异步构造 [英] Asynchronous constructor

问题描述

我怎样才能最好地处理类似下面的情况呢？

How can I best handle a situation like the following?

我有一个构造函数，需要一段时间才能完成。

I have a constructor that takes a while to complete.

var Element = function Element(name){
   this.name = name;
   this.nucleus = {};

   this.load_nucleus(name); // This might take a second.
}

var oxygen = new Element('oxygen');
console.log(oxygen.nucleus); // Returns {}, because load_nucleus hasn't finished.

我看到三个选项，每个看起来与众不同。

I see three options, each of which seem out of the ordinary.

一，回调添加到构造。

var Element = function Element(name, fn){
   this.name = name;
   this.nucleus = {};

   this.load_nucleus(name, function(){
      fn(); // Now continue.
   });
}

Element.prototype.load_nucleus(name, fn){
   fs.readFile(name+'.json', function(err, data) {
      this.nucleus = JSON.parse(data); 
      fn();
   });
}

var oxygen = new Element('oxygen', function(){  
   console.log(oxygen.nucleus);
});

两个后，使用EventEmitter发出一个'装'事件。

Two, use EventEmitter to emit a 'loaded' event.

var Element = function Element(name){
   this.name = name;
   this.nucleus = {};

   this.load_nucleus(name); // This might take a second.
}

Element.prototype.load_nucleus(name){
   var self = this;
   fs.readFile(name+'.json', function(err, data) {
      self.nucleus = JSON.parse(data); 
      self.emit('loaded');
   });
}

util.inherits(Element, events.EventEmitter);

var oxygen = new Element('oxygen');
oxygen.on('loaded', function(){
   console.log(this.nucleus);
});

也可能是3 ，阻止构造函数。

var Element = function Element(name){
   this.name = name;
   this.nucleus = {};

   this.load_nucleus(name); // This might take a second.
}

Element.prototype.load_nucleus(name, fn){
   this.nucleus = JSON.parse(fs.readFileSync(name+'.json'));
}

var oxygen = new Element('oxygen');
console.log(oxygen.nucleus)

但我还没有看到任何这样做过。

But I haven't seen any of this done before.

什么其他的选择呢我？

推荐答案

由于必要性，以避免堵塞节点，使用事件或回调不那么陌生^（1）。

Given the necessity to avoid blocking in Node, the use of events or callbacks isn't so strange⁽¹⁾.

通过两个轻微的编辑，你可以用一个合并的：

With a slight edit of Two, you could merge it with One:

var Element = function Element(name, fn){
    this.name = name;
    this.nucleus = {};

    if (fn) this.on('loaded', fn);

    this.load_nucleus(name); // This might take a second.
}

...

不过，像 fs.readFile 在你的榜样，核心节点API（至少）经常跟随时的数据，揭露实例静态函数模式准备：

Though, like the fs.readFile in your example, the core Node APIs (at least) often follow the pattern of static functions that expose the instance when the data is ready:

var Element = function Element(name, nucleus) {
    this.name = name;
    this.nucleus = nucleus;
};

Element.create = function (name, fn) {
    fs.readFile(name+'.json', function(err, data) {
        var nucleus = err ? null : JSON.parse(data);
        fn(err, new Element(name, nucleus));
    });
};

Element.create('oxygen', function (err, elem) {
    if (!err) {
        console.log(elem.name, elem.nucleus);
    }
});

---

（1）它应该不会很长读取JSON文件。如果是，也许在存储系统中的改变是为了使数据

---

(1) It shouldn't take very long to read a JSON file. If it is, perhaps a change in storage system is in order for the data.

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