CODEWITHSUNDEEP
javascriptdojocallbackdeferred
antonpug
antonpug

Reputation: 14296

How to return a value from a deferred?

I am calling a method that has some logic in a deferred, and when that logic finishes, I want to return the value to the callee. See below:

//Callee.js
var myAssistant = new Assistant();
console.log(myAssistant.whatIsTheValue());



//Assistant.js
whatIsTheValue : function(someArg) {
   var deferred = someService.getSomething();
   deferred.then(lang.hitch(this, this._getTheValue));

   //In theory, I want to return whatever this._getTheValue returns, how can I do that?!
}

_getTheValue() {
   ...
   ...
   return xyz;
}

Upvotes: 3

Views: 1168

Answers (3)

Ben McCormick
Ben McCormick

Reputation: 25728

Deferreds are asynchronous operations. Therefore you can't return a variable in a normal way from them because they will not execute until after the current function context completes.

If you want to do more with that value you will need to do so in terms of another callback (IE chaining the then statements.)

The point of deferreds is to provide sequential operations for callbacks. So you can chain them to achieve the results you want. If you need the results to be available in your current execution context, you're going to have to find a synchronous (not deferreds) method of doing what you want.

So something like this

//Assistant.js
whatIsTheValue : function(someArg) {
   var deferred = someService.getSomething();
   var next = deferred.then(lang.hitch(this, this._getTheValue));
   next.then(/*insert next function here*/);
}

You need to understand that using a deferred lang.hitch isn't going to execute until after whatistheValue is done operating. So instead of returning the value to whatever function called whatisthevalue, you're going to have to put the logic of processing that value into a new function and use that as an additional callback for your deferred. This will probably require some restructuring of your program.

Upvotes: 2

Sully
Sully

Reputation: 14943

use JQuery's $when instead.

Example

// assuming both getData and getLocation return their respective Promise
var combinedPromise = $.when(getData(), getLocation())

// function will be called when both getData and getLocation resolve
combinePromise.done(function(data,location){
  alert("We got data: " + dataResult + " and location: " + location);
});

http://www.html5rocks.com/en/tutorials/async/deferred/

Upvotes: 1

Bergi
Bergi

Reputation: 664356

I do not know what your lang.hitch does, but the solution should look like that:

Assistant.prototype.whatIsTheValue = function(someArg) {
    var deferred = someService.getSomething();
    return deferred.then(lang.hitch(this, this._getTheValue));
//  ^^^^^^
};

var myAssistant = new Assistant();
myAssistant.whatIsTheValue().then(console.log); // use console.log.bind(console) in Chrome
//                           ^^^^ - it is a promise you return

Upvotes: 1

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