# Dart Quick Reference

*Types, functions, classes, async, null safety, collections*

> Source: Dart Documentation (dart.dev) · MIT

## Basics

### Hello World

```
void main() {
  print('Hello, Dart!');
}
```

### Variables

```
var name = 'Dart';       // type inferred
String lang = 'Dart';    // explicit type
final pi = 3.14;         // runtime constant
const max = 100;         // compile-time constant
```

### String Interpolation

```
var name = 'World';
print('Hello, $name!');
print('1 + 1 = ${1 + 1}');
```

## Types

### Built-in Types

| Command | Description |
|---------|-------------|
| `int` | 64-bit integer |
| `double` | 64-bit floating point |
| `num` | Supertype of int and double |
| `String` | UTF-16 string |
| `bool` | true or false |
| `List` | Ordered collection (array) |
| `Set` | Unordered unique collection |
| `Map` | Key-value pairs |
| `dynamic` | Any type, disables static checking |
| `void` | No return value |

### Type Checks & Casts

```
if (obj is String) print(obj.length);
var s = obj as String;   // cast
print(obj.runtimeType);  // runtime type
```

## Functions

### Function Syntax

```
int add(int a, int b) => a + b;
void greet({required String name}) {
  print('Hello, $name');
}
```

### Parameters

| Command | Description |
|---------|-------------|
| `int f(int a)` | Required positional parameter |
| `int f([int a = 0])` | Optional positional with default |
| `f({required int a})` | Required named parameter |
| `f({int a = 0})` | Optional named with default |

### Closures & Tearoffs

```
var square = (int n) => n * n;
[1, 2, 3].map((e) => e * 2);
[1, 2, 3].forEach(print);  // tearoff
```

## Control Flow

### Conditionals

```
if (x > 0) { print('pos'); }
else if (x == 0) { print('zero'); }
else { print('neg'); }
var result = x > 0 ? 'pos' : 'neg';
```

### Loops

```
for (var i = 0; i < 5; i++) { }
for (var item in list) { }
while (x > 0) { x--; }
do { x--; } while (x > 0);
```

### Switch & Pattern Matching

```
switch (color) {
  case 'red':   print('R'); break;
  case 'blue':  print('B'); break;
  default:      print('?');
}
```

## Classes

### Class Definition

```
class Point {
  final double x, y;
  Point(this.x, this.y);
  double distanceTo(Point p) =>
    sqrt(pow(x - p.x, 2) + pow(y - p.y, 2));
}
```

### Named & Factory Constructors

```
class Point {
  double x, y;
  Point(this.x, this.y);
  Point.origin() : x = 0, y = 0;
  factory Point.fromJson(Map j) =>
    Point(j['x'], j['y']);
}
```

### Inheritance

```
class Animal { void speak() {} }
class Dog extends Animal {
  @override
  void speak() => print('Woof');
}
```

## Mixins & Extensions

### Mixins

```
mixin Flyable {
  void fly() => print('Flying');
}
class Bird with Flyable {}
```

### Extension Methods

```
extension StringX on String {
  String capitalize() =>
    '${this[0].toUpperCase()}${substring(1)}';
}
print('hello'.capitalize()); // Hello
```

### Abstract & Implements

```
abstract class Shape {
  double area();
}
class Circle implements Shape {
  final double r;
  Circle(this.r);
  @override double area() => pi * r * r;
}
```

## Async/Await

### Futures

```
Future<String> fetchData() async {
  var res = await http.get(uri);
  return res.body;
}
```

### Streams

```
Stream<int> count(int n) async* {
  for (var i = 0; i < n; i++) {
    yield i;
  }
}
```

### Error Handling

```
try {
  var data = await fetchData();
} on HttpException catch (e) {
  print('HTTP error: $e');
} catch (e) {
  print('Error: $e');
}
```

## Collections

### List Operations

```
var nums = [1, 2, 3];
nums.add(4);
nums.where((n) => n > 2);     // [3, 4]
nums.map((n) => n * 2);        // [2,4,6,8]
var sorted = nums..sort();
```

### Map Operations

```
var m = {'a': 1, 'b': 2};
m['c'] = 3;
m.containsKey('a');  // true
m.entries.map((e) => '${e.key}=${e.value}');
```

### Spread & Collection If/For

```
var all = [0, ...nums];
var nav = ['Home', if (isAdmin) 'Admin'];
var sq = [for (var i in nums) i * i];
```

## Null Safety

### Nullable Types

| Command | Description |
|---------|-------------|
| `int?` | Nullable int (can be null) |
| `int` | Non-nullable int (never null) |
| `!` | Null assertion operator |
| `?.` | Null-aware access |
| `??` | Default if null |
| `??=` | Assign if null |
| `late` | Deferred initialization |

### Null Safety Examples

```
String? name;            // nullable
int len = name?.length ?? 0;
late final String title; // set before use
name ??= 'default';     // assign if null
```

## Common Patterns

### Enum with Values

```
enum Color {
  red('FF0000'), green('00FF00');
  final String hex;
  const Color(this.hex);
}
```

### Records & Destructuring

```
(String, int) userInfo() => ('Alice', 30);
var (name, age) = userInfo();
print('$name is $age');
```

### Sealed Classes

```
sealed class Shape {}
class Circle extends Shape { final double r; Circle(this.r); }
class Rect extends Shape { final double w, h; Rect(this.w, this.h); }
```