Constructors

🎯 Mục tiêu: Các loại constructors trong Dart.

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💡 Default Constructor

class Person {
  String name;
  int age;

  // Short syntax
  Person(this.name, this.age);

  // Full syntax
  // Person(String name, int age) {
  //   this.name = name;
  //   this.age = age;
  // }
}

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📝 Named Constructor

class Point {
  double x, y;

  Point(this.x, this.y);

  Point.origin() : x = 0, y = 0;
  Point.fromJson(Map<String, double> json)
      : x = json['x']!,
        y = json['y']!;
}

var p1 = Point(10, 20);
var p2 = Point.origin();
var p3 = Point.fromJson({'x': 5, 'y': 10});

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🔧 Const Constructor

class ImmutablePoint {
  final double x, y;

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

// Compile-time constant
const origin = ImmutablePoint(0, 0);

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🎯 Factory Constructor

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

  final String name;

  factory Logger(String name) {
    return _cache.putIfAbsent(name, () => Logger._internal(name));
  }

  Logger._internal(this.name);
}

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✅ Checklist

• this. shorthand
• Named constructors
• const constructors
• factory cho singletons/caching

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