StickySectionItemDecoration Kotlin

StickySectionItemDecoration.kt

class StickySectionItemDecoration(
  private val recyclerView: RecyclerView,
  private val callback: Callback
) : RecyclerView.ItemDecoration() {

  private var headerViewCache: View? = null
  private var isInLayout = false

  interface Callback {
    fun isHeader(position: Int): Boolean
    fun getHeaderView(): View?
    fun onHeaderViewClick()
  }

  init {
    recyclerView.addOnItemTouchListener(object : OnItemTouchListener {
      override fun onTouchEvent(
        rv: RecyclerView,
        e: MotionEvent
      ) {
      }

      override fun onInterceptTouchEvent(
        rv: RecyclerView,
        e: MotionEvent
      ): Boolean {
        if (!isInLayout && isNotNull(headerViewCache) && e.y < headerViewCache!!.height) {
          if (e.action == MotionEvent.ACTION_UP) callback.onHeaderViewClick()
          return true
        }
        return false
      }

      override fun onRequestDisallowInterceptTouchEvent(disallowIntercept: Boolean) {}
    })
  }

  override fun onDrawOver(
    c: Canvas,
    parent: RecyclerView,
    state: RecyclerView.State
  ) {

    if (isNull(headerViewCache) || headerViewCache != callback.getHeaderView()) {
      headerViewCache = callback.getHeaderView()
      headerViewCache?.let {
        calculateLayoutSize(it, parent)
      }
    }

    for (i in 0 until parent.childCount) {
      val child = parent.getChildAt(i)
      val position = parent.getChildAdapterPosition(child)

      if (callback.isHeader(position)) {
        val firstCompletelyVisiblePosition = (parent.layoutManager as LinearLayoutManager).findFirstCompletelyVisibleItemPosition()
        isInLayout = firstCompletelyVisiblePosition <= position
        break
      }
    }

    if (!isInLayout and isNotNull(headerViewCache)) {
      drawHeader(c, headerViewCache!!)
    } else {
      super.onDrawOver(c, parent, state)
    }
  }

  private fun drawHeader(
    c: Canvas,
    headerView: View
  ) {
    c.save()
    c.translate(recyclerView.paddingLeft.toFloat(), if (recyclerView.clipToPadding) recyclerView.paddingTop.toFloat() else 0f)
    headerView.draw(c)
    c.restore()
  }

  private fun calculateLayoutSize(
    view: View,
    parent: ViewGroup
  ) {
    val widthSpec = View.MeasureSpec.makeMeasureSpec(parent.width, View.MeasureSpec.EXACTLY)
    val heightSpec = View.MeasureSpec.makeMeasureSpec(parent.height, View.MeasureSpec.UNSPECIFIED)
    val childWidth = ViewGroup.getChildMeasureSpec(widthSpec, parent.paddingStart + parent.paddingEnd, view.layoutParams.width)
    val childHeight = ViewGroup.getChildMeasureSpec(heightSpec, parent.paddingTop + parent.paddingBottom, view.layoutParams.height)

    view.apply {
      measure(childWidth, childHeight)
      layout(0, 0, view.measuredWidth, view.measuredHeight)
    }
  }
}