixin A {
  void hello() => print('A');
}
mixin B {
  void hello() => print('B');
}

class X with A, B {}
class Y with B, A {}

X().hello(); // B (later mixin wins)
Y().hello(); // A (later mixin wins)

// resolution order: class -> last mixin -> ... -> first mixin

class Stack<T> {
  final List<T> _items = [];

  void push(T item) => _items.add(item);
  T pop() => _items.removeLast();
  bool get isEmpty => _items.isEmpty;
}

var s = Stack<int>();
s.push(1);
s.push(2);
print(s.pop()); // 2

var names = Stack<String>();
names.push('Al');

// generic method with its own type parameter
T firstOrDefault<T>(List<T> list, T defaultValue) {
  return list.isEmpty ? defaultValue : list.first;
}

print(firstOrDefault<int>([1, 2, 3], 0)); // 1
print(firstOrDefault([], 'none'));        // none (type inferred)

// type often inferred from arguments

class Comparable<T> {
  int compareTo(T other);
}

// constrain T to subtypes of Comparable<T>
T max<T extends Comparable<T>>(T a, T b) {
  return a.compareTo(b) >= 0 ? a : b;
}

// number sum constraint
num sum<T extends num>(List<T> nums) =>
    nums.fold(0, (a, b) => a + b);

print(sum([1, 2.5, 3])); // 6.5

List<int> nums = [1, 2, 3];
Map<String, int> scores = {'a': 1, 'b': 2};
Set<double> uniq = {1.1, 2.2, 1.1};

// runtime type checks work (reified generics)
print(nums is List<int>);    // true
print(nums is List<String>); // false
print(scores is Map);        // true

// generic variance
void process(List<num> list) => print(list);
process(nums); // OK: int is a num

// generic function type alias
typedef Transformer<T> = T Function(T input);

int doubler(int x) => x * 2;
String upper(String s) => s.toUpperCase();

Transformer<int> dt = doubler;
Transformer<String> ut = upper;
print(dt(5));  // 10
print(ut('hi')); // HI

// generic class alias
typedef IntList = List<int>;
IntList xs = [1, 2, 3];

var nums = [3, 1, 2];
nums.sort();
print(nums);            // [1, 2, 3]
print(nums.reversed.toList()); // [3, 2, 1]
print(nums.indexOf(2)); // 1
print(nums.contains(3));// true
print(nums.where((n) => n > 1).toList()); // [2, 3]
print(nums.map((n) => n * 2).toList());   // [2, 4, 6]
print(nums.fold(0, (a, b) => a + b));     // 6

var a = {1, 2, 3};
var b = {2, 3, 4};
print(a.union(b));        // {1, 2, 3, 4}
print(a.intersection(b)); // {2, 3}
print(a.difference(b));   // {1}

var dedup = [1, 1, 2, 3, 3].toSet();
print(dedup.toList());    // [1, 2, 3]
print(dedup.contains(2)); // true

var ages = {'Alice': 30, 'Bob': 25};
ages['Carol'] = 28;
print(ages.keys);    // (Alice, Bob, Carol)
print(ages.values);  // (30, 25, 28)
print(ages.length);  // 3

ages.update('Bob', (v) => v + 1);
print(ages['Bob']);  // 26

ages.remove('Alice');
ages.forEach((k, v) => print('$k=$v'));

var mapped = ages.map((k, v) => MapEntry(k, v + 100));

var a = [1, 2];
var b = [0, ...a, 3];      // [0, 1, 2, 3]
print(b);

List<int>? maybe;
var c = [0, ...?maybe, 4]; // [0, 4] (null-spread is safe)
print(c);

var m1 = {'a': 1};
var m2 = {'b': 2, ...m1};  // {b: 2, a: 1}
print(m2);

var promo = true;
var menu = [
  'home',
  'products',
  if (promo) 'sale',
  'about',
];
print(menu); // [home, products, sale, about]

var nums = [1, 2, 3];
var doubled = [
  for (var n in nums) n * 2,
];
print(doubled); // [2, 4, 6]

// combine: [for (var x in xs) if (x > 0) x]

var nums = [1, 2, 3, 4, 5];
print(nums.any((n) => n > 4));   // true
print(nums.every((n) => n > 0)); // true
print(nums.firstWhere((n) => n > 2)); // 3
print(nums.reduce((a, b) => a + b));  // 15
var byParity = nums.fold(<bool, List<int>>{}, (m, n) {
  m[n.isOdd] = [...?m[n.isOdd], n]; return m;
});
print(byParity); // {true: [1,3,5], false: [2,4]}

Future<String> fetchUser() {
  return Future.delayed(Duration(seconds: 1), () => 'Alice');
}

void main() {
  fetchUser().then((name) {
    print('Got: $name'); // Got: Alice (after 1s)
  });
  print('waiting...');
}

Future<String> fetchUser() async {
  await Future.delayed(Duration(seconds: 1));
  return 'Alice';
}

Future<void> main() async {
  print('start');
  String name = await fetchUser();
  print('Got: $name');
  print('done');
}

Future<int> divide(int a, int b) async {
  if (b == 0) throw Exception('divide by zero');
  return a ~/ b;
}

Future<void> main() async {
  try {
    var result = await divide(10, 0);
    print(result);
  } catch (e) {
    print('Error: $e'); // Error: Exception: divide by zero
  } finally {
    print('done');
  }
}

Future<int> compute() async => 42;

compute()
    .then((v) => v * 2)
    .then((v) => print(v))      // 84
    .catchError((e) => print('err: $e'))
    .whenComplete(() => print('cleanup'));

// chaining transforms the result type
Future<String> fetch() async => 'data';
fetch().then((s) => s.length).then(print); // 4

Future<int> task(int n) async {
  await Future.delayed(Duration(milliseconds: n));
  return n;
}

// run in parallel, wait for all
var all = await Future.wait([task(100), task(50), task(200)]);
print(all); // [100, 50, 200]

// resolve with the first to complete
var first = await Future.any([task(100), task(50), task(200)]);
print(first); // 50 (fastest)

import 'dart:async';

Completer<String> completer = Completer<String>();

// complete the future from elsewhere
Future<String> get value => completer.future;

completer.complete('resolved!');

void main() async {
  print(await value); // resolved!
}

// useful when bridging callback-based APIs to Futures

Stream<int> countDown(int from) async* {
  while (from > 0) {
    await Future.delayed(Duration(seconds: 1));
    yield from;
    from--;
  }
}

void main() async {
  await for (var n in countDown(3)) {
    print(n); // 3, 2, 1 (one per second)
  }
}

var sub = countDown(3).listen(
  (n) => print('got $n'),
  onDone: () => print('done'),
  onError: (e) => print('err: $e'),
);

// pause/resume/cancel
sub.pause();
sub.resume();
// sub.cancel(); // stop listening

Stream<int> countDown(int from) async* {
  while (from > 0) yield from--;
}

var stream = Stream.fromIterable([1, 2, 3, 4]);

// transform like an Iterable
var evens = stream.where((n) => n.isEven);
var doubled = stream.map((n) => n * 2);

await for (var n in Stream.fromIterable([1,2,3]).map((n) => n * 10)) {
  print(n); // 10, 20, 30
}

print(await stream.first);  // 1
print(await stream.last);   // 4
print(await stream.length); // 4

import 'dart:async';

var controller = StreamController<int>();

// add events manually
controller.add(1);
controller.add(2);
controller.addError('oops');
controller.close();

controller.stream.listen(
  (n) => print(n),           // 1, 2
  onError: (e) => print(e),  // oops
  onDone: () => print('done'),
);

// use controller.addError/sink.add for errors

// await for consumes a stream like a loop
Future<int> sumStream(Stream<int> s) async {
  var total = 0;
  await for (var n in s) {
    total += n;
  }
  return total;
}

print(await sumStream(Stream.fromIterable([1, 2, 3]))); // 6

// broadcast stream: multiple listeners
var bc = StreamController<int>.broadcast();
bc.stream.listen(print);
bc.stream.listen((n) => print('got $n'));
bc.add(5); // both listeners receive 5

void checkAge(int age) {
  if (age < 0) {
    throw ArgumentError('age cannot be negative');
  }
  if (age > 150) {
    throw StateError('unrealistic age: $age');
  }
}

// you can throw any non-null object
void fail() => throw 'something went wrong';

try {
  checkAge(-5);
} on ArgumentError catch (e) {
  print('argument error: $e');
} on StateError catch (e) {
  print('state error: $e');
} catch (e, stackTrace) {
  print('unknown: $e');
  print(stackTrace);
} finally {
  print('always runs');
}

class InvalidCredentialsException implements Exception {
  final String message;
  InvalidCredentialsException(this.message);

  @override
  String toString() => 'InvalidCredentialsException: $message';
}

void login(String user, String pass) {
  if (user.isEmpty) {
    throw InvalidCredentialsException('username required');
  }
}

try {
  login('', 'x');
} on InvalidCredentialsException catch (e) {
  print(e); // InvalidCredentialsException: username required
}

