ViewPager2嵌套RecyclerView滑动冲突解决办法

在嵌套的滚动视图与包含它的 ViewPager2 对象方向相同的情况下，ViewPager2 本身并不支持该滚动视图。例如，在垂直方向的 ViewPager2 对象内，垂直滚动视图无法滚动。

为了支持方向相同的 ViewPager2 对象内的滚动视图，如果您希望改为滚动嵌套的元素，则必须对 ViewPager2 对象调用 requestDisallowInterceptTouchEvent()。ViewPager2 嵌套滚动示例展示了一种使用通用自定义封装容器布局解决此问题的办法。

由于ViewPager2是个final类，无法重写，而RecyclerViewImpl是ViewPager2的私有类，也无法被继承，所以要解决滑动冲突，只能通过监听子类RecyclerView的分发事件通过requestDisallowInterceptTouchEvent来限制父布类的拦截事件

Child can scroll, disallow all parents to intercept
parent.requestDisallowInterceptTouchEvent(true)

• 方案一：
  使用自定义View包裹子类的RecyclerView，实现如下：

/**
 * Layout to wrap a scrollable component inside a ViewPager2. Provided as a solution to the problem
 * where pages of ViewPager2 have nested scrollable elements that scroll in the same direction as
 * ViewPager2. The scrollable element needs to be the immediate and only child of this host layout.
 *
 * This solution has limitations when using multiple levels of nested scrollable elements
 * (e.g. a horizontal RecyclerView in a vertical RecyclerView in a horizontal ViewPager2).
 */
class NestedScrollableHost : FrameLayout {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)

    private var touchSlop = 0
    private var initialX = 0f
    private var initialY = 0f
    private val parentViewPager: ViewPager2?
        get() {
            var v: View? = parent as? View
            while (v != null && v !is ViewPager2) {
                v = v.parent as? View
            }
            return v as? ViewPager2
        }

    private val child: View? get() = if (childCount > 0) getChildAt(0) else null

    init {
        touchSlop = ViewConfiguration.get(context).scaledTouchSlop
    }

    private fun canChildScroll(orientation: Int, delta: Float): Boolean {
        val direction = -delta.sign.toInt()
        return when (orientation) {
            0 -> child?.canScrollHorizontally(direction) ?: false
            1 -> child?.canScrollVertically(direction) ?: false
            else -> throw IllegalArgumentException()
        }
    }

    override fun onInterceptTouchEvent(e: MotionEvent): Boolean {
        handleInterceptTouchEvent(e)
        return super.onInterceptTouchEvent(e)
    }

    private fun handleInterceptTouchEvent(e: MotionEvent) {
        val orientation = parentViewPager?.orientation ?: return

        // Early return if child can't scroll in same direction as parent
        if (!canChildScroll(orientation, -1f) && !canChildScroll(orientation, 1f)) {
            return
        }

        if (e.action == MotionEvent.ACTION_DOWN) {
            initialX = e.x
            initialY = e.y
            parent.requestDisallowInterceptTouchEvent(true)
        } else if (e.action == MotionEvent.ACTION_MOVE) {
            val dx = e.x - initialX
            val dy = e.y - initialY
            val isVpHorizontal = orientation == ORIENTATION_HORIZONTAL

            // assuming ViewPager2 touch-slop is 2x touch-slop of child
            val scaledDx = dx.absoluteValue * if (isVpHorizontal) .5f else 1f
            val scaledDy = dy.absoluteValue * if (isVpHorizontal) 1f else .5f

            if (scaledDx > touchSlop || scaledDy > touchSlop) {
                if (isVpHorizontal == (scaledDy > scaledDx)) {
                    // Gesture is perpendicular, allow all parents to intercept
                    parent.requestDisallowInterceptTouchEvent(false)
                } else {
                    // Gesture is parallel, query child if movement in that direction is possible
                    if (canChildScroll(orientation, if (isVpHorizontal) dx else dy)) {
                        // Child can scroll, disallow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(true)
                    } else {
                        // Child cannot scroll, allow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(false)
                    }
                }
            }
        }
    }
}

使用如上自定义View包裹RecyclerView

• 方案二
  重写RecyclerView的dispatchTouchEvent方法，实现如下：

public class RecyclerViewAtViewPager2 extends RecyclerView {

    public RecyclerViewAtViewPager2(@NonNull Context context) {
        super(context);
    }

    public RecyclerViewAtViewPager2(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public RecyclerViewAtViewPager2(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    private int startX, startY;

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                startX = (int) ev.getX();
                startY = (int) ev.getY();
                getParent().requestDisallowInterceptTouchEvent(true);
                break;
            case MotionEvent.ACTION_MOVE:
                int endX = (int) ev.getX();
                int endY = (int) ev.getY();
                int disX = Math.abs(endX - startX);
                int disY = Math.abs(endY - startY);
                LogUtils.debugInfo("DispatchTouchEvent disX="+ disX + "; disY" + disY + "; canScrollHorizontally(startX - endX) = " + canScrollHorizontally(startX - endX) + "; canScrollVertically(startY - endY)" + canScrollVertically(startY - endY));
                if (disX > disY) {
                    //如果是纵向滑动，告知父布局不进行时间拦截，交由子布局消费，　requestDisallowInterceptTouchEvent(true)
                    getParent().requestDisallowInterceptTouchEvent(canScrollHorizontally(startX - endX));
                } else {
                    getParent().requestDisallowInterceptTouchEvent(canScrollVertically(startX - endX));
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                getParent().requestDisallowInterceptTouchEvent(false);
                break;
        }
        return super.dispatchTouchEvent(ev);
    }
}

使用此自定义View代替RecyclerView

refrence: https://developer.android.google.cn/training/animation/vp2-migration?hl=zh_cn