<view class="mask-con {{!hidden " bindtap="sendDynamic">
  <image class="sendDynamic mask-con 
	{
	{
	!hidden " src="/UploadFiles/2021-04-02/发布.png">

wxss：


.sendDynamic{
 height: 100rpx;
 width: 100rpx;
 bottom:100rpx;
 right: 60rpx;
 border-radius: 50%;
 position: fixed;
 box-shadow:5rpx 5rpx 5rpx #fccee5;
}
.mask-con{
 transition: 0.5s; 
 position: fixed;
 width: 100rpx;
 height: 100rpx;
 bottom:-100rpx;
 right: 60rpx;
 text-align: center;
 line-height: 100rpx;
 }
.mask-con-show{
 bottom: 100rpx;
}


js:


data: {
 hidden:false,
 scrollTop: 0
 },
 onPageScroll(ev){
 var _this = this;
 if (ev.scrollTop <= 0) {
  ev.scrollTop = 0;
 } else if (ev.scrollTop > wx.getSystemInfoSync().windowHeight) {
  ev.scrollTop = wx.getSystemInfoSync().windowHeight;
 }
 if (ev.scrollTop > this.data.scrollTop || ev.scrollTop == wx.getSystemInfoSync().windowHeight) {
  this.setData({
  hidden:true
  })
 } else {
  this.setData({
  hidden:false
  })
 }
 setTimeout(function () {
  _this.setData({
  scrollTop: ev.scrollTop
  })
 }, 0)
 },


以上简单三步，完成目标。
总结