javascript - "Cannot read property ' 内容 ' of null": Psuedo Elements

Question

我从 here 改编了一些代码为了检测我的 Phonegap 应用程序中的方向变化(为横向和纵向加载不同的 CSS 文件)。我收到“无法读取空错误的属性‘内容’，暗示 DOM 调用有问题。代码涉及将这些伪元素添加到主 CSS 文件:

/*Orientation Detection Hack*/
body:after {
    content: "";
    position: absolute;
    color: transparent;
}

@media all and (orientation: portrait) {
    body:after {
        content: "p";
    }
}

@media all and (orientation: landscape) {
    body:after {
        content: "l";
    }
}
/*end of orientation detection*/

这个事件监听器在方向改变时检查伪元素:

/*ORIENTATION CHANGES*/
    //get style of the body:after element
var bodyAfterStyle = window.getComputedStyle(document.body, ":after");

window.onorientationchange = function() {


  navigator.notification.alert(
        'ORIENTATION CHANGELOL',  // message
        'Credits',            // title
        'Credits');              // buttonName

  if(bodyAfterStyle.content == 'p') {

/*whatever
    navigator.notification.alert("Now in Portrait");
    var sheet = document.createElement('link');
    sheet.innerHTML = 'href=\"portrait.css\" rel=\"stylesheet\"  type=\"text/css\"';
    document.body.appendChild(sheet);
end whatever*/
} 
    else if(bodyAfterStyle.content == 'l') {
    //do landscape stuff
  }
}
    /*END OF ORIENTATION STUFF*/

Answer 1

bodyAfterStyle 似乎丢失了，我建议将它的赋值移动到事件函数中。