RecyclerView实现抖音纵向滚动ViewPager效果

class MyLinearLayoutManager : LinearLayoutManager {

  private lateinit var mPagerSnapHelper: PagerSnapHelper
  private var mOnViewPagerListener: OnViewPagerListener? = null
  private lateinit var mRecyclerView: RecyclerView
  private var mDrift: Int = 0//位移，用来判断移动方向

  constructor(context: Context) : this(context, OrientationHelper.VERTICAL)
  constructor(context: Context, orientation: Int) : this(context, orientation, false)
  constructor(context: Context, orientation: Int, reverseLayout: Boolean) : super(context, orientation, reverseLayout) {
    mPagerSnapHelper = PagerSnapHelper()
  }

  override fun onAttachedToWindow(view: RecyclerView) {
    super.onAttachedToWindow(view)
    mPagerSnapHelper.attachToRecyclerView(view)//设置RecyclerView每次滚动一页
    mRecyclerView = view
    mRecyclerView.addOnChildAttachStateChangeListener(mChildAttachStateChangeListener)
  }


  /**
   * 滑动状态的改变
   * 缓慢拖拽-> SCROLL_STATE_DRAGGING
   * 快速滚动-> SCROLL_STATE_SETTLING
   * 空闲状态-> SCROLL_STATE_IDLE
   * @param state
   */
  override fun onScrollStateChanged(state: Int) {
    if (state == RecyclerView.SCROLL_STATE_IDLE){
      val viewIdle = mPagerSnapHelper.findSnapView(this)
      val positionIdle = getPosition(viewIdle!!)
      if (mOnViewPagerListener != null && childCount == 1) {
        mOnViewPagerListener!!.onPageSelected(positionIdle, positionIdle == itemCount - 1)
      }
    }
  }

  /**
   * 布局完成后调用
   * @param state
   */
  override fun onLayoutCompleted(state: RecyclerView.State?) {
    super.onLayoutCompleted(state)
    if (mOnViewPagerListener != null) mOnViewPagerListener!!.onLayoutComplete()
  }

  /**
   * 监听竖直方向的相对偏移量
   */
  override fun scrollVerticallyBy(dy: Int, recycler: RecyclerView.Recycler?, state: RecyclerView.State?): Int {
    this.mDrift = dy
    return super.scrollVerticallyBy(dy, recycler, state)
  }


  /**
   * 监听水平方向的相对偏移量
   */
  override fun scrollHorizontallyBy(dx: Int, recycler: RecyclerView.Recycler?, state: RecyclerView.State?): Int {
    this.mDrift = dx
    return super.scrollHorizontallyBy(dx, recycler, state)
  }

  /**
   * 设置监听
   * @param listener
   */
  fun setOnViewPagerListener(listener: OnViewPagerListener) {
    this.mOnViewPagerListener = listener
  }

  private val mChildAttachStateChangeListener = object : RecyclerView.OnChildAttachStateChangeListener {
    override fun onChildViewAttachedToWindow(view: View) {
    }

    override fun onChildViewDetachedFromWindow(view: View) {
      if (mDrift >= 0) {
        if (mOnViewPagerListener != null) mOnViewPagerListener!!.onPageRelease(true, getPosition(view))
      } else {
        if (mOnViewPagerListener != null) mOnViewPagerListener!!.onPageRelease(false, getPosition(view))
      }

    }
  }

  interface OnViewPagerListener{
    /*释放的监听*/
    fun onPageRelease(isNext: Boolean, position: Int)

    /*选中的监听以及判断是否滑动到底部*/
    fun onPageSelected(position: Int, isBottom: Boolean)

    /*布局完成的监听*/
    fun onLayoutComplete()
  }
}

class MyImageView : ImageView {
  constructor(context: Context) : this(context, null!!)
  constructor(context: Context, attr: AttributeSet) : this(context, attr, 0)
  constructor(context: Context, attr: AttributeSet, defStyleAttr: Int) : super(context, attr, defStyleAttr)

  override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
    val width = View.getDefaultSize(0, widthMeasureSpec)
    val height = View.getDefaultSize(0, heightMeasureSpec)
    setMeasuredDimension(width, height)
  }

}