angular.js/src/Deferred.js

'use strict';

/**
 * @ngdoc service
 * @name angular.module.ng.$q
 * @requires $rootScope
 *
 * @description
 * A promise/deferred implementation inspired by [Kris Kowal's Q](https://github.com/kriskowal/q).
 *
 * [The CommonJS Promise proposal](http://wiki.commonjs.org/wiki/Promises) describes a promise as an
 * interface for interacting with an object that represents the result of an action that is
 * performed asynchronously, and may or may not be finished at any given point in time.
 *
 * From the perspective of dealing with error handling, deferred and promise apis are to
 * asynchronous programing what `try`, `catch` and `throw` keywords are to synchronous programing.
 *
 * <pre>
 *   // for the purpose of this example let's assume that variables `$q` and `scope` are
 *   // available in the current lexical scope (they could have been injected or passed in).
 *
 *   function asyncGreet(name) {
 *     var deferred = $q.defer();
 *
 *     setTimeout(function() {
 *       // since this fn executes async in a future turn of the event loop, we need to wrap
 *       // our code into an $apply call so that the model changes are properly observed.
 *       scope.$apply(function() {
 *         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);
 *   );
 * </pre>
 *
 * 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](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 `$q.defer()`.
 *
 * The purpose of the deferred object is to expose the associated Promise instance as well as apis
 * that can be used for signaling the successful or unsuccessful completion of the task.
 *
 * **Methods**
 *
 * - `resolve(value)` – resolves the derived promise with the `value`. If the value is a rejection
 *   constructed via `$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`.
 *
 * **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)` – regardless of when the promise was or will be resolved
 *   or rejected 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.
 *
 *   This method *returns a new promise* which is resolved or rejected via the return value of the
 *   `successCallback` or `errorCallback`.
 *
 *
 * # Chaining promises
 *
 * Because calling `then` api of a promise returns a new derived promise, it is easily possible
 * to create a chain of promises:
 *
 * <pre>
 *   promiseB = promiseA.then(function(result) {
 *     return result + 1;
 *   });
 *
 *   // promiseB will be resolved immediately after promiseA is resolved and it's value will be
 *   // the result of promiseA incremented by 1
 * </pre>
 *
 * 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 three main differences:
 *
 * - $q is integrated with the {@link angular.module.ng.$rootScope.Scope} Scope model observation
 *   mechanism in angular, which means faster propagation of resolution or rejection into your
 *   models and avoiding unnecessary browser repaints, which would result in flickering UI.
 * - $q promises are recognized by the templating engine in angular, which means that in templates
 *   you can treat promises attached to a scope as if they were the resulting values.
 * - Q has many more features that $q, but that comes at a cost of bytes. $q is tiny, but contains
 *   all the important functionality needed for common async tasks.
 */
function $QProvider() {

  this.$get = ['$rootScope', '$exceptionHandler', function($rootScope, $exceptionHandler) {
    return qFactory(function(callback) {
      $rootScope.$evalAsync(callback);
    }, $exceptionHandler);
  }];
}


/**
 * Constructs a promise manager.
 *
 * @param {function(function)} nextTick Function for executing functions in the next turn.
 * @param {function(...*)} exceptionHandler Function into which unexpected exceptions are passed for
 *     debugging purposes.
 * @returns {object} Promise manager.
 */
function qFactory(nextTick, exceptionHandler) {

  /**
   * @ngdoc
   * @name angular.module.ng.$q#defer
   * @methodOf angular.module.ng.$q
   * @description
   * Creates a `Deferred` object which represents a task which will finish in the future.
   *
   * @returns {Deferred} Returns a new instance of deferred.
   */
  var defer = function() {
    var pending = [],
        value, deferred;

    deferred = {

      resolve: function(val) {
        if (pending) {
          var callbacks = pending;
          pending = undefined;
          value = ref(val);

          if (callbacks.length) {
            nextTick(function() {
              var callback;
              for (var i = 0, ii = callbacks.length; i < ii; i++) {
                callback = callbacks[i];
                value.then(callback[0], callback[1]);
              }
            });
          }
        }
      },


      reject: function(reason) {
        deferred.resolve(reject(reason));
      },


      promise: {
        then: function(callback, errback) {
          var result = defer();

          var wrappedCallback = function(value) {
            try {
              result.resolve((callback || defaultCallback)(value));
            } catch(e) {
              exceptionHandler(e);
              result.reject(e);
            }
          };

          var wrappedErrback = function(reason) {
            try {
              result.resolve((errback || defaultErrback)(reason));
            } catch(e) {
              exceptionHandler(e);
              result.reject(e);
            }
          };

          if (pending) {
            pending.push([wrappedCallback, wrappedErrback]);
          } else {
            value.then(wrappedCallback, wrappedErrback);
          }

          return result.promise;
        }
      }
    };

    return deferred;
  };


  var ref = function(value) {
    if (value && value.then) return value;
    return {
      then: function(callback) {
        var result = defer();
        nextTick(function() {
          result.resolve(callback(value));
        });
        return result.promise;
      }
    };
  };


