backbone.js

//     Backbone.js 0.9.2

//     (c) 2010-2012 Jeremy Ashkenas, DocumentCloud Inc.
//     Backbone may be freely distributed under the MIT license.
//     For all details and documentation:
//     http://backbonejs.org

(function(){

  // Initial Setup
  // -------------

  // Save a reference to the global object (`window` in the browser, `exports`
  // on the server).
  var root = this;

  // Save the previous value of the `Backbone` variable, so that it can be
  // restored later on, if `noConflict` is used.
  var previousBackbone = root.Backbone;

  // Create a local reference to array methods.
  var ArrayProto = Array.prototype;
  var push = ArrayProto.push;
  var slice = ArrayProto.slice;
  var splice = ArrayProto.splice;

  // The top-level namespace. All public Backbone classes and modules will
  // be attached to this. Exported for both CommonJS and the browser.
  var Backbone;
  if (typeof exports !== 'undefined') {
    Backbone = exports;
  } else {
    Backbone = root.Backbone = {};
  }

  // Current version of the library. Keep in sync with `package.json`.
  Backbone.VERSION = '0.9.2';

  // Require Underscore, if we're on the server, and it's not already present.
  var _ = root._;
  if (!_ && (typeof require !== 'undefined')) _ = require('underscore');

  // For Backbone's purposes, jQuery, Zepto, or Ender owns the `$` variable.
  Backbone.$ = root.jQuery || root.Zepto || root.ender;

  // Runs Backbone.js in *noConflict* mode, returning the `Backbone` variable
  // to its previous owner. Returns a reference to this Backbone object.
  Backbone.noConflict = function() {
    root.Backbone = previousBackbone;
    return this;
  };

  // Turn on `emulateHTTP` to support legacy HTTP servers. Setting this option
  // will fake `"PUT"` and `"DELETE"` requests via the `_method` parameter and
  // set a `X-Http-Method-Override` header.
  Backbone.emulateHTTP = false;

  // Turn on `emulateJSON` to support legacy servers that can't deal with direct
  // `application/json` requests ... will encode the body as
  // `application/x-www-form-urlencoded` instead and will send the model in a
  // form param named `model`.
  Backbone.emulateJSON = false;

  // Backbone.Events
  // ---------------

  // Regular expression used to split event strings
  var eventSplitter = /\s+/;

  // A module that can be mixed in to *any object* in order to provide it with
  // custom events. You may bind with `on` or remove with `off` callback functions
  // to an event; `trigger`-ing an event fires all callbacks in succession.
  //
  //     var object = {};
  //     _.extend(object, Backbone.Events);
  //     object.on('expand', function(){ alert('expanded'); });
  //     object.trigger('expand');
  //
  var Events = Backbone.Events = {

    // Bind one or more space separated events, `events`, to a `callback`
    // function. Passing `"all"` will bind the callback to all events fired.
    on: function(events, callback, context) {
      var calls, event, list;
      if (!callback) return this;

      events = events.split(eventSplitter);
      calls = this._callbacks || (this._callbacks = {});

      while (event = events.shift()) {
        list = calls[event] || (calls[event] = []);
        list.push(callback, context);
      }

      return this;
    },

    // Remove one or many callbacks. If `context` is null, removes all callbacks
    // with that function. If `callback` is null, removes all callbacks for the
    // event. If `events` is null, removes all bound callbacks for all events.
    off: function(events, callback, context) {
      var event, calls, list, i;

      // No events, or removing *all* events.
      if (!(calls = this._callbacks)) return this;
      if (!(events || callback || context)) {
        delete this._callbacks;
        return this;
      }

      events = events ? events.split(eventSplitter) : _.keys(calls);

      // Loop through the callback list, splicing where appropriate.
      while (event = events.shift()) {
        if (!(list = calls[event]) || !(callback || context)) {
          delete calls[event];
          continue;
        }

        for (i = list.length - 2; i >= 0; i -= 2) {
          if (!(callback && list[i] !== callback || context && list[i + 1] !== context)) {
            list.splice(i, 2);
          }
        }
      }

      return this;
    },

    // Trigger one or many events, firing all bound callbacks. Callbacks are
    // passed the same arguments as `trigger` is, apart from the event name
    // (unless you're listening on `"all"`, which will cause your callback to
    // receive the true name of the event as the first argument).
    trigger: function(events) {
      var event, calls, list, i, length, args, all, rest;
      if (!(calls = this._callbacks)) return this;

      rest = [];
      events = events.split(eventSplitter);

      // Fill up `rest` with the callback arguments.  Since we're only copying
      // the tail of `arguments`, a loop is much faster than Array#slice.
      for (i = 1, length = arguments.length; i < length; i++) {
        rest[i - 1] = arguments[i];
      }

      // For each event, walk through the list of callbacks twice, first to
      // trigger the event, then to trigger any `"all"` callbacks.
      while (event = events.shift()) {
        // Copy callback lists to prevent modification.
        if (all = calls.all) all = all.slice();
        if (list = calls[event]) list = list.slice();

        // Execute event callbacks.
        if (list) {
          for (i = 0, length = list.length; i < length; i += 2) {
            list[i].apply(list[i + 1] || this, rest);
          }
        }

        // Execute "all" callbacks.
        if (all) {
          args = [event].concat(rest);
          for (i = 0, length = all.length; i < length; i += 2) {
            all[i].apply(all[i + 1] || this, args);
          }
        }
      }

      return this;
    }

  };

  // Aliases for backwards compatibility.
  Events.bind   = Events.on;
  Events.unbind = Events.off;

  // Backbone.Model
  // --------------

  // Create a new model, with defined attributes. A client id (`cid`)
  // is automatically generated and assigned for you.
  var Model = Backbone.Model = function(attributes, options) {
    var defaults;
    var attrs = attributes || {};
    if (options && options.collection) this.collection = options.collection;
    this.attributes = {};
    this._escapedAttributes = {};
    this.cid = _.uniqueId('c');
    this.changed = {};
    this._changes = {};
    this._pending = {};
    if (options && options.parse) attrs = this.parse(attrs);
    if (defaults = _.result(this, 'defaults')) {
      attrs = _.extend({}, defaults, attrs);
    }
    this.set(attrs, {silent: true});
    // Reset change tracking.
    this.changed = {};
    this._changes = {};
    this._pending = {};
    this._previousAttributes = _.clone(this.attributes);
    this.initialize.apply(this, arguments);
  };

