Question

var o = new X();
o.setFooCallback1(function(result1){
});
o.setFooCallback2(function(result2){
});
o.foo("xxx");  

as you can see, when I call o.foo(), there’re two callbacks will be fired with two results, result1 and result2, what I want to do is use pass result1 and result2 to my constructor function to create an object:

var y = new Y(result1, result2);  

But result1 and result2 come in different time(asynchronous)， how could I handle this?
ps: the class X is from others’ library, I can’t modify it’s implemention

Answer 1

You need to implement what is known as the semaphore pattern

Here is a hands-on implementation:

var o = new X()
  , n = 0
  , result1, result2

function checkResults(){
    if (--n > 0) return;
    var y = new Y(result1, result2)
}

o.setFooCallback1(function(res){
    result1 = res
    checkResults()
})

o.setFooCallback2(function(res){
    result2 = res
    checkResults()
})

Or an object-oriented approach:

function Semaphore(callback){
    this.callback = callback
    this.count = 0
    this.args = []
}
Semaphore.prototype.check = function(){
    if (--this.count <= 0)
        this.callback.apply(null, this.args)
}
Semaphore.prototype.queue = function(){
    var self = this
    self.count++
    return function(res){
        self.args.push(res)
        self.check()
    }
}

var fooResults = new Semaphore(function(res1, res2){
    var y = new Y(res1, res2)
})

o.setFooCallback1(fooResults.queue())
o.setFooCallback2(fooResults.queue())

Note that it only captures the first callback arguments, but you can easily extend this to whatever you need.