@ngdoc service
@name $q
@requires $rootScope

@description
A service that helps you run functions asynchronously, and use their return values (or exceptions)
when they are done processing.

This is a [Promises/A+](https://promisesaplus.com/)-compliant implementation of promises/deferred
objects inspired by [Kris Kowal's Q](https://github.com/kriskowal/q).

$q can be used in two fashions --- one which is more similar to Kris Kowal's Q or jQuery's Deferred
implementations, and the other which resembles ES6 (ES2015) promises to some degree.

## $q constructor

The streamlined ES6 style promise is essentially just using $q as a constructor which takes a `resolver`
function as the first argument. This is similar to the native Promise implementation from ES6,
see [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).

While the constructor-style use is supported, not all of the supporting methods from ES6 promises are
available yet.

It can be used like so:

```js
// for the purpose of this example let's assume that variables `$q` and `okToGreet`
// are available in the current lexical scope (they could have been injected or passed in).

function asyncGreet(name) {
// perform some asynchronous operation, resolve or reject the promise when appropriate.
return $q(function(resolve, reject) {
setTimeout(function() {
if (okToGreet(name)) {
resolve('Hello, ' + name + '!');
} else {
reject('Greeting ' + name + ' is not allowed.');
}
}, 1000);
});
}

var promise = asyncGreet('Robin Hood');
promise.then(function(greeting) {
alert('Success: ' + greeting);
}, function(reason) {
alert('Failed: ' + reason);
});
```

Note: progress/notify callbacks are not currently supported via the ES6-style interface.

Note: unlike ES6 behavior, an exception thrown in the constructor function will NOT implicitly reject the promise.

However, the more traditional CommonJS-style usage is still available, and documented below.

[The CommonJS Promise proposal](http://wiki.commonjs.org/wiki/Promises) describes a promise as an
interface for interacting with an object that represents the result of an action that is
performed asynchronously, and may or may not be finished at any given point in time.

From the perspective of dealing with error handling, deferred and promise APIs are to
asynchronous programming what `try`, `catch` and `throw` keywords are to synchronous programming.

```js
// for the purpose of this example let's assume that variables `$q` and `okToGreet`
// are available in the current lexical scope (they could have been injected or passed in).

function asyncGreet(name) {
var deferred = $q.defer();

setTimeout(function() {
deferred.notify('About to greet ' + name + '.');

if (okToGreet(name)) {
deferred.resolve('Hello, ' + name + '!');
} else {
deferred.reject('Greeting ' + name + ' is not allowed.');
}
}, 1000);

return deferred.promise;
}

var promise = asyncGreet('Robin Hood');
promise.then(function(greeting) {
alert('Success: ' + greeting);
}, function(reason) {
alert('Failed: ' + reason);
}, function(update) {
alert('Got notification: ' + update);
});
```

At first it might not be obvious why this extra complexity is worth the trouble. The payoff
comes in the way of guarantees that promise and deferred APIs make, see
https://github.com/kriskowal/uncommonjs/blob/master/promises/specification.md.

Additionally the promise api allows for composition that is very hard to do with the
traditional callback ([CPS](http://en.wikipedia.org/wiki/Continuation-passing_style)) approach.
For more on this please see the [Q documentation](https://github.com/kriskowal/q) especially the
section on serial or parallel joining of promises.

## The Deferred API

A new instance of deferred is constructed by calling `$q.defer()`.

The purpose of the deferred object is to expose the associated Promise instance as well as APIs
that can be used for signaling the successful or unsuccessful completion, as well as the status
of the task.

**Methods**

- `resolve(value)` – resolves the derived promise with the `value`. If the value is a rejection
constructed via `$q.reject`, the promise will be rejected instead.
- `reject(reason)` – rejects the derived promise with the `reason`. This is equivalent to
resolving it with a rejection constructed via `$q.reject`.
- `notify(value)` - provides updates on the status of the promise's execution. This may be called
multiple times before the promise is either resolved or rejected.

**Properties**

- promise – `{Promise}` – promise object associated with this deferred.


## The Promise API

A new promise instance is created when a deferred instance is created and can be retrieved by
calling `deferred.promise`.

The purpose of the promise object is to allow for interested parties to get access to the result
of the deferred task when it completes.

