此篇文章给出在Android上用Kotlin实现ScrollView和RecyclerView的嵌套滚动。
首先看一下实现后的效果：
我们需要了解的是Android已为我们实现了ScrollView的嵌套类NestedScrollView和RecyclerView的嵌套。NestedScrollView实现了NestedScrollingParent3和NestedScrollingChild3这两个支持嵌套的接口；RecyclerView实现了NestedScrollingChild2, NestedScrollingChild3这两个支持嵌套的接口。为了实现以上效果，简单的方式就是继承以上类来自定义实现。
实际上只需要自定义NestedScrollView就行了，原理涉及到事件分发，分发原理本文不涉及。
NestedScrollViewFather类的代码：
class NestedScrollViewFather : NestedScrollView {
private val TAG = "NestedScrollViewFather"
private val DEBUG = false
constructor(context: Context) :super(context) {
init()
}
constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
init()
}
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(
context,
attrs,
defStyleAttr
) {
init()
}
private lateinit var topView: View
private lateinit var contentView: ViewGroup
private lateinit var mFlingHelper: FlingHelper
private var totalDy = 0
/**
* 用于判断RecyclerView是否在fling
*/
private var isStartingFling = false
/**
* 记录当前滑动的y轴加速度
*/
private var velocityY = 0
private fun init() {
mFlingHelper = FlingHelper(context)
setOnScrollChangeListener { v, scrollX, scrollY, oldScrollX, oldScrollY ->
if (isStartingFling) {
totalDy = 0
isStartingFling = false
}
if(0 == scrollY) {
if (DEBUG) {
Log.d(TAG, "TOP SCROLL")
}
}
if (scrollY == (getChildAt(0).measuredHeight - v.measuredHeight)) {
if (DEBUG) {
Log.d(TAG, "BOTTOM SCROLL")
}
dispatchChildFling()
}
//在RecyclerView fling情况下，记录当前RecyclerView在y轴的偏移
totalDy += scrollY - oldScrollY
}
}
private fun dispatchChildFling() {
if (0 != velocityY) {
val splineFlingDistance = mFlingHelper.getSplineFlingDistance(velocityY)
if (splineFlingDistance > totalDy) {
childFling(mFlingHelper.getVelocityByDistance(splineFlingDistance - totalDy))
}
}
totalDy = 0
velocityY = 0
}
private fun childFling(velY: Int) {
getChildRecyclerView(contentView)?.fling(0, velY)
}
override fun fling(velocityY: Int) {
super.fling(velocityY)
if (velocityY <= 0) {
this.velocityY = 0
} else {
isStartingFling = true
this.velocityY = velocityY
}
}
override fun onFinishInflate() {
super.onFinishInflate()
topView = (getChildAt(0) as ViewGroup).getChildAt(0)
contentView = (getChildAt(0) as ViewGroup).getChildAt(1) as ViewGroup
}
override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec)
val lp = contentView.layoutParams!!.also {
it.height = measuredHeight
}
if (DEBUG) {
Log.e(TAG, "lp.height=${lp.height}")
}
contentView.layoutParams = lp
}
override fun onNestedPreScroll(target: View, dx: Int, dy: Int, consumed: IntArray, type: Int) {
val hideTop = dy > 0 && scrollY < topView.measuredHeight
if (hideTop) {
scrollBy(0, dy)
consumed[1] = dy
}
}
companion object {
fun getChildRecyclerView(viewGroup: ViewGroup) : RecyclerView? {
for (i in 0 until viewGroup.childCount) {
val view = viewGroup.getChildAt(i)
if (view is NestLogRecyclerView) {
return view
} else if (view is ViewGroup) {
return getChildRecyclerView(view)
}
}
return null
}
}
}
列出速度与距离换算的工具类：
class FlingHelper constructor(context: Context) {
companion object {
private val DECELERATION_RATE = ln(.78) / ln(.9)
private val mFlingFriction = ViewConfiguration.getScrollFriction()
}
private var mPhysicalCoeff = context.resources.displayMetrics.density * 160 * 386.0878f * .84f
private fun getSplineDeceleration(i: Int): Double {
return ln((.35 * abs(i)) / (mFlingFriction * mPhysicalCoeff))
}
private fun getSplineDecelerationByDistance(d: Double) : Double {
return (DECELERATION_RATE - 1) * ln((d / (mFlingFriction * mPhysicalCoeff))) / DECELERATION_RATE
}
/**
* 速度 转换成 距离
*/
fun getSplineFlingDistance(i: Int): Double {
return exp(getSplineDeceleration(i) * (DECELERATION_RATE / (DECELERATION_RATE - 1)) * mFlingFriction * mPhysicalCoeff)
}
/**
* 距离 转换成 速度
*/
fun getVelocityByDistance(d: Double): Int {
return abs(exp(getSplineDecelerationByDistance(d)) * mFlingFriction * mPhysicalCoeff / .3499999940395355).toInt()
}
}
配套的XML布局文件代码：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/conflict_root"
android:layout_width="match_parent"
android:layout_height="match_parent">
<com.tikeda.binley.conflictdemo.nested.NestedScrollViewFather
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:layout_width="match_parent"
android:layout_height="60dp"
android:textSize="24sp"
android:textColor="@color/black"
android:text="这是第一个"
android:gravity="center"/>
<TextView
android:layout_width="match_parent"
android:layout_height="60dp"
android:textSize="24sp"
android:textColor="@color/black"
android:text="这是第二个"
android:gravity="center"/>
<TextView
android:layout_width="match_parent"
android:layout_height="60dp"
android:textSize="24sp"
android:textColor="@color/black"
android:text="这是第三个"
android:gravity="center"/>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="46dp"
android:orientation="horizontal"
android:background="@color/purple_200">
<TextView
android:layout_width="0dp"
android:layout_height="match_parent"
android:text="表1"
android:textColor="@color/black"
android:gravity="center"
android:layout_weight="1"/>
<TextView
android:layout_width="0dp"
android:layout_height="match_parent"
android:text="表2"
android:textColor="@color/black"
android:gravity="center"
android:layout_weight="1"/>
<TextView
android:layout_width="0dp"
android:layout_height="match_parent"
android:text="表3"
android:textColor="@color/black"
android:gravity="center"
android:layout_weight="1"/>
</LinearLayout>
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/rv_demo"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
</LinearLayout>
</LinearLayout>
</com.tikeda.binley.conflictdemo.nested.NestedScrollViewFather>
</LinearLayout>
NestedScrollViewFather由于内部写死了获取topView和contentView的规则，所以仅针对给出的XML代码有效
标签：velocityY,return,Kotlin,private,ScrollView,context,fun,RecyclerView From： https://www.cnblogs.com/swalka/p/16815187.html