  // Attach all inheritable methods to the Model prototype.
  _.extend(Model.prototype, Events, {

    // A hash of attributes whose current and previous value differ.
    changed: null,

    // A hash of attributes that have changed since the last time `change`
    // was called.
    _changes: null,

    // A hash of attributes that have changed since the last `change` event
    // began.
    _pending: null,

    // A hash of attributes with the current model state to determine if
    // a `change` should be recorded within a nested `change` block.
    _changing : null,

    // The default name for the JSON `id` attribute is `"id"`. MongoDB and
    // CouchDB users may want to set this to `"_id"`.
    idAttribute: 'id',

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // Return a copy of the model's `attributes` object.
    toJSON: function(options) {
      return _.clone(this.attributes);
    },

    // Proxy `Backbone.sync` by default.
    sync: function() {
      return Backbone.sync.apply(this, arguments);
    },

    // Get the value of an attribute.
    get: function(attr) {
      return this.attributes[attr];
    },

    // Get the HTML-escaped value of an attribute.
    escape: function(attr) {
      var html;
      if (html = this._escapedAttributes[attr]) return html;
      var val = this.get(attr);
      return this._escapedAttributes[attr] = _.escape(val == null ? '' : '' + val);
    },

    // Returns `true` if the attribute contains a value that is not null
    // or undefined.
    has: function(attr) {
      return this.get(attr) != null;
    },

    // Set a hash of model attributes on the object, firing `"change"` unless
    // you choose to silence it.
    set: function(key, val, options) {
      var attr, attrs;
      if (key == null) return this;

      // Handle both `"key", value` and `{key: value}` -style arguments.
      if (_.isObject(key)) {
        attrs = key;
        options = val;
      } else {
        (attrs = {})[key] = val;
      }

      // Extract attributes and options.
      var silent = options && options.silent;
      var unset = options && options.unset;
      if (attrs instanceof Model) attrs = attrs.attributes;
      if (unset) for (attr in attrs) attrs[attr] = void 0;

      // Run validation.
      if (!this._validate(attrs, options)) return false;

      // Check for changes of `id`.
      if (this.idAttribute in attrs) this.id = attrs[this.idAttribute];

      var changing = this._changing;
      var now = this.attributes;
      var escaped = this._escapedAttributes;
      var prev = this._previousAttributes || {};

      // For each `set` attribute...
      for (attr in attrs) {
        val = attrs[attr];

        // If the new and current value differ, record the change.
        if (!_.isEqual(now[attr], val) || (unset && _.has(now, attr))) {
          delete escaped[attr];
          this._changes[attr] = true;
        }

        // Update or delete the current value.
        unset ? delete now[attr] : now[attr] = val;

        // If the new and previous value differ, record the change.  If not,
        // then remove changes for this attribute.
        if (!_.isEqual(prev[attr], val) || (_.has(now, attr) !== _.has(prev, attr))) {
          this.changed[attr] = val;
          if (!silent) this._pending[attr] = true;
        } else {
          delete this.changed[attr];
          delete this._pending[attr];
          if (!changing) delete this._changes[attr];
        }

        if (changing && _.isEqual(now[attr], changing[attr])) delete this._changes[attr];
      }

      // Fire the `"change"` events.
      if (!silent) this.change(options);
      return this;
    },

    // Remove an attribute from the model, firing `"change"` unless you choose
    // to silence it. `unset` is a noop if the attribute doesn't exist.
    unset: function(attr, options) {
      options = _.extend({}, options, {unset: true});
      return this.set(attr, null, options);
    },

    // Clear all attributes on the model, firing `"change"` unless you choose
    // to silence it.
    clear: function(options) {
      options = _.extend({}, options, {unset: true});
      return this.set(_.clone(this.attributes), options);
    },

    // Fetch the model from the server. If the server's representation of the
    // model differs from its current attributes, they will be overriden,
    // triggering a `"change"` event.
    fetch: function(options) {
      options = options ? _.clone(options) : {};
      var model = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        if (!model.set(model.parse(resp, xhr), options)) return false;
        if (success) success(model, resp, options);
      };
      return this.sync('read', this, options);
    },

    // Set a hash of model attributes, and sync the model to the server.
    // If the server returns an attributes hash that differs, the model's
    // state will be `set` again.
    save: function(key, val, options) {
      var attrs, current, done;

      // Handle both `"key", value` and `{key: value}` -style arguments.
      if (key == null || _.isObject(key)) {
        attrs = key;
        options = val;
      } else if (key != null) {
        (attrs = {})[key] = val;
      }
      options = options ? _.clone(options) : {};

      // If we're "wait"-ing to set changed attributes, validate early.
      if (options.wait) {
        if (!this._validate(attrs, options)) return false;
        current = _.clone(this.attributes);
      }

      // Regular saves `set` attributes before persisting to the server.
      var silentOptions = _.extend({}, options, {silent: true});
      if (attrs && !this.set(attrs, options.wait ? silentOptions : options)) {
        return false;
      }

      // Do not persist invalid models.
      if (!attrs && !this._validate(null, options)) return false;

      // After a successful server-side save, the client is (optionally)
      // updated with the server-side state.
      var model = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        done = true;
        var serverAttrs = model.parse(resp, xhr);
        if (options.wait) serverAttrs = _.extend(attrs || {}, serverAttrs);
        if (!model.set(serverAttrs, options)) return false;
        if (success) success(model, resp, options);
      };

      // Finish configuring and sending the Ajax request.
      var xhr = this.sync(this.isNew() ? 'create' : 'update', this, options);

      // When using `wait`, reset attributes to original values unless
      // `success` has been called already.
      if (!done && options.wait) {
        this.clear(silentOptions);
        this.set(current, silentOptions);
      }

      return xhr;
    },

    // Destroy this model on the server if it was already persisted.
    // Optimistically removes the model from its collection, if it has one.
    // If `wait: true` is passed, waits for the server to respond before removal.
    destroy: function(options) {
      options = options ? _.clone(options) : {};
      var model = this;
      var success = options.success;

      var destroy = function() {
        model.trigger('destroy', model, model.collection, options);
      };

      options.success = function(resp) {
        if (options.wait || model.isNew()) destroy();
        if (success) success(model, resp, options);
      };

      if (this.isNew()) {
        options.success();
        return false;
      }

      var xhr = this.sync('delete', this, options);
      if (!options.wait) destroy();
      return xhr;
    },

    // Default URL for the model's representation on the server -- if you're
    // using Backbone's restful methods, override this to change the endpoint
    // that will be called.
    url: function() {
      var base = _.result(this, 'urlRoot') || _.result(this.collection, 'url') || urlError();
      if (this.isNew()) return base;
      return base + (base.charAt(base.length - 1) === '/' ? '' : '/') + encodeURIComponent(this.id);
    },

    // **parse** converts a response into the hash of attributes to be `set` on
    // the model. The default implementation is just to pass the response along.
    parse: function(resp, xhr) {
      return resp;
    },

    // Create a new model with identical attributes to this one.
    clone: function() {
      return new this.constructor(this.attributes);
    },

    // A model is new if it has never been saved to the server, and lacks an id.
    isNew: function() {
      return this.id == null;
    },

    // Call this method to manually fire a `"change"` event for this model and
    // a `"change:attribute"` event for each changed attribute.
    // Calling this will cause all objects observing the model to update.
    change: function(options) {
      var changing = this._changing;
      var current = this._changing = {};

      // Silent changes become pending changes.
      for (var attr in this._changes) this._pending[attr] = true;

      // Trigger 'change:attr' for any new or silent changes.
      var changes = this._changes;
      this._changes = {};

      // Set the correct state for this._changing values
      var triggers = [];
      for (var attr in changes) {
        current[attr] = this.get(attr);
        triggers.push(attr);
      }

      for (var i=0, l=triggers.length; i < l; i++) {
        this.trigger('change:' + triggers[i], this, current[triggers[i]], options);
      }
      if (changing) return this;

      // Continue firing `"change"` events while there are pending changes.
      while (!_.isEmpty(this._pending)) {
        this._pending = {};
        this.trigger('change', this, options);
        // Pending and silent changes still remain.
        for (var attr in this.changed) {
          if (this._pending[attr] || this._changes[attr]) continue;
          delete this.changed[attr];
        }
        this._previousAttributes = _.clone(this.attributes);
      }

      this._changing = null;
      return this;
    },

    // Determine if the model has changed since the last `"change"` event.
    // If you specify an attribute name, determine if that attribute has changed.
    hasChanged: function(attr) {
      if (attr == null) return !_.isEmpty(this.changed);
      return _.has(this.changed, attr);
    },

    // Return an object containing all the attributes that have changed, or
    // false if there are no changed attributes. Useful for determining what
    // parts of a view need to be updated and/or what attributes need to be
    // persisted to the server. Unset attributes will be set to undefined.
    // You can also pass an attributes object to diff against the model,
    // determining if there *would be* a change.
    changedAttributes: function(diff) {
      if (!diff) return this.hasChanged() ? _.clone(this.changed) : false;
      var val, changed = false, old = this._previousAttributes;
      for (var attr in diff) {
        if (_.isEqual(old[attr], (val = diff[attr]))) continue;
        (changed || (changed = {}))[attr] = val;
      }
      return changed;
    },

    // Get the previous value of an attribute, recorded at the time the last
    // `"change"` event was fired.
    previous: function(attr) {
      if (attr == null || !this._previousAttributes) return null;
      return this._previousAttributes[attr];
    },

    // Get all of the attributes of the model at the time of the previous
    // `"change"` event.
    previousAttributes: function() {
      return _.clone(this._previousAttributes);
    },

    // Check if the model is currently in a valid state. It's only possible to
    // get into an *invalid* state if you're using silent changes.
    isValid: function(options) {
      return !this.validate || !this.validate(this.attributes, options);
    },

    // Run validation against the next complete set of model attributes,
    // returning `true` if all is well. If a specific `error` callback has
    // been passed, call that instead of firing the general `"error"` event.
    _validate: function(attrs, options) {
      if (options && options.silent || !this.validate) return true;
      attrs = _.extend({}, this.attributes, attrs);
      var error = this.validate(attrs, options);
      if (!error) return true;
      if (options && options.error) options.error(this, error, options);
      this.trigger('error', this, error, options);
      return false;
    }

  });

  // Backbone.Collection
  // -------------------

  // Provides a standard collection class for our sets of models, ordered
  // or unordered. If a `comparator` is specified, the Collection will maintain
  // its models in sort order, as they're added and removed.
  var Collection = Backbone.Collection = function(models, options) {
    options || (options = {});
    if (options.model) this.model = options.model;
    if (options.comparator !== void 0) this.comparator = options.comparator;
    this._reset();
    this.initialize.apply(this, arguments);
    if (models) {
      if (options.parse) models = this.parse(models);
      this.reset(models, {silent: true, parse: options.parse});
    }
  };

  // Define the Collection's inheritable methods.
  _.extend(Collection.prototype, Events, {

    // The default model for a collection is just a **Backbone.Model**.
    // This should be overridden in most cases.
    model: Model,

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // The JSON representation of a Collection is an array of the
    // models' attributes.
    toJSON: function(options) {
      return this.map(function(model){ return model.toJSON(options); });
    },

    // Proxy `Backbone.sync` by default.
    sync: function() {
      return Backbone.sync.apply(this, arguments);
    },

    // Add a model, or list of models to the set. Pass **silent** to avoid
    // firing the `add` event for every new model.
    add: function(models, options) {
      var i, args, length, model, existing;
      var at = options && options.at;
      models = _.isArray(models) ? models.slice() : [models];