**Methods**

- `then(successCallback, [errorCallback], [notifyCallback])` – regardless of when the promise was or
will be resolved or rejected, `then` calls one of the success or error callbacks asynchronously
as soon as the result is available. The callbacks are called with a single argument: the result
or rejection reason. Additionally, the notify callback may be called zero or more times to
provide a progress indication, before the promise is resolved or rejected.

This method *returns a new promise* which is resolved or rejected via the return value of the
`successCallback`, `errorCallback` (unless that value is a promise, in which case it is resolved
with the value which is resolved in that promise using
[promise chaining](http://www.html5rocks.com/en/tutorials/es6/promises/#toc-promises-queues)).
It also notifies via the return value of the `notifyCallback` method. The promise cannot be
resolved or rejected from the notifyCallback method. The errorCallback and notifyCallback
arguments are optional.

- `catch(errorCallback)` – shorthand for `promise.then(null, errorCallback)`

- `finally(callback, notifyCallback)` – allows you to observe either the fulfillment or rejection of a promise,
but to do so without modifying the final value. This is useful to release resources or do some
clean-up that needs to be done whether the promise was rejected or resolved. See the [full
specification](https://github.com/kriskowal/q/wiki/API-Reference#promisefinallycallback) for
more information.

## Chaining promises

Because calling the `then` method of a promise returns a new derived promise, it is easily
possible to create a chain of promises:

```js
promiseB = promiseA.then(function(result) {
return result + 1;
});

// promiseB will be resolved immediately after promiseA is resolved and its value
// will be the result of promiseA incremented by 1
```

It is possible to create chains of any length and since a promise can be resolved with another
promise (which will defer its resolution further), it is possible to pause/defer resolution of
the promises at any point in the chain. This makes it possible to implement powerful APIs like
$http's response interceptors.


## Differences between Kris Kowal's Q and $q

There are two main differences:

- $q is integrated with the {@link ng.$rootScope.Scope} Scope model observation
mechanism in AngularJS, which means faster propagation of resolution or rejection into your
models and avoiding unnecessary browser repaints, which would result in flickering UI.
- Q has many more features than $q, but that comes at a cost of bytes. $q is tiny, but contains
all the important functionality needed for common async tasks.

## Testing

```js
it('should simulate promise', inject(function($q, $rootScope) {
var deferred = $q.defer();
var promise = deferred.promise;
var resolvedValue;

promise.then(function(value) { resolvedValue = value; });
expect(resolvedValue).toBeUndefined();

// Simulate resolving of promise
deferred.resolve(123);
// Note that the 'then' function does not get called synchronously.
// This is because we want the promise API to always be async, whether or not
// it got called synchronously or asynchronously.
expect(resolvedValue).toBeUndefined();

// Propagate promise resolution to 'then' functions using $apply().
$rootScope.$apply();
expect(resolvedValue).toEqual(123);
}));
```

@param {function(function, function)} resolver Function which is responsible for resolving or
rejecting the newly created promise. The first parameter is a function which resolves the
promise, the second parameter is a function which rejects the promise.

@returns {Promise} The newly created promise.
 
@ngdoc provider
@name $qProvider
@this

@description
function $QProvider() {
  var errorOnUnhandledRejections = true;
  this.$get = ['$rootScope', '$exceptionHandler', function($rootScope, $exceptionHandler) {
    return qFactory(function(callback) {
      $rootScope.$evalAsync(callback);
    }, $exceptionHandler, errorOnUnhandledRejections);
  }];

  /**
   * @ngdoc method
   * @name $qProvider#errorOnUnhandledRejections
   * @kind function
   *
   * @description
   * Retrieves or overrides whether to generate an error when a rejected promise is not handled.
   * This feature is enabled by default.
   *
   * @param {boolean=} value Whether to generate an error when a rejected promise is not handled.
   * @returns {boolean|ng.$qProvider} Current value when called without a new value or self for
   *    chaining otherwise.
   */
  this.errorOnUnhandledRejections = function(value) {
    if (isDefined(value)) {
      errorOnUnhandledRejections = value;
      return this;
    } else {
      return errorOnUnhandledRejections;
    }
  };
}