The core problem
You're basically trying to convert an existing callback API to promises. In Angular $q.when
is used for promise aggregation, and for thenable assimilation (that is, working with another promise library). Fear not, as what you want is perfectly doable without the cruft of a manual deferred each time.
Deferred objects, and the promise constructor
Sadly, with Angular 1.x you're stuck with the outdated deferred interface, that not only like you said is ugly, it's also unsafe (it's risky and throws synchronously).
What you'd like is called the promise constructor, it's what all implementations (Bluebird, Q, When, RSVP, native promises, etc) are switching to since it's nicer and safer.
Here is how your method would look with native promises:
var promise = new Promise(function(resolve,reject){
notAPromise(
function success(res) { resolve("Special reason") },
function failure(res) { reject(new Error('failure')); } // Always reject
) // with errors!
);
You can replicate this functionality in $q
of course:
function resolver(handler){
try {
var d = $q.defer();
handler(function(v){ d.resolve(v); }, function(r){ d.reject(r); });
return d.promise;
} catch (e) {
return $q.reject(e);
// $exceptionHandler call might be useful here, since it's a throw
}
}
Which would let you do:
var promise = resolver(function(resolve,reject){
notAPromise(function success(res){ resolve("Special reason"),
function failure(res){ reject(new Error("failure")); })
});
promise.then(function(){
});
An automatic promisification helper
Of course, it's equally easy to write an automatic promisification method for your specific case. If you work with a lot of APIs with the callback convention fn(onSuccess, onError)
you can do:
function promisify(fn){
return function promisified(){
var args = Array(arguments.length + 2);
for(var i = 0; i < arguments.length; i++){
args.push(arguments[i]);
}
var d = $q.defer();
args.push(function(r){ d.resolve(r); });
args.push(function(r){ d.reject(r); });
try{
fn.call(this, args); // call with the arguments
} catch (e){ // promise returning functions must NEVER sync throw
return $q.reject(e);
// $exceptionHandler call might be useful here, since it's a throw
}
return d.promise; // return a promise on the API.
};
}
This would let you do:
var aPromise = promisify(notAPromise);
var promise = aPromise.then(function(val){
// access res here
return "special reason";
}).catch(function(e){
// access rejection value here
return $q.reject(new Error("failure"));
});
Which is even neater