RVPagerStateListenerSubscriber.kt

package com.d4vidi

import android.support.annotation.Px
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.PagerSnapHelper
import android.support.v7.widget.RecyclerView
import android.view.View
import android.view.ViewTreeObserver

data class VisiblePageState(
        var index: Int,
        var view: View,
        @Px var viewCenterX: Int,
        @Px var distanceToSettledPixels: Int,
        var distanceToSettled: Float)

interface RVPagerStateListener {
    fun onPageScroll(pagesState: List<VisiblePageState>) {}
    fun onScrollStateChanged(state: Int) {}
    fun onPageSelected(index: Int) {}
}

open class RVPagerSnapHelperListenable(private val maxPages: Int = 3) : RecyclerView.OnScrollListener() {
    fun attachToRecyclerView(recyclerView: RecyclerView, listener: RVPagerStateListener) {
        this.assertRecyclerViewSetup(recyclerView)

        setUpSnapHelper(recyclerView, listener)
        setUpScrollListener(recyclerView, listener)
    }

    protected fun setUpScrollListener(recyclerView: RecyclerView, listener: RVPagerStateListener) {
        PagerSnapOnScrollListener(recyclerView, listener, maxPages)
    }

    protected fun setUpSnapHelper(recyclerView: RecyclerView, listener: RVPagerStateListener) {
        PagerSnapHelperListenable(recyclerView, listener).attachToRecyclerView(recyclerView)
    }

    protected fun assertRecyclerViewSetup(recyclerView: RecyclerView) {
        if (recyclerView.layoutManager !is LinearLayoutManager) {
            throw IllegalArgumentException("RVPagerStateListenerSubscriber can only work with a linear layout manager")
        }

        if ((recyclerView.layoutManager as LinearLayoutManager).orientation != LinearLayoutManager.HORIZONTAL) {
            throw IllegalArgumentException("RVPagerStateListenerSubscriber can only work with a horizontal orientation")
        }
    }
}

open class PagerSnapHelperListenable(protected val recyclerView: RecyclerView, val externalListener: RVPagerStateListener)
    : PagerSnapHelper()
    , ViewTreeObserver.OnGlobalLayoutListener {

    var lastPage = RecyclerView.NO_POSITION

    init {
        recyclerView.viewTreeObserver.addOnGlobalLayoutListener(this)
    }

    override fun onGlobalLayout() {
        val position = (recyclerView.layoutManager as LinearLayoutManager).findFirstCompletelyVisibleItemPosition()
        if (position != RecyclerView.NO_POSITION) {
            notifyNewPageIfNeeded(position)
            recyclerView.viewTreeObserver.removeOnGlobalLayoutListener(this)
        }
    }

    override fun findSnapView(layoutManager: RecyclerView.LayoutManager?): View? {
        val view = super.findSnapView(layoutManager)
        notifyNewