backbone 学习之sync

sync 是 Backbone 每次向服务器读取或保存模型时都要调用执行的函数。 默认情况下，它使用 (jQuery/Zepto).ajax 方法请求。 如果想采用其他的方案也可以，比如 WebSockets, XML, 或 Local Storage，我们可以重载该函数即可大道我们的需要。

下面分析下它的代码逻辑：

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

  // 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.
    // 确保有url 没在option中的话 从model获取
    if (!options.url) {
      params.url = _.result(model, 'url') || urlError();
    }

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

    // 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' || type === 'PATCH')) {
      params.type = 'POST';
      if (options.emulateJSON) params.data._method = type;
      // 在请求之前需要做的处理
      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;
    }

    // If we're sending a `PATCH` request, and we're in an old Internet Explorer
    // that still has ActiveX enabled by default, override jQuery to use that
    // for XHR instead. Remove this line when jQuery supports `PATCH` on IE8.
    // 兼容jquery的patch请求
    if (params.type === 'PATCH' && window.ActiveXObject &&
          !(window.external && window.external.msActiveXFilteringEnabled)) {
      params.xhr = function() {
        return new ActiveXObject("Microsoft.XMLHTTP");
      };
    }

    // Make the request, allowing the user to override any Ajax options.
    var xhr = options.xhr = Backbone.ajax(_.extend(params, options));
    model.trigger('request', model, xhr, options);// 触发request请求
    return xhr;
  };

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

  // Set the default implementation of `Backbone.ajax` to proxy through to `$`.
  // Override this if you'd like to use a different library.
  // 默认jquery zepto的ajax请求
  Backbone.ajax = function() {
    return Backbone.$.ajax.apply(Backbone.$, arguments);
  };

欢迎指导、纠错、建议。