javascript - 检测 Web 下拉菜单是否要显示在屏幕外

Question

我有一个基于 CSS 的简单多级下拉菜单。第二级或第三级可能会超出可见窗口，具有特定的分辨率和窗口大小组合。

一些预建的菜单控件如果检测到这种情况，只会向左而不是向右打开下拉菜单。

我该如何测试(使用 JS/jQuery)这种情况？

Answer 1

您可以使用以下函数测试菜单项是否在屏幕外:

/*---   function bIsNodeClippedOrOffscreen returns true if a node
        is offscreen (without scrolling).
        Requires jQuery.
*/
function bIsNodeClippedOrOffscreen (zJnode)
{
    var aDivPos             = zJnode.offset ();
    var iLeftPos            = aDivPos.left;
    var iTopPos             = aDivPos.top;

    var iDivWidth           = zJnode.outerWidth  (true);
    var iDivHeight          = zJnode.outerHeight (true);

    var bOffScreen          = CheckIfPointIsOffScreen (iLeftPos, iTopPos);
    var bClipped            = CheckIfPointIsOffScreen (iLeftPos + iDivWidth, iTopPos + iDivHeight);

    return (bOffScreen || bClipped);
}


function CheckIfPointIsOffScreen (iLeftPos, iTopPos)
{
    var iBrowserWidth       = $(window).width()  - 16;   //-- 16 is fudge for scrollbars, refine later
    var iBrowserHeight      = $(window).height() - 16;   //-- 16 is fudge for scrollbars, refine later
    var bOffScreen          = false;

    if (iLeftPos < 0  ||  iLeftPos >= iBrowserWidth)
        bOffScreen          = true;

    if (iTopPos < 0   ||  iTopPos >= iBrowserHeight)
        bOffScreen          = true;

    return bOffScreen;
}

.
示例用法:

<li id="SomeMenuItem"> Get your shopping cart for free!
...
...

var Node            = $("#SomeMenuItem");

var NeedToMoveIt    = bIsNodeClippedOrOffscreen (Node);