      // Begin by turning bare objects into model references, and preventing
      // invalid models from being added.
      for (i = 0, length = models.length; i < length; i++) {
        if (models[i] = this._prepareModel(models[i], options)) continue;
        throw new Error("Can't add an invalid model to a collection");
      }

      for (i = models.length - 1; i >= 0; i--) {
        model = models[i];
        existing = model.id != null && this._byId[model.id];

        // If a duplicate is found, splice it out and optionally merge it into
        // the existing model.
        if (existing || this._byCid[model.cid]) {
          if (options && options.merge && existing) {
            existing.set(model, options);
          }
          models.splice(i, 1);
          continue;
        }

        // Listen to added models' events, and index models for lookup by
        // `id` and by `cid`.
        model.on('all', this._onModelEvent, this);
        this._byCid[model.cid] = model;
        if (model.id != null) this._byId[model.id] = model;
      }

      // Update `length` and splice in new models.
      this.length += models.length;
      args = [at != null ? at : this.models.length, 0];
      push.apply(args, models);
      splice.apply(this.models, args);

      // Sort the collection if appropriate.
      if (this.comparator && at == null) this.sort({silent: true});

      if (options && options.silent) return this;

      // Trigger `add` events.
      while (model = models.shift()) {
        model.trigger('add', model, this, options);
      }

      return this;
    },

    // Remove a model, or a list of models from the set. Pass silent to avoid
    // firing the `remove` event for every model removed.
    remove: function(models, options) {
      var i, l, index, model;
      options || (options = {});
      models = _.isArray(models) ? models.slice() : [models];
      for (i = 0, l = models.length; i < l; i++) {
        model = this.getByCid(models[i]) || this.get(models[i]);
        if (!model) continue;
        delete this._byId[model.id];
        delete this._byCid[model.cid];
        index = this.indexOf(model);
        this.models.splice(index, 1);
        this.length--;
        if (!options.silent) {
          options.index = index;
          model.trigger('remove', model, this, options);
        }
        this._removeReference(model);
      }
      return this;
    },

    // Add a model to the end of the collection.
    push: function(model, options) {
      model = this._prepareModel(model, options);
      this.add(model, options);
      return model;
    },

    // Remove a model from the end of the collection.
    pop: function(options) {
      var model = this.at(this.length - 1);
      this.remove(model, options);
      return model;
    },

    // Add a model to the beginning of the collection.
    unshift: function(model, options) {
      model = this._prepareModel(model, options);
      this.add(model, _.extend({at: 0}, options));
      return model;
    },

    // Remove a model from the beginning of the collection.
    shift: function(options) {
      var model = this.at(0);
      this.remove(model, options);
      return model;
    },

    // Slice out a sub-array of models from the collection.
    slice: function(begin, end) {
      return this.models.slice(begin, end);
    },

    // Get a model from the set by id.
    get: function(id) {
      if (id == null) return void 0;
      return this._byId[id.id != null ? id.id : id];
    },

    // Get a model from the set by client id.
    getByCid: function(cid) {
      return cid && this._byCid[cid.cid || cid];
    },

    // Get the model at the given index.
    at: function(index) {
      return this.models[index];
    },

    // Return models with matching attributes. Useful for simple cases of `filter`.
    where: function(attrs) {
      if (_.isEmpty(attrs)) return [];
      return this.filter(function(model) {
        for (var key in attrs) {
          if (attrs[key] !== model.get(key)) return false;
        }
        return true;
      });
    },

    // Force the collection to re-sort itself. You don't need to call this under
    // normal circumstances, as the set will maintain sort order as each item
    // is added.
    sort: function(options) {
      if (!this.comparator) {
        throw new Error('Cannot sort a set without a comparator');
      }

      if (_.isString(this.comparator) || this.comparator.length === 1) {
        this.models = this.sortBy(this.comparator, this);
      } else {
        this.models.sort(_.bind(this.comparator, this));
      }

      if (!options || !options.silent) this.trigger('reset', this, options);
      return this;
    },

    // Pluck an attribute from each model in the collection.
    pluck: function(attr) {
      return _.invoke(this.models, 'get', attr);
    },

    // When you have more items than you want to add or remove individually,
    // you can reset the entire set with a new list of models, without firing
    // any `add` or `remove` events. Fires `reset` when finished.
    reset: function(models, options) {
      for (var i = 0, l = this.models.length; i < l; i++) {
        this._removeReference(this.models[i]);
      }
      this._reset();
      if (models) this.add(models, _.extend({silent: true}, options));
      if (!options || !options.silent) this.trigger('reset', this, options);
      return this;
    },

    // When you want to update a subset of models for this collection,
    // adding missing models, update existing and and optionally remove missing models.
    // If `prune: true` is passed, the collection is pruned of the models not present in
    // the method's models argument.
    update: function(models, options) {
      var i, index, length, dummyModel, model, index, stack = [];
      options = options ? _.clone(options) : {};
      models = _.isArray(models) ? models.slice() : [models];
      if (this.models.length) {
        for (i = 0, length = this.models.length; i < length; i++) {
          stack.push(this.models[i].id);
        }
      }
      for (i = 0, length = models.length; i < length; i++) {
        dummyModel = this._prepareModel(models[i], options);
        model = this.get(dummyModel);
        if (model) {
          model.set(models[i], options);
          index = _.indexOf(stack, model.id);
          stack.splice(index, 1);
        } else {
          this.add(dummyModel, options);
        }
      }
      if (options.prune) {
        for (i = 0, length = stack.length; i < length; i++) {
          this.remove( stack[i] );
        }
      }
      return this;
    },

    // Fetch the default set of models for this collection, resetting the
    // collection when they arrive. If `add: true` is passed, appends the
    // models to the collection instead of resetting.
    fetch: function(options) {
      options = options ? _.clone(options) : {};
      if (options.parse === void 0) options.parse = true;
      var collection = this;
      var success = options.success;
      options.success = function(resp, status, xhr) {
        collection[options.update ? 'update' : options.add ? 'add' : 'reset'](collection.parse(resp, xhr), options);
        if (success) success(collection, resp, options);
      };
      return this.sync('read', this, options);
    },

