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Deferred.when/dojo.when

since:V1.5

dojo.when provides the ability to define a callback (and error handler) for the eventual completion of a Deferred or promise object, or any standard synchronous value.

As of Dojo 1.7, the when function is exposed as a member of the dojo/_base/Deferred module return (which is also the Deferred constructor). It is aliased to dojo.when for backward-compatibility.

Usage

The first argument to dojo.when may be any value, the second argument is the callback, and the optional third argument is the error callback. If the first argument is a promise (like a Deferred object), then the callback (or error callback) will be called when the promise is resolved. If the first argument is not a promise, the callback will be call immediately (with the value passed as the first argument.

Example 1: Creating and calling a deferred (1.7+ AMD).

require(["dojo/_base/Deferred"], function(Deferred){

  Deferred.when(4, print); // this will print 4 immediately

  var fourAsync = new Deferred();
  Deferred.when(fourAsync, print); // this will print 4, one second later when the Deferred is resolved
  setTimeout(function(){
    fourAsync.resolve(4);
  }, 1000);

  function print(value){
    console.log(value);
  };
});

Example 2: Creating and calling a deferred (Dojo < 1.7 style).

dojo.when(4, print); // this will print 4 immediately

var fourAsync = new dojo.Deferred();
dojo.when(fourAsync, print); // this will print 4, one second later when the Deferred is resolved
setTimeout(function(){
  fourAsync.resolve(4);
}, 1000);

function print(value){
  console.log(value);
}