【每日一包0026】delegates

本文详细介绍了koa2源码中的委托机制,通过delegates模块实现对象属性代理,包括方法代理、属性获取代理、属性赋值代理及双向代理等。并通过示例展示了如何将request上的方法直接代理到ctx上,以及如何对属性进行读写操作。

[github地址:https://github.com/ABCDdouyae...]

delegates (koa2源码依赖)
委托机制,用于对象属性代理
Delegate(proto, prop)创建一个代理实例,使用proto对象下的prop对象作为被代理者
method(name) 接受一个方法,进行方法代理

将request上的方法直接代理到ctx上

const delegate = require('delegates');

var ctx = {};

ctx.request = {
  fn: function(i){return i}
};

delegate(ctx, 'request')
    .method('fn');

console.log(ctx.fn(1))
getter(name) 属性的获取被代理
var ctx = {
    request:{
        url: 'localhost:8080'
    }
};

delegate(ctx, 'request')
     .getter('url')


console.log(ctx.url);//localhost:8080
setter(name) 属性的赋值代理
var ctx = {
    request:{}
}

delegate(ctx, 'request')
    .setter('other')

ctx.other = '1';

console.log(ctx.request.other)//1
access(name) 赋值和获取值得双向代理
var ctx = {
    request: {}
}

delegate(ctx, 'request')
   .access('method')

ctx.method = 'POST';
console.log(ctx.request.method);//'POST'

ctx.request.method = 'GET';

console.log(ctx.method);//'GET'
fluent(name) 被代理者中该属性有值就返回该值,没有的话可以通过函数调用设置,返回ctx对象
var ctx = {
    request:{
        a : 1
    }
}

delegate(ctx, 'request')
    .fluent('a')

console.log(ctx.a())//1
console.log(ctx.a(2))//{ request: { a: 2 }, a: [Function] }
console.log(ctx.a())//2
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### C# Delegates vs Python Function Pointers and Callable Objects In the context of programming languages like C# and Python, both offer mechanisms to pass functions as arguments or store them in variables. However, these two languages implement this concept differently. #### C# Delegates A delegate in C# is a type that represents references to methods with a particular parameter list and return type[^1]. The primary use case for delegates includes event handling within .NET applications but extends far beyond just events. A simple example demonstrates how one can define and invoke a delegate: ```csharp // Define a delegate that takes no parameters and returns void. public delegate void SimpleDelegate(); class Program { static void Main() { // Instantiate the delegate with a method reference. SimpleDelegate myDelegate = new SimpleDelegate(SayHello); // Invoke the delegate. myDelegate(); } public static void SayHello() { Console.WriteLine("Hello from Delegate!"); } } ``` Delegates are strongly typed which means they enforce compile-time checking on signatures ensuring only compatible methods get assigned to them. #### Python Function Pointers and Callable Objects Unlike C#, where delegates provide strong typing over function pointers, Python uses dynamic typing allowing any object implementing `__call__` protocol to be treated as callable. This flexibility enables not only regular functions but also classes defining custom behavior through special methods such as `__call__`. Here's an illustration using plain functions alongside class-based callables: ```python def greet(name="World"): """Simple greeting function.""" print(f"Hello {name}!") class Greeter: def __init__(self, message): self.message = message def __call__(self, name="Everyone"): """Callable instance method""" print(f"{self.message}, {name}") greet() greeter_instance = Greeter("Good morning") greeter_instance() # Assigning either directly works due to duck-typing nature of Python. callable_objects = [greet, greeter_instance] for obj in callable_objects: obj() ``` This approach provides greater flexibility at runtime since there isn't strict enforcement regarding argument count/types until actual invocation occurs. #### Comparison Summary While both approaches allow passing around executable code blocks, key differences lie in their design philosophies: - **Type Safety**: C#'s delegate system enforces stricter rules about what kinds of methods may be referenced by each delegate signature versus Python’s more permissive model relying heavily upon conventions rather than compiler checks. - **Syntax Complexity**: Defining and working with delegates requires explicit syntax elements (`delegate`, instantiation), whereas Python leverages its general-purpose calling mechanism without requiring additional keywords specific solely to callbacks. - **Use Cases**: Although originally intended primarily for GUI frameworks and asynchronous operations, modern usage spans across various domains including functional reactive programming patterns; meanwhile, Python treats all first-class citizens equally making it suitable even outside traditional callback scenarios. --related questions-- 1. How does lambda expression support differ between C# and Python? 2. What advantages do anonymous inner classes have over delegates when used inside Java compared to C#? 3. Can you explain multicast delegates in C#? Are similar constructs available in Python? 4. In terms of performance characteristics, how do C# delegates compare against Python's built-in types supporting the callable interface? 5. Explore advanced features provided by C# language related specifically towards enhancing delegate functionality.
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