    // Create a new instance of a model in this collection. Add the model to the
    // collection immediately, unless `wait: true` is passed, in which case we
    // wait for the server to agree.
    create: function(model, options) {
      var collection = this;
      options = options ? _.clone(options) : {};
      model = this._prepareModel(model, options);
      if (!model) return false;
      if (!options.wait) collection.add(model, options);
      var success = options.success;
      options.success = function(model, resp, options) {
        if (options.wait) collection.add(model, options);
        if (success) success(model, resp, options);
      };
      model.save(null, options);
      return model;
    },

    // **parse** converts a response into a list of models to be added to the
    // collection. The default implementation is just to pass it through.
    parse: function(resp, xhr) {
      return resp;
    },

    // Create a new collection with an identical list of models as this one.
    clone: function() {
      return new this.constructor(this.models);
    },

    // Proxy to _'s chain. Can't be proxied the same way the rest of the
    // underscore methods are proxied because it relies on the underscore
    // constructor.
    chain: function() {
      return _(this.models).chain();
    },

    // Reset all internal state. Called when the collection is reset.
    _reset: function(options) {
      this.length = 0;
      this.models = [];
      this._byId  = {};
      this._byCid = {};
    },

    // Prepare a model or hash of attributes to be added to this collection.
    _prepareModel: function(attrs, options) {
      if (attrs instanceof Model) {
        if (!attrs.collection) attrs.collection = this;
        return attrs;
      }
      options || (options = {});
      options.collection = this;
      var model = new this.model(attrs, options);
      if (!model._validate(model.attributes, options)) return false;
      return model;
    },

    // Internal method to remove a model's ties to a collection.
    _removeReference: function(model) {
      if (this === model.collection) delete model.collection;
      model.off('all', this._onModelEvent, this);
    },

    // Internal method called every time a model in the set fires an event.
    // Sets need to update their indexes when models change ids. All other
    // events simply proxy through. "add" and "remove" events that originate
    // in other collections are ignored.
    _onModelEvent: function(event, model, collection, options) {
      if ((event === 'add' || event === 'remove') && collection !== this) return;
      if (event === 'destroy') this.remove(model, options);
      if (model && event === 'change:' + model.idAttribute) {
        delete this._byId[model.previous(model.idAttribute)];
        if (model.id != null) this._byId[model.id] = model;
      }
      this.trigger.apply(this, arguments);
    }

  });

  // Underscore methods that we want to implement on the Collection.
  var methods = ['forEach', 'each', 'map', 'collect', 'reduce', 'foldl',
    'inject', 'reduceRight', 'foldr', 'find', 'detect', 'filter', 'select',
    'reject', 'every', 'all', 'some', 'any', 'include', 'contains', 'invoke',
    'max', 'min', 'sortedIndex', 'toArray', 'size', 'first', 'head', 'take',
    'initial', 'rest', 'tail', 'last', 'without', 'indexOf', 'shuffle',
    'lastIndexOf', 'isEmpty'];

  // Mix in each Underscore method as a proxy to `Collection#models`.
  _.each(methods, function(method) {
    Collection.prototype[method] = function() {
      var args = slice.call(arguments);
      args.unshift(this.models);
      return _[method].apply(_, args);
    };
  });

  // Underscore methods that take a property name as an argument.
  var attributeMethods = ['groupBy', 'countBy', 'sortBy'];

  // Use attributes instead of properties.
  _.each(attributeMethods, function(method) {
    Collection.prototype[method] = function(value, context) {
      var iterator = _.isFunction(value) ? value : function(model) {
        return model.get(value);
      };
      return _[method](this.models, iterator, context);
    };
  });

  // Backbone.Router
  // ---------------

  // Routers map faux-URLs to actions, and fire events when routes are
  // matched. Creating a new one sets its `routes` hash, if not set statically.
  var Router = Backbone.Router = function(options) {
    options || (options = {});
    if (options.routes) this.routes = options.routes;
    this._bindRoutes();
    this.initialize.apply(this, arguments);
  };

  // Cached regular expressions for matching named param parts and splatted
  // parts of route strings.
  var optionalParam = /\((.*?)\)/g;
  var namedParam    = /:\w+/g;
  var splatParam    = /\*\w+/g;
  var escapeRegExp  = /[-{}[\]+?.,\\^$|#\s]/g;

  // Set up all inheritable **Backbone.Router** properties and methods.
  _.extend(Router.prototype, Events, {

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // Manually bind a single named route to a callback. For example:
    //
    //     this.route('search/:query/p:num', 'search', function(query, num) {
    //       ...
    //     });
    //
    route: function(route, name, callback) {
      if (!_.isRegExp(route)) route = this._routeToRegExp(route);
      if (!callback) callback = this[name];
      Backbone.history.route(route, _.bind(function(fragment) {
        var args = this._extractParameters(route, fragment);
        callback && callback.apply(this, args);
        this.trigger.apply(this, ['route:' + name].concat(args));
        Backbone.history.trigger('route', this, name, args);
      }, this));
      return this;
    },

    // Simple proxy to `Backbone.history` to save a fragment into the history.
    navigate: function(fragment, options) {
      Backbone.history.navigate(fragment, options);
      return this;
    },

    // Bind all defined routes to `Backbone.history`. We have to reverse the
    // order of the routes here to support behavior where the most general
    // routes can be defined at the bottom of the route map.
    _bindRoutes: function() {
      if (!this.routes) return;
      var route, routes = _.keys(this.routes);
      while ((route = routes.pop()) != null) {
        this.route(route, this.routes[route]);
      }
    },

    // Convert a route string into a regular expression, suitable for matching
    // against the current location hash.
    _routeToRegExp: function(route) {
      route = route.replace(escapeRegExp, '\\$&')
                   .replace(optionalParam, '(?:$1)?')
                   .replace(namedParam, '([^\/]+)')
                   .replace(splatParam, '(.*?)');
      return new RegExp('^' + route + '$');
    },

