承诺不可变及其保证价值意味着什么? [英] What does it mean for promises to be immutable and their guaranteed value?

问题描述

我试图了解 es6承诺与常规回调之间的区别，但不要不能得到下面的例子.有人可以显示在回调中执行以下操作的样子吗?

I'm trying to understand the differences between es6 promises and regular callbacks but don't get the examples below. Can someone show what it would look like to do the below with callbacks?

// an immediately resolved promise
var p2 = Promise.resolve("foo"); 

// can get it after the fact, unlike events
p2.then((res) => console.log(res)); 

var p = new Promise(function(resolve, reject) {  
   setTimeout(() => resolve(4), 2000);
});

// handler can't change promise, just value
p.then((res) => {  
  res += 2;  
  console.log(res);
});

// still gets 4
p.then((res) => console.log(res));

推荐答案

要与标准回调系统进行比较，让我们创建一个可以生成此类通知程序对象的类.它的接口非常类似于Promise的接口，但是它实现了一个简单的回调系统:

To make the comparison with a standard callback system, let's create a class that can produce such notifier objects. It will have an interface much like Promise has, but which implements a simple callback system:

class Notifier {
    constructor(executor) {
        // The object will maintain a list of callbacks
        this.callbacks = [];
        // The executor is executed now and will allow the client
        // to determine the logic of this notifier object:
        // ...when the callbacks need to be called and with which value:
        executor( (res) => {
            // The client can call this resolve function to indicate
            // the value. So now the callbacks need to be called with it:
            this.callbacks.forEach(callback => callback(res));
        });
    }
    addListener(callback) {
        // This method resembles the `then` of promises: it allows
        // a client to pass a callback function, that should be called
        // when the value becomes available (i.e. when the event triggers).
        this.callbacks.push(callback);
    }
}; 

因此，与Promise一样，您可以将一个函数传递给此类的构造函数以执行一些工作并在适当的时间指示该值.您还可以将侦听器附加到该侦听器，该侦听器将在值可用时被调用.

So, like with Promise, you can pass to the constructor of this class a function to do some work and indicate the value at the appropriate time. You can also attach listeners to it, which will be called at the moment the value becomes available.

这最后一句话强调了一个重要的事实:如果该值可用，但是您尚未附加侦听器(回调)，即使您在事实之后附加了侦听器，也会错过通知.

This last phrase highlights an important fact: if the value becomes available, but you did not attach a listener (callback) yet, you'll miss the notification, even if you attach the listener after the facts.

这是基于回调的代码，您可以将其与本文引用的代码进行比较:

Here is the callback-based code which you could compare with the code you quoted from the article:

class Notifier {
    constructor(executor) {
        // The object will maintain a list of callbacks
        this.callbacks = [];
        // The executor is executed now and will allow the client
        // to determine the logic of this notifier object:
        // ...when the callbacks need to be called and with which value:
        executor( (res) => {
            // The client can call this resolve function to indicate
            // the value. So now the callbacks need to be called with it:
            this.callbacks.forEach(callback => callback(res));
        });
    }
    addListener(callback) {
        // This method resembles the `then` of promises: it allows
        // a client to pass a callback function, that should be called
        // when the value becomes available (i.e. when the event triggers).
        this.callbacks.push(callback);
    }
}; 

// a notifier that immediately notifies the result
f2 = new Notifier( (resolve) => resolve("foo") );
// but since no-one was listening, no callback is called.

// canNOT get it after the fact, unlike promises
f2.addListener((res) => console.log(res));
// ... nothing gets called or printed: we are too late.

// 
var f = new Notifier(function(resolve) {  
   setTimeout(() => resolve({ data: 4}), 2000);
});

// handler CAN change the outcome
f.addListener((res) => {  
  res.data += 2;
  console.log(res.data);
});

// ... now also this one gets 6!
f.addListener((res) => console.log(res.data));

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