最近遇到需要做ajax延迟的需求，上一段代码记录下：

var XHRService = function(options) {
    this.options = options;
    this.reqTimer = null;
};

XHRService.prototype = {
    constructor: XHRService,

    req: function(options) {
        var that = this,
            defer = $.Deferred();

        clearTimeout(this.reqTimer);
        this.reqTimer = setTimeout(function() {
            that._req(options, defer);
        }, this.options.reqDelay);

        return defer.promise();
    },

    _req: function(options, defer) {
        var that = this;

        if (this.xhr) {
            this.xhr.abort();
        }

        if (typeof this.options.before === 'function') {
            clearTimeout(this.loadingTimer);
            this.loadingTimer = setTimeout(function() {
                that.options.before();
            }, this.options.loadingDelay);
        }

        this.xhr = $.ajax(options).done(function(data) {
                defer.resolve(data);
            })
            .always(function(res, status, xhrObj) {
                clearTimeout(that.loadingTimer);

                if (typeof that.options.after === 'function') {
                    that.options.after();
                }

                if (xhrObj === that.xhr) {
                    that.xhr = null;
                }
            });
    }
};
var xhr = new XHRService({
    reqDelay: 10,
    loadingDelay: 10,
    before: function() {
        console.log('show loading...');//显示loadingbar
    },
    after: function() {
        console.log('hide loading...');//隐藏loadingbar
    }
});
 xhr.req({
        url: url,
        dataType: 'json'
    }).done(function(data) {
        console.log('done!');
    });