Dart / Flutter 泛型（Generics）

泛型类

class Box<T> {
  T value;

  Box(this.value);

  void show() {
    print("Value: $value");
  }
}

var intBox = Box<int>(123);
intBox.show();  // Value: 123

var strBox = Box<String>("hello");
strBox.show();  // Value: hello

泛型方法

T echo<T>(T data) {
  return data;
}

print(echo<int>(456));       // 456
print(echo<String>("dart")); // dart

泛型约束

class Animal {
  void eat() {
    print("Animal eats");
  }
}

class Dog extends Animal {
  void bark() {
    print("Dog barks");
  }
}

// 泛型约束 T 必须继承自 Animal
class Cage<T extends Animal> {
  T occupant;

  Cage(this.occupant);

  void show() {
    occupant.eat();
  }
}

var dogCage = Cage<Dog>(Dog());
dogCage.show();  // Animal eats

多泛型

class Pair<K, V> {
  K key;
  V value;

  Pair(this.key, this.value);

  void show() {
    print("$key : $value");
  }
}

var pair = Pair<String, int>("age", 18);
pair.show();   // age : 18