异步构造函数 [英] Asynchronous constructor

问题描述

我如何才能最好地处理以下情况?

我有一个需要一段时间才能完成的构造函数.

var Element = function Element(name){this.name = 名称；this.nucleus = {};this.load_nucleus(name);//这可能需要一秒钟.}var 氧 = 新元素(‘氧’)；控制台日志(氧气.核)；//返回 {}，因为 load_nucleus 还没有完成.

我看到了三个选项，每个选项都显得与众不同.

一，给构造函数添加一个回调.

var Element = function Element(name, fn){this.name = 名称；this.nucleus = {};this.load_nucleus(名称，函数(){fn();//现在继续.});}Element.prototype.load_nucleus(name, fn){fs.readFile(name+'.json', function(err, data) {this.nucleus = JSON.parse(data);fn();});}var 氧气 = 新元素('氧气'，函数(){控制台日志(氧气.核)；});

二，使用 EventEmitter 来发出一个 'loaded' 事件.

var Element = function Element(name){this.name = 名称；this.nucleus = {};this.load_nucleus(name);//这可能需要一秒钟.}Element.prototype.load_nucleus(名称){var self = this;fs.readFile(name+'.json', function(err, data) {self.nucleus = JSON.parse(data);self.emit('加载');});}util.inherits(Element, events.EventEmitter);var 氧 = 新元素(‘氧’)；氧气.一次('加载'，功能(){控制台日志(this.nucleus)；});

或者三个，阻塞构造函数.

var Element = function Element(name){this.name = 名称；this.nucleus = {};this.load_nucleus(name);//这可能需要一秒钟.}Element.prototype.load_nucleus(name, fn){this.nucleus = JSON.parse(fs.readFileSync(name+'.json'));}var 氧 = 新元素(‘氧’)；控制台.日志(氧.核)

但我以前从未见过这样做过.

我还有哪些选择?

解决方案

鉴于在 Node 中避免阻塞的必要性，事件或回调的使用并不奇怪⁽¹⁾.>

稍微编辑两个，你可以将它与一个合并:

var Element = function Element(name, fn){this.name = 名称；this.nucleus = {};if (fn) this.on('loaded', fn);this.load_nucleus(name);//这可能需要一秒钟.}...

虽然，就像示例中的 fs.readFile 一样，核心 Node API(至少)通常遵循静态函数的模式，在数据准备好时公开实例:

var Element = function Element(name,nucleus) {this.name = 名称；this.nucleus = 细胞核；};Element.create = 函数(名称，fn){fs.readFile(name+'.json', function(err, data) {无功核= 错误?null : JSON.parse(data);fn(错误，新元素(名称，核))；});};Element.create('oxygen', function (err, elem) {如果(！错误){控制台日志(元素名称，元素.核)；}});

<小时>

(1) 读取 JSON 文件应该不会花费很长时间.如果是的话，也许是为了数据而改变了存储系统.

How can I best handle a situation like the following?

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.

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

One, add a callback to the constructor.

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);
});

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.once('loaded', function(){
   console.log(this.nucleus);
});

Or three, block the constructor.

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)

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

What other options do I have?

解决方案

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

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.
}

...

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) 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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