@ngdoc method
@name $q#defer
@kind function

@description
Creates a `Deferred` object which represents a task which will finish in the future.

@returns {Deferred} Returns a new instance of deferred.
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], callback[2]);
              }
            });
          }
        }
      },


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


      notify: function(progress) {
        if (pending) {
          var callbacks = pending;

          if (pending.length) {
            nextTick(function() {
              var callback;
              for (var i = 0, ii = callbacks.length; i < ii; i++) {
                callback = callbacks[i];
                callback[2](progress);
              }
            });
          }
        }
      },


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

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

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

          var wrappedProgressback = function(progress) {
            try {
              result.notify((isFunction(progressback) ? progressback : defaultCallback)(progress));
            } catch(e) {
              exceptionHandler(e);
            }
          };

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

          return result.promise;
        },

        "catch": function(callback) {
          return this.then(null, callback);
        },

        "finally": function(callback) {

          function makePromise(value, resolved) {
            var result = defer();
            if (resolved) {
              result.resolve(value);
            } else {
              result.reject(value);
            }
            return result.promise;
          }

          function handleCallback(value, isResolved) {
            var callbackOutput = null;
            try {
              callbackOutput = (callback ||defaultCallback)();
            } catch(e) {
              return makePromise(e, false);
            }
            if (isPromiseLike(callbackOutput)) {
              return callbackOutput.then(function() {
                return makePromise(value, isResolved);
              }, function(error) {
                return makePromise(error, false);
              });
            } else {
              return makePromise(value, isResolved);
            }
          }

          return this.then(function(value) {
            return handleCallback(value, true);
          }, function(error) {
            return handleCallback(error, false);
          });
        }
      }
    };

    return deferred;
  }