function(callback) {     
            var settings        = this.settings;
            
            if ($.inArray(settings.mode, ["gfm", "markdown"]) < 0)
            {
                return this;
            }
            
            this.state.watching = settings.watch = true;
            this.preview.show();
            
            if (this.toolbar)
            {
                var watchIcon   = settings.toolbarIconsClass.watch;
                var unWatchIcon = settings.toolbarIconsClass.unwatch;
                
                var icon        = this.toolbar.find(".fa[name=watch]");
                icon.parent().attr("title", settings.lang.toolbar.watch);
                icon.removeClass(unWatchIcon).addClass(watchIcon);
            }
            
            this.codeMirror.css("border-right", "1px solid #ddd").width(this.editor.width() / 2); 
            
            timer = 0;
            
            this.save().resize();
            
            if (!settings.onwatch)
            {
                settings.onwatch = callback || function() {};
            }
            
            $.proxy(settings.onwatch, this)();
            
            return this;
        }