//待拖拽的元素
let wf = document.getElementById('waifu');
//获取元素与浏览器的宽高
setTimeout(main,5000);//延时3秒执行页面跳转方法
    
function main() {
    
	let winWidth = window.innerWidth,
		winHeight = window.innerHeight,
		imgWidth = wf.offsetWidth,
		imgHeight = wf.offsetHeight;
	//在待拖拽的元素上绑定鼠标按下事件
	wf.onmousedown = function (event) {
		//兼容IE
		event = event || window.event;
		//阻止浏览器默认行为兼容IE的写法
		event.preventDefault ? event.preventDefault() : event.returnValue = false;
		//记录光标在图片按下时的坐标
		let _offsetX = event.offsetX,
			_offsetY = event.offsetY;
		//绑定鼠标移动事件
		document.onmousemove = function (event) {
			//获取光标在可视窗口中的坐标
			let _x = event.clientX,
				_y = event.clientY;
			//计算拖动的图片的定位的位置
			let _left = _x - _offsetX,
				_top = _y - _offsetY;
			//判断是否在窗口范围内
			if (_top < 0) { //上
				_top = 0;
			} else if (_top >= winHeight - imgHeight) { //下
				_top = winHeight - imgHeight;
			}
			if (_left < 0) { //左
				_left = 0;
			} else if (_left >= winWidth - imgWidth) { //右
				_left = winWidth - imgWidth;
			}
			//设置拖动过程中图片的定位
			wf.style.top = _top + 'px';
			wf.style.left = _left + 'px';
		}
		//绑定鼠标弹起事件
		document.onmouseup = function () {
			document.onmousemove = null;
		}
	}
	//当浏览器的窗口大小被改变时重设宽高
	window.onresize = function(){
		winWidth = window.innerWidth;
		winHeight = window.innerHeight;	
	}
}