Future<void> logErrors(Future<void> Function() action) async {
  try {
    await action();
  } catch (e) {
    print('logging error: $e');
    rethrow; // re-throw the caught exception
  }
}

void main() async {
  try {
    await logErrors(() async => throw Exception('fail'));
  } catch (e) {
    print('handled upstream: $e');
  }
}

// Exception: recoverable, expected to be caught
class MyException implements Exception {}

// Error: programming bugs, not meant to be caught
class MyError extends Error {
  @override
  String toString() => 'MyError: invalid state';
}

void risky() {
  throw MyError();   // bug: should fix the code
  throw MyException(); // runtime condition: catch it
}

// assert failures, type errors, range errors are Errors

// alias for a function type
typedef IntOperator = int Function(int, int);

int add(int a, int b) => a + b;
int mul(int a, int b) => a * b;

IntOperator op = add;
print(op(2, 3)); // 5
op = mul;
print(op(2, 3)); // 6

// pass as a parameter
void apply(IntOperator f, int a, int b) => print(f(a, b));

// generic function type alias
typedef Mapper<T, R> = R Function(T input);

String stringify(int n) => n.toString();
int lenOf(String s) => s.length;

Mapper<int, String> intToStr = stringify;
Mapper<String, int> strToLen = lenOf;

print(intToStr(42));  // '42'
print(strToLen('hi')); // 2

// Dart 2.13+: alias for any type, not just functions
typedef IntList = List<int>;
typedef StringMap<V> = Map<String, V>;

IntList nums = [1, 2, 3];
StringMap<int> scores = {'a': 1};

// alias for a record type (Dart 3)
typedef Point = ({double x, double y});
Point p = (x: 1.0, y: 2.0);
print(p.x); // 1.0

typedef Predicate<T> = bool Function(T);

bool isEven(int n) => n.isEven;

List<T> filter<T>(List<T> list, Predicate<T> test) {
  return list.where(test).toList();
}

print(filter([1, 2, 3, 4], isEven)); // [2, 4]
print(filter(['', 'a', ''], (s) => s.isNotEmpty)); // [a]

// typedef makes callback contracts explicit

// these two are equivalent
typedef Handler = void Function(String event);

class EventBus {
  // using typedef
  void on(Handler handler) {}
  // using inline type
  void on2(void Function(String) handler) {}
}

// both accept the same functions
void myHandler(String e) => print(e);
EventBus().on(myHandler);
EventBus().on2(myHandler);

enum Color { red, green, blue }

var c = Color.red;
print(c);           // Color.red
print(c.name);      // 'red'
print(c.index);     // 0
print(Color.values); // [Color.red, Color.green, Color.blue]
print(Color.green.index); // 1

enum Vehicle {
  car('Car', 4),
  bike('Bike', 2),
  truck('Truck', 6);

  final String label;
  final int wheels;
  const Vehicle(this.label, this.wheels);

  int get axles => wheels ~/ 2;
}

print(Vehicle.car.label);   // Car
print(Vehicle.bike.wheels); // 2
print(Vehicle.truck.axles); // 3

enum Status { pending, active, done }

// iterate all values
for (var s in Status.values) {
  print(s.name);
}

// exhaustive switch (no default needed)
String label(Status s) => switch (s) {
  Status.pending => 'Waiting',
  Status.active => 'Running',
  Status.done => 'Finished',
};

print(label(Status.active)); // Running

enum HttpStatus {
  ok(200),
  notFound(404),
  serverError(500);

  final int code;
  const HttpStatus(this.code);

  bool get isSuccess => code >= 200 && code < 300;
  String get reason => switch (this) {
    ok => 'OK',
    notFound => 'Not Found',
    serverError => 'Internal Server Error',
  };
}

print(HttpStatus.ok.isSuccess);   // true
print(HttpStatus.notFound.reason); // Not Found

enum Priority { low, medium, high }

var a = Priority.low;
var b = Priority.high;

print(a == b);           // false
print(a == Priority.low);// true
print(a.index < b.index);// true
print(identical(a, Priority.low)); // true

// enums are singletons: only one instance per value
// use == for equality, not identical (though both work)

class Animal {
  @override
  String toString() => 'Animal';

  @Deprecated('use newName instead')
  String oldName = 'x';
  String newName = 'x';

  @protected
  void internalMethod() {}

  @visibleForTesting
  String testHook() => 'test';
}

class Base {
  void greet() {}
  String name = 'base';
}

class Derived extends Base {
  @override
  void greet() => print('hi');

  // @override verifies the parent method exists
  // typo here would be a compile error:
  // @override void greeet() {}
}

// @Deprecated emits a warning at the call site
@Deprecated('use bar()')
void foo() {}
void bar() {}