  /**
   * @ngdoc
   * @name angular.module.ng.$q#reject
   * @methodOf angular.module.ng.$q
   * @description
   * Creates a promise that is resolved as rejected with the specified `reason`. This api should be
   * used to forward rejection in a chain of promises. If you are dealing with the last promise in
   * a promise chain, you don't need to worry about it.
   *
   * When comparing deferreds/promises to the familiar behavior of try/catch/throw, think of
   * `reject` as the `throw` keyword in JavaScript. This also means that if you "catch" an error via
   * a promise error callback and you want to forward the error to the promise derived from the
   * current promise, you have to "rethrow" the error by returning a rejection constructed via
   * `reject`.
   *
   * <pre>
   *   promiseB = promiseA.then(function(result) {
   *     // success: do something and resolve promiseB
   *     //          with the old or a new result
   *     return result;
   *   }, function(reason) {
   *     // error: handle the error if possible and
   *     //        resolve promiseB with newPromiseOrValue,
   *     //        otherwise forward the rejection to promiseB
   *     if (canHandle(reason)) {
   *      // handle the error and recover
   *      return newPromiseOrValue;
   *     }
   *     return $q.reject(reason);
   *   });
   * </pre>
   *
   * @param {*} reason Constant, message, exception or an object representing the rejection reason.
   * @returns {Promise} Returns a promise that was already resolved as rejected with the `reason`.
   */
  var reject = function(reason) {
    return {
      then: function(callback, errback) {
        var result = defer();
        nextTick(function() {
          result.resolve(errback(reason));
        });
        return result.promise;
      }
    };
  };


  /**
   * @ngdoc
   * @name angular.module.ng.$q#when
   * @methodOf angular.module.ng.$q
   * @description
   * Wraps an object that might be a value or a (3rd party) then-able promise into a $q promise.
   * This is useful when you are dealing with on object that might or might not be a promise, or if
   * the promise comes from a source that can't be trusted.
   *
   * @param {*} value Value or a promise
   * @returns {Promise} Returns a single promise that will be resolved with an array of values,
   *   each value coresponding to the promise at the same index in the `promises` array. If any of
   *   the promises is resolved with a rejection, this resulting promise will be resolved with the
   *   same rejection.
   */
  var when = function(value, callback, errback) {
    var result = defer(),
        done;

    var wrappedCallback = function(value) {
      try {
        return (callback || defaultCallback)(value);
      } catch (e) {
        exceptionHandler(e);
        return reject(e);
      }
    };

    var wrappedErrback = function(reason) {
      try {
        return (errback || defaultErrback)(reason);
      } catch (e) {
        exceptionHandler(e);
        return reject(e);
      }
    };

    nextTick(function() {
      ref(value).then(function(value) {
        if (done) return;
        done = true;
        result.resolve(ref(value).then(wrappedCallback, wrappedErrback));
      }, function(reason) {
        if (done) return;
        done = true;
        result.resolve(wrappedErrback(reason));
      });
    });

    return result.promise;
  };


  function defaultCallback(value) {
    return value;
  }


  function defaultErrback(reason) {
    return reject(reason);
  }


  /**
   * @ngdoc
   * @name angular.module.ng.$q#all
   * @methodOf angular.module.ng.$q
   * @description
   * Combines multiple promises into a single promise that is resolved when all of the input
   * promises are resolved.
   *
   * @param {Array.<Promise>} promises An array of promises.
   * @returns {Promise} Returns a single promise that will be resolved with an array of values,
   *   each value coresponding to the promise at the same index in the `promises` array. If any of
   *   the promises is resolved with a rejection, this resulting promise will be resolved with the
   *   same rejection.
   */
  function all(promises) {
    var deferred = defer(),
        counter = promises.length,
        results = [];

    forEach(promises, function(promise, index) {
      promise.then(function(value) {
        if (index in results) return;
        results[index] = value;
        if (!(--counter)) deferred.resolve(results);
      }, function(reason) {
        if (index in results) return;
        deferred.reject(reason);
      });
    });

    return deferred.promise;
  }

  return {
    defer: defer,
    reject: reject,
    when: when,
    all: all
  };
}