【C#】委托

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委托

委托（delegate）是一种类型，定义了一种方法签名，因此可以将方法作为参数进行传递。
委托类似于 C++ 中的函数指针，但比函数指针更加安全和灵活。
使用委托，可以实现事件处理、回调函数等功能。
建议：使用接口取代委托

• Action
  • 无参无返回
• Func
  • 有参有返回

        Calaculator calaculator = new Calaculator();
        
        // Action
        new Action(calaculator.Report).Invoke();
        
        // Func
		Func<int, int, int> func1 = new Func<int, int, int>(calaculator.Add);
		Func<int, int, int> func2 = new Func<int, int, int>(calaculator.Sub);
		
		int x = 100;
		int y = 200;
		int z = 0;

		// 可以省略invoke
		z = func1(x, y);
		Console.WriteLine(z);

		z = func2.Invoke(x, y);
		Console.WriteLine(z);

class Calaculator {
    public void Report()
    {
        Console.WriteLine("i have 3 methods");
    }

    public int Add(int a, int b)
    {
        return a + b;
    }

    public int Sub(int a, int b)
    {
        return a - b;
    }
}
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自定义委托

// 与类平级，放到类中属于嵌套
public delegate double Calc(double x,double y);
internal class Program
{
    static void Main(string[] args)
    {
        Calaculator calaculator = new Calaculator();
        Calc calc1 = new Calc(calaculator.Add1);
        Calc calc2 = new Calc(calaculator.Mul);

        double x = 100;
        double y = 100;
        double z = 0;
        z = calc1.Invoke(x, y);
        Console.WriteLine(z);
        z = calc2.Invoke(x, y);
        Console.WriteLine(z);
	}
}
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模板方法（工厂模式

”借用“指定的外部方法来产生结果 ， 提高对代码的复用
ProductFactory() 只需要扩展这个方法，其他方法都不用动

    static void Main(string[] args)
    {
        ProductFactory productFactory = new ProductFactory();
        WrapFactory wrapFactory = new WrapFactory();

        Func<Product> func1 = new Func<Product>(productFactory.MakeToy);
        Func<Product> func2 = new Func<Product>(productFactory.MakePizza);

        Box box1 = wrapFactory.WrapProduct(func1);
        Box box2 = wrapFactory.WrapProduct(func2);

        Console.WriteLine(box1.Product.Name);
        Console.WriteLine(box2.Product.Name);
    }
}

class Product() { 
    public string Name { get; set; }
}

class Box() { 
    public Product Product { get; set; }
}

class WrapFactory {
    public Box WrapProduct(Func<Product> getProduct) {
        Box box = new Box();
        // invoke 执行拿到产品（不用管是什么产品
        Product product = getProduct.Invoke();
        box.Product = product;
        return box;
    }
}

class ProductFactory() {
    public Product MakePizza() {
        Product product = new Product();
        product.Name = "Pizza";
        return product;
    }
    public Product MakeToy() {
        Product product = new Product();
        product.Name = "Toy";
        return product;
    }
}
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回调(callback)函数（观察者模式

调用指定的外部方法
在模板方法基础上添加

internal class Program
{
    static void Main(string[] args)
    {
        ProductFactory productFactory = new ProductFactory();
        WrapFactory wrapFactory = new WrapFactory();

        Func<Product> func1 = new Func<Product>(productFactory.MakeToy);
        Func<Product> func2 = new Func<Product>(productFactory.MakePizza);

        Logger logger = new Logger();
        Action<Product> action = new Action<Product>(logger.Log);

        Box box1 = wrapFactory.WrapProduct(func1,action);
        Box box2 = wrapFactory.WrapProduct(func2,action);

        Console.WriteLine(box1.Product.Name);
        Console.WriteLine(box2.Product.Name);
    }
}

class Logger {
    public void Log(Product product) {
        //DateTime.UtcNow 无时区时间 ; DateTime.Now 有时区
        Console.WriteLine("Product '{0}' created at {1}.Price is {2}",product.Name,DateTime.UtcNow,product.Price);
    }
}

class Product() { 
    public string Name { get; set; }
    public double Price{ get; set; }
}

class Box() { 
    public Product Product { get; set; }
}

class WrapFactory {
    public Box WrapProduct(Func<Product> getProduct,Action<Product> logCallback) {
        Box box = new Box();
        Product product = getProduct.Invoke();

        // 回调函数：触发某种条件就自动执行
        if (product.Price >= 50) {
            logCallback(product);
        }

        box.Product = product;
        return box;
    }
}

class ProductFactory() {
    public Product MakePizza() {
        Product product = new Product();
        product.Name = "Pizza";
        product.Price = 12;
        return product;
    }
    public Product MakeToy() {
        Product product = new Product();
        product.Name = "Toy";
        product.Price=120;
        return product;
    }
}
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多播（multicast）委托

委托的高级使用

使用接口 重构 模板方法代码

直接用接口，不使用委托。

方法形参为接口，类继承接口，new不同的类传进同个方法

internal class Program
{
    static void Main(string[] args)
    {
        WrapFactory wrapFactory = new WrapFactory();

        Logger logger = new Logger();
        Action<Product> action = new Action<Product>(logger.Log);

        Box box1 = wrapFactory.WrapProduct(new PizzaFactory());
        Box box2 = wrapFactory.WrapProduct(new ToyCarFactory());

        Console.WriteLine(box1.Product.Name);
        Console.WriteLine(box2.Product.Name);
    }
}

interface IProductFactory {
    Product Make();
}

class PizzaFactory : IProductFactory
{
    public Product Make()
    {
        Product product = new Product();
        product.Name = "Pizza";
        product.Price = 12;
        return product;
    }
}

class ToyCarFactory : IProductFactory
{
    public Product Make()
    {
        Product product = new Product();
        product.Name = "Toy";
        product.Price = 120;
        return product;
    }
}
class WrapFactory {
    public Box WrapProduct(IProductFactory productFactory) {
        Box box = new Box();
        Product product = productFactory.Make();
        box.Product = product;
        return box;
    }
}
}
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注意

委托：

参考

刘铁猛——C#