// a custom annotation is just a const constructor class
class Todo {
  final String msg;
  const Todo(this.msg);
}

class Service {
  @Todo('refactor to use cache')
  void fetchData() {}

  @Todo('add tests')
  void process() {}
}

// annotations are accessed via dart:mirrors (VM) or
// code generation (build_runner) in practice

import 'package:meta/meta.dart';
// requires the 'meta' package

@immutable
class User {
  final String name;
  final int age;
  const User(this.name, this.age);
}

// with package:json_annotation / json_serializable
// @JsonSerializable()
// class Product {
//   final String id;
//   Product(this.id);
//   factory Product.fromJson(Map<String, dynamic> j) => ...;
// }

class Required {
  const Required([this.reason]);
  final String? reason;
}

class Service {
  // annotation on a parameter
  void create({
    @Required('name is mandatory') String? name,
    @protected int? internal,
  }) {
    print(name);
  }
}

// @required is built into Dart (the 'required' keyword
// is preferred in Dart 2.12+ for null safety)

// import an entire library
import 'dart:io';
import 'package:http/http.dart';

// import only specific names
import 'dart:math' show Random, pi;

// hide specific names
import 'dart:async' hide Timer;

// prefix to avoid name clashes
import 'package:http/http.dart' as http;
http.get(Uri.parse('https://example.com'));

// library.dart
library my_lib;

// split implementation across files
part 'src/widget_a.dart';
part 'src/widget_b.dart';

// re-export another library's API
export 'src/utils.dart' show formatDate, parseDate;

// users import library.dart and get everything

// names starting with _ are library-private
class _Internal {
  void _helper() {}
}

class Public {
  String _secret = 'hidden';   // private field
  String name = 'visible';     // public field

  String _process() => 'internal';
  String reveal() => _process();
}

// _secret is accessible anywhere in the SAME library/file
// but not from other libraries that import this file

// load a library on demand (web only)
import 'package:heavy_lib/heavy.dart' deferred as heavy;

Future<void> main() async {
  // library is NOT loaded until this call
  await heavy.loadLibrary();
  heavy.SomeClass().doWork();
}

// useful for splitting large web bundles and
// loading rarely-used features only when needed

// explicit library declaration
library my_package.utils;

import 'dart:math';

part 'src/helper.dart';

const double version = 1.0;

// a 'library' name is optional in modern Dart;
// it's mainly used with part/part-of and tooling
// most files omit it and are treated as anonymous libraries

// shapes.dart
library shapes;
part 'circle.dart';
part 'square.dart';

class Shape {}

// circle.dart
part of shapes;
class Circle extends Shape {}

// files in the same library share private members
// (_name visible across all parts)

int a = 42;          // non-nullable: cannot be null
int? b;              // nullable: can be null
print(b);            // null
b = 10;
print(b);            // 10

String name = 'Al';  // non-nullable
String? middle;      // nullable

// non-nullable types are guaranteed non-null
// int x = null;      // ERROR
// print(a.length);   // safe: a is non-null

String? maybeName;
// int len = maybeName.length; // ERROR: maybeName is nullable

maybeName = 'Alice';
int len = maybeName!.length;  // ! asserts non-null
print(len); // 5

// throws if null at runtime:
// String? n; print(n!.length); // NoSuchMethodError/null

String? name;
print(name?.length);     // null (safe access)
print(name?.length ?? 0);// 0 (default if null)

name = 'Alice';
print(name?.length);     // 5

name ??= 'Bob';          // assign only if null
print(name);             // Alice (already set)

List<int>? list;
print(list?.first);      // null
print(list?.first ?? -1);// -1

String? name;
if (name != null) {
  // name is promoted to non-nullable String here
  print(name.length); // safe, no ! needed
}

// promoted via is check
Object obj = 'hello';
if (obj is String) {
  print(obj.length); // smart-cast to String
}

// promoted via assignment
int? x;
x = 5;
print(x.abs()); // x is non-null after assignment

class Config {
  // late: non-nullable, initialized later
  late final String value = _load();

  // late without initializer: assign before first use
  late final int computed;
  Config() {
    computed = expensive();
  }

  String _load() => 'loaded';
  int expensive() => 42;
}

// required: mandatory named parameter
void build({required String name}) {}
build(name: 'Al'); // OK
// build(); // ERROR: missing required

class Service {
  // lazy: _expensive runs only on first access
  late final int cache = _expensive();

  int _expensive() {
    print('computing...');
    return 42;
  }
}

var s = Service();
print('created');
print(s.cache); // computing... 42
print(s.cache); // 42 (cached, no recompute)

// late fields with initializers are evaluated lazily