原理

利用滑动的距离控制toolbar的alpha，达到上拉显示，下拉隐藏效果

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/mRv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        />

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/mToolbar"
        android:background="@color/purple_200"
        app:title="hello"
        android:alpha="0"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>
</RelativeLayout>

 mRv.addOnScrollListener(object :RecyclerView.OnScrollListener(){
            //dy 垂直滚动的距离
            override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
                super.onScrolled(recyclerView, dx, dy)
                "dy---->$totalDy".e()
                //totalDy 总的垂直滑动距离
                totalDy += dy
                if(totalDy < 0){
                    return
                }
                if(totalDy <= 180){
                    val alpha = totalDy / 180.0f
                    "alpha---->$alpha".e()
                    mToolbar.alpha = alpha
                }else{
                    mToolbar.alpha = 1.0f
                }
            }
        })