    // Given a route, and a URL fragment that it matches, return the array of
    // extracted parameters.
    _extractParameters: function(route, fragment) {
      return route.exec(fragment).slice(1);
    }

  });

  // Backbone.History
  // ----------------

  // Handles cross-browser history management, based on URL fragments. If the
  // browser does not support `onhashchange`, falls back to polling.
  var History = Backbone.History = function() {
    this.handlers = [];
    _.bindAll(this, 'checkUrl');

    // #1653 - Ensure that `History` can be used outside of the browser.
    if (typeof window !== 'undefined') {
      this.location = window.location;
      this.history = window.history;
    }
  };

  // Cached regex for cleaning leading hashes and slashes.
  var routeStripper = /^[#\/]|\s+$/;

  // Cached regex for stripping leading and trailing slashes.
  var rootStripper = /^\/+|\/+$/g;

  // Cached regex for detecting MSIE.
  var isExplorer = /msie [\w.]+/;

  // Cached regex for removing a trailing slash.
  var trailingSlash = /\/$/;

  // Has the history handling already been started?
  History.started = false;

  // Set up all inheritable **Backbone.History** properties and methods.
  _.extend(History.prototype, Events, {

    // The default interval to poll for hash changes, if necessary, is
    // twenty times a second.
    interval: 50,

    // Gets the true hash value. Cannot use location.hash directly due to bug
    // in Firefox where location.hash will always be decoded.
    getHash: function(window) {
      var match = (window || this).location.href.match(/#(.*)$/);
      return match ? match[1] : '';
    },

    // Get the cross-browser normalized URL fragment, either from the URL,
    // the hash, or the override.
    getFragment: function(fragment, forcePushState) {
      if (fragment == null) {
        if (this._hasPushState || !this._wantsHashChange || forcePushState) {
          fragment = this.location.pathname;
          var root = this.root.replace(trailingSlash, '');
          if (!fragment.indexOf(root)) fragment = fragment.substr(root.length);
        } else {
          fragment = this.getHash();
        }
      }
      return decodeURIComponent(fragment.replace(routeStripper, ''));
    },

    // Start the hash change handling, returning `true` if the current URL matches
    // an existing route, and `false` otherwise.
    start: function(options) {
      if (History.started) throw new Error("Backbone.history has already been started");
      History.started = true;

      // Figure out the initial configuration. Do we need an iframe?
      // Is pushState desired ... is it available?
      this.options          = _.extend({}, {root: '/'}, this.options, options);
      this.root             = this.options.root;
      this._wantsHashChange = this.options.hashChange !== false;
      this._wantsPushState  = !!this.options.pushState;
      this._hasPushState    = !!(this.options.pushState && this.history && this.history.pushState);
      var fragment          = this.getFragment();
      var docMode           = document.documentMode;
      var oldIE             = (isExplorer.exec(navigator.userAgent.toLowerCase()) && (!docMode || docMode <= 7));

      // Normalize root to always include a leading and trailing slash.
      this.root = ('/' + this.root + '/').replace(rootStripper, '/');

      if (oldIE && this._wantsHashChange) {
        this.iframe = Backbone.$('<iframe src="javascript:0" tabindex="-1" />').hide().appendTo('body')[0].contentWindow;
        this.navigate(fragment);
      }

      // Depending on whether we're using pushState or hashes, and whether
      // 'onhashchange' is supported, determine how we check the URL state.
      if (this._hasPushState) {
        Backbone.$(window).bind('popstate', this.checkUrl);
      } else if (this._wantsHashChange && ('onhashchange' in window) && !oldIE) {
        Backbone.$(window).bind('hashchange', this.checkUrl);
      } else if (this._wantsHashChange) {
        this._checkUrlInterval = setInterval(this.checkUrl, this.interval);
      }

      // Determine if we need to change the base url, for a pushState link
      // opened by a non-pushState browser.
      this.fragment = fragment;
      var loc = this.location;
      var atRoot = loc.pathname.replace(/[^\/]$/, '$&/') === this.root;

      // If we've started off with a route from a `pushState`-enabled browser,
      // but we're currently in a browser that doesn't support it...
      if (this._wantsHashChange && this._wantsPushState && !this._hasPushState && !atRoot) {
        this.fragment = this.getFragment(null, true);
        this.location.replace(this.root + this.location.search + '#' + this.fragment);
        // Return immediately as browser will do redirect to new url
        return true;

      // Or if we've started out with a hash-based route, but we're currently
      // in a browser where it could be `pushState`-based instead...
      } else if (this._wantsPushState && this._hasPushState && atRoot && loc.hash) {
        this.fragment = this.getHash().replace(routeStripper, '');
        this.history.replaceState({}, document.title, this.root + this.fragment + loc.search);
      }

      if (!this.options.silent) return this.loadUrl();
    },

    // Disable Backbone.history, perhaps temporarily. Not useful in a real app,
    // but possibly useful for unit testing Routers.
    stop: function() {
      Backbone.$(window).unbind('popstate', this.checkUrl).unbind('hashchange', this.checkUrl);
      clearInterval(this._checkUrlInterval);
      History.started = false;
    },

    // Add a route to be tested when the fragment changes. Routes added later
    // may override previous routes.
    route: function(route, callback) {
      this.handlers.unshift({route: route, callback: callback});
    },

    // Checks the current URL to see if it has changed, and if it has,
    // calls `loadUrl`, normalizing across the hidden iframe.
    checkUrl: function(e) {
      var current = this.getFragment();
      if (current === this.fragment && this.iframe) {
        current = this.getFragment(this.getHash(this.iframe));
      }
      if (current === this.fragment) return false;
      if (this.iframe) this.navigate(current);
      this.loadUrl() || this.loadUrl(this.getHash());
    },

    // Attempt to load the current URL fragment. If a route succeeds with a
    // match, returns `true`. If no defined routes matches the fragment,
    // returns `false`.
    loadUrl: function(fragmentOverride) {
      var fragment = this.fragment = this.getFragment(fragmentOverride);
      var matched = _.any(this.handlers, function(handler) {
        if (handler.route.test(fragment)) {
          handler.callback(fragment);
          return true;
        }
      });
      return matched;
    },

    // Save a fragment into the hash history, or replace the URL state if the
    // 'replace' option is passed. You are responsible for properly URL-encoding
    // the fragment in advance.
    //
    // The options object can contain `trigger: true` if you wish to have the
    // route callback be fired (not usually desirable), or `replace: true`, if
    // you wish to modify the current URL without adding an entry to the history.
    navigate: function(fragment, options) {
      if (!History.started) return false;
      if (!options || options === true) options = {trigger: options};
      fragment = this.getFragment(fragment || '');
      if (this.fragment === fragment) return;
      this.fragment = fragment;
      var url = this.root + fragment;

      // If pushState is available, we use it to set the fragment as a real URL.
      if (this._hasPushState) {
        this.history[options.replace ? 'replaceState' : 'pushState']({}, document.title, url);

      // If hash changes haven't been explicitly disabled, update the hash
      // fragment to store history.
      } else if (this._wantsHashChange) {
        this._updateHash(this.location, fragment, options.replace);
        if (this.iframe && (fragment !== this.getFragment(this.getHash(this.iframe)))) {
          // Opening and closing the iframe tricks IE7 and earlier to push a
          // history entry on hash-tag change.  When replace is true, we don't
          // want this.
          if(!options.replace) this.iframe.document.open().close();
          this._updateHash(this.iframe.location, fragment, options.replace);
        }

      // If you've told us that you explicitly don't want fallback hashchange-
      // based history, then `navigate` becomes a page refresh.
      } else {
        return this.location.assign(url);
      }
      if (options.trigger) this.loadUrl(fragment);
    },

    // Update the hash location, either replacing the current entry, or adding
    // a new one to the browser history.
    _updateHash: function(location, fragment, replace) {
      if (replace) {
        var href = location.href.replace(/(javascript:|#).*$/, '');
        location.replace(href + '#' + fragment);
      } else {
        // #1649 - Some browsers require that `hash` contains a leading #.
        location.hash = '#' + fragment;
      }
    }

  });

  // Create the default Backbone.history.
  Backbone.history = new History;

  // Backbone.View
  // -------------

  // Creating a Backbone.View creates its initial element outside of the DOM,
  // if an existing element is not provided...
  var View = Backbone.View = function(options) {
    this.cid = _.uniqueId('view');
    this._configure(options || {});
    this._ensureElement();
    this.initialize.apply(this, arguments);
    this.delegateEvents();
  };

  // Cached regex to split keys for `delegate`.
  var delegateEventSplitter = /^(\S+)\s*(.*)$/;

  // List of view options to be merged as properties.
  var viewOptions = ['model', 'collection', 'el', 'id', 'attributes', 'className', 'tagName'];

  // Set up all inheritable **Backbone.View** properties and methods.
  _.extend(View.prototype, Events, {

    // The default `tagName` of a View's element is `"div"`.
    tagName: 'div',

    // jQuery delegate for element lookup, scoped to DOM elements within the
    // current view. This should be prefered to global lookups where possible.
    $: function(selector) {
      return this.$el.find(selector);
    },

    // Initialize is an empty function by default. Override it with your own
    // initialization logic.
    initialize: function(){},

    // **render** is the core function that your view should override, in order
    // to populate its element (`this.el`), with the appropriate HTML. The
    // convention is for **render** to always return `this`.
    render: function() {
      return this;
    },

    // Clean up references to this view in order to prevent latent effects and
    // memory leaks.
    dispose: function() {
      this.undelegateEvents();
      if (this.model && this.model.off) this.model.off(null, null, this);
      if (this.collection && this.collection.off) this.collection.off(null, null, this);
      return this;
    },

    // Remove this view from the DOM. Note that the view isn't present in the
    // DOM by default, so calling this method may be a no-op.
    remove: function() {
      this.dispose();
      this.$el.remove();
      return this;
    },

    // For small amounts of DOM Elements, where a full-blown template isn't
    // needed, use **make** to manufacture elements, one at a time.
    //
    //     var el = this.make('li', {'class': 'row'}, this.model.escape('title'));
    //
    make: function(tagName, attributes, content) {
      var el = document.createElement(tagName);
      if (attributes) Backbone.$(el).attr(attributes);
      if (content != null) Backbone.$(el).html(content);
      return el;
    },

    // Change the view's element (`this.el` property), including event
    // re-delegation.
    setElement: function(element, delegate) {
      if (this.$el) this.undelegateEvents();
      this.$el = element instanceof Backbone.$ ? element : Backbone.$(element);
      this.el = this.$el[0];
      if (delegate !== false) this.delegateEvents();
      return this;
    },

    // Set callbacks, where `this.events` is a hash of
    //
    // *{"event selector": "callback"}*
    //
    //     {
    //       'mousedown .title':  'edit',
    //       'click .button':     'save'
    //       'click .open':       function(e) { ... }
    //     }
    //
    // pairs. Callbacks will be bound to the view, with `this` set properly.
    // Uses event delegation for efficiency.
    // Omitting the selector binds the event to `this.el`.
    // This only works for delegate-able events: not `focus`, `blur`, and
    // not `change`, `submit`, and `reset` in Internet Explorer.
    delegateEvents: function(events) {
      if (!(events || (events = _.result(this, 'events')))) return;
      this.undelegateEvents();
      for (var key in events) {
        var method = events[key];
        if (!_.isFunction(method)) method = this[events[key]];
        if (!method) throw new Error('Method "' + events[key] + '" does not exist');
        var match = key.match(delegateEventSplitter);
        var eventName = match[1], selector = match[2];
        method = _.bind(method, this);
        eventName += '.delegateEvents' + this.cid;
        if (selector === '') {
          this.$el.bind(eventName, method);
        } else {
          this.$el.delegate(selector, eventName, method);
        }
      }
    },

    // Clears all callbacks previously bound to the view with `delegateEvents`.
    // You usually don't need to use this, but may wish to if you have multiple
    // Backbone views attached to the same DOM element.
    undelegateEvents: function() {
      this.$el.unbind('.delegateEvents' + this.cid);
    },

    // Performs the initial configuration of a View with a set of options.
    // Keys with special meaning *(model, collection, id, className)*, are
    // attached directly to the view.
    _configure: function(options) {
      if (this.options) options = _.extend({}, this.options, options);
      _.extend(this, _.pick(options, viewOptions));
      this.options = options;
    },

    // Ensure that the View has a DOM element to render into.
    // If `this.el` is a string, pass it through `$()`, take the first
    // matching element, and re-assign it to `el`. Otherwise, create
    // an element from the `id`, `className` and `tagName` properties.
    _ensureElement: function() {
      if (!this.el) {
        var attrs = _.extend({}, _.result(this, 'attributes'));
        if (this.id) attrs.id = _.result(this, 'id');
        if (this.className) attrs['class'] = _.result(this, 'className');
        this.setElement(this.make(_.result(this, 'tagName'), attrs), false);
      } else {
        this.setElement(_.result(this, 'el'), false);
      }
    }

  });

  // Backbone.sync
  // -------------

  // Map from CRUD to HTTP for our default `Backbone.sync` implementation.
  var methodMap = {
    'create': 'POST',
    'update': 'PUT',
    'delete': 'DELETE',
    'read':   'GET'
  };

  // Override this function to change the manner in which Backbone persists
  // models to the server. You will be passed the type of request, and the
  // model in question. By default, makes a RESTful Ajax request
  // to the model's `url()`. Some possible customizations could be:
  //
  // * Use `setTimeout` to batch rapid-fire updates into a single request.
  // * Send up the models as XML instead of JSON.
  // * Persist models via WebSockets instead of Ajax.
  //
  // Turn on `Backbone.emulateHTTP` in order to send `PUT` and `DELETE` requests
  // as `POST`, with a `_method` parameter containing the true HTTP method,
  // as well as all requests with the body as `application/x-www-form-urlencoded`
  // instead of `application/json` with the model in a param named `model`.
  // Useful when interfacing with server-side languages like **PHP** that make
  // it difficult to read the body of `PUT` requests.
  Backbone.sync = function(method, model, options) {
    var type = methodMap[method];

    // Default options, unless specified.
    _.defaults(options || (options = {}), {
      emulateHTTP: Backbone.emulateHTTP,
      emulateJSON: Backbone.emulateJSON
    });

    // Default JSON-request options.
    var params = {type: type, dataType: 'json'};

    // Ensure that we have a URL.
    if (!options.url) {
      params.url = _.result(model, 'url') || urlError();
    }

    // Ensure that we have the appropriate request data.
    if (!options.data && model && (method === 'create' || method === 'update')) {
      params.contentType = 'application/json';
      params.data = JSON.stringify(model);
    }

    // For older servers, emulate JSON by encoding the request into an HTML-form.
    if (options.emulateJSON) {
      params.contentType = 'application/x-www-form-urlencoded';
      params.data = params.data ? {model: params.data} : {};
    }

    // For older servers, emulate HTTP by mimicking the HTTP method with `_method`
    // And an `X-HTTP-Method-Override` header.
    if (options.emulateHTTP && (type === 'PUT' || type === 'DELETE')) {
      params.type = 'POST';
      if (options.emulateJSON) params.data._method = type;
      var beforeSend = options.beforeSend;
      options.beforeSend = function(xhr) {
        xhr.setRequestHeader('X-HTTP-Method-Override', type);
        if (beforeSend) return beforeSend.apply(this, arguments);
      };
    }

    // Don't process data on a non-GET request.
    if (params.type !== 'GET' && !options.emulateJSON) {
      params.processData = false;
    }

    var success = options.success;
    options.success = function(resp, status, xhr) {
      if (success) success(resp, status, xhr);
      model.trigger('sync', model, resp, options);
    };

    var error = options.error;
    options.error = function(xhr, status, thrown) {
      if (error) error(model, xhr, options);
      model.trigger('error', model, xhr, options);
    };

    // Make the request, allowing the user to override any Ajax options.
    return Backbone.ajax(_.extend(params, options));
  };

  // Set the default implementation of `Backbone.ajax` to proxy through to `$`.
  Backbone.ajax = function() {
    return Backbone.$.ajax.apply(Backbone.$, arguments);
  };

  // Helpers
  // -------

  // Helper function to correctly set up the prototype chain, for subclasses.
  // Similar to `goog.inherits`, but uses a hash of prototype properties and
  // class properties to be extended.
  var extend = function(protoProps, staticProps) {
    var parent = this;
    var child;

    // The constructor function for the new subclass is either defined by you
    // (the "constructor" property in your `extend` definition), or defaulted
    // by us to simply call the parent's constructor.
    if (protoProps && _.has(protoProps, 'constructor')) {
      child = protoProps.constructor;
    } else {
      child = function(){ parent.apply(this, arguments); };
    }

    // Add static properties to the constructor function, if supplied.
    _.extend(child, parent, staticProps);

    // Set the prototype chain to inherit from `parent`, without calling
    // `parent`'s constructor function.
    var Surrogate = function(){ this.constructor = child; };
    Surrogate.prototype = parent.prototype;
    child.prototype = new Surrogate;

    // Add prototype properties (instance properties) to the subclass,
    // if supplied.
    if (protoProps) _.extend(child.prototype, protoProps);

    // Set a convenience property in case the parent's prototype is needed
    // later.
    child.__super__ = parent.prototype;

    return child;
  };

  // Set up inheritance for the model, collection, router, view and history.
  Model.extend = Collection.extend = Router.extend = View.extend = History.extend = extend;

  // Throw an error when a URL is needed, and none is supplied.
  var urlError = function() {
    throw new Error('A "url" property or function must be specified');
  };

}).call(this);