基于Javascript的媒体查询

Question

我正在尝试开发一个响应式导航菜单，它可以动态地创建一个“更多..”当屏幕尺寸低于特定宽度时显示菜单项。

到目前为止，我的代码如下：

Html：

<ul id="menuElem" class="clearfix">
    <li class="HighLighted"><a href="#">Menu Item 1</a></li>
    <li><a href="#">Menu Item 2</a></li>
    <li><a href="#">Menu Item 3</a></li>
    <li><a href="#">Menu Item 4</a></li>
    <li><a href="#">Menu Item 5</a></li>
    <li><a href="#">Menu Item 6</a></li>
</ul>

Javascript：

function MoreMenu () {
    //Checks if sub-menu exsists
    if ($(".sub-menu").length > 0) {
        //if it does then prepends second-last menu item to sub menu
        $(".sub-menu").prepend($("#menuElem > li:nth-last-child(2)"));
    }
    else {
        //if it doesn't exsist then appends a list item with a menu-item "more" having a sub-menu and then prepends second-last menu item to this sub menu.
        $("#menuElem").append("<li class='more'><a href='#'>More...</a><ul class='sub-menu'></ul></li>");
        $(".sub-menu").prepend($("#menuElem > li:nth-last-child(2)"));
    }
}

function RemoveMoreMenu () {
    //checks if sub-menu has something
    if ($(".sub-menu li").length > 0) {
        //if it does then the first child is taken out from the sub-menu and added back to the main menu.   
        $(".sub-menu li:first-child").insertBefore($("#menuElem > li:last-child"));

        //if sub-menu doesn't have any more children then it removes the "more" menu item.
        if ($(".sub-menu li").length === 0) {
            $(".more").remove();
        }   
    }
}

function Resize() {
    benchmark = 800; //Maximum width required to run the function
    $(window).resize((function() {
        currentWidth = $(window).width(); //Current browser width
        if (benchmark - currentWidth > 0) { 
            //checks if the browser width is less than maximum width required and if true it trigers the MoreMenu function              
            MoreMenu ();
            console.log("screen size resized down");
        } 
        else {
        }
    }));
}

问题是，当我运行Resize()函数时，它实际上会为每个小于800px的窗口调整活动运行MoreMenu()函数-这是不理想的。

那么，有没有办法在屏幕尺寸低于800时只运行一次MoreMenu()函数呢？

提前感谢-Struggling让我明白了javascript :)

Answer 1

跟踪resize事件处理程序之前的宽度，以便只在传递宽度限制时调用MoreMenu和RemoveMoreMenu。

var previousWidth = $(window).width();
var benchmark = 800;

$(window).resize(function() {
    var newWidth = $(window).width();
    if (newWidth < benchmark && previousWidth >= benchmark) {
        MoreMenu();
    }
    else if (newWidth >= benchmark && previousWidth < benchmark) {
        RemoveMoreMenu();
    }
    previousWidth = newWidth;
});

如果previousWidth从一开始就低于基准测试，那么您可能还希望在开始时运行MoreMenu。