如何检测是否有应用程序进入全屏或退出全屏？

// 进入全屏
function enterFullscreen(element) {
  if (element.requestFullscreen) {
    element.requestFullscreen();
  } else if (element.mozRequestFullScreen) {
    element.mozRequestFullScreen();
  } else if (element.webkitRequestFullscreen) {
    element.webkitRequestFullscreen();
  } else if (element.msRequestFullscreen) {
    element.msRequestFullscreen();
  }
}

// 退出全屏
function exitFullscreen() {
  if (document.exitFullscreen) {
    document.exitFullscreen();
  } else if (document.mozCancelFullScreen) {
    document.mozCancelFullScreen();
  } else if (document.webkitExitFullscreen) {
    document.webkitExitFullscreen();
  } else if (document.msExitFullscreen) {
    document.msExitFullscreen();
  }
}

// 检测是否处于全屏状态
function isFullscreen() {
  return document.fullscreenElement ||
         document.mozFullScreenElement ||
         document.webkitFullscreenElement ||
         document.msFullscreenElement;
}

document.addEventListener('fullscreenchange', function() {
  if (isFullscreen()) {
    // 进入全屏
    console.log('进入全屏');
  } else {
    // 退出全屏
    console.log('退出全屏');
  }
});

/* 进入全屏时，设置元素的样式 */
:fullscreen {
  background-color: #000;
}

/* 退出全屏时，设置元素的样式 */
:not(:fullscreen) {
  background-color: #fff;
}