Window.screenTop

Window.screenTop 只读属性返回垂直距离,单位是CSS 像素, 从用户浏览器的上边界到屏幕最顶端。

Note: screenTop is an alias of the older Window.screenY property. screenTop was originally supported only in IE but was introduced everywhere due to popularity.

语法

topWindowPos = window.screenTop

返回值

A number equal to the number of CSS pixels from the top edge of the browser viewport to the  top edge of the screen.

例子

In our screenleft-screentop example, you'll see a canvas onto which has been drawn a circle. In this example we are using screenLeft/screenTop plus Window.requestAnimationFrame() to constantly redraw the circle in the same physical position on the screen, even if the window position is moved.

initialLeft = window.screenLeft + canvasElem.offsetLeft;
initialTop = window.screenTop + canvasElem.offsetTop;

function positionElem() {
  let newLeft = window.screenLeft + canvasElem.offsetLeft;
  let newTop = window.screenTop + canvasElem.offsetTop;

  let leftUpdate = initialLeft - newLeft;
  let topUpdate = initialTop - newTop;

  ctx.fillStyle = 'rgb(0, 0, 0)';
  ctx.fillRect(0, 0, width, height);
  ctx.fillStyle = 'rgb(0, 0, 255)';
  ctx.beginPath();
  ctx.arc(leftUpdate + (width/2), topUpdate + (height/2) + 35, 50, degToRad(0), degToRad(360), false);
  ctx.fill();

  pElem.textContent = 'Window.screenLeft: ' + window.screenLeft + ', Window.screenTop: ' + window.screenTop;

  window.requestAnimationFrame(positionElem);
}

window.requestAnimationFrame(positionElem);

Also in the code we include a snippet that detects whether screenLeft is supported, and if not, polyfills in screenLeft/screenTop using Window.screenX/Window.screenY.

if(!window.screenLeft) {
  window.screenLeft = window.screenX;
  window.screenTop = window.screenY;
}

说明

Specification Status Comment
CSS Object Model (CSSOM) View Module
Window.screenTop
Working Draft Initial definition.

浏览器兼容性

BCD tables only load in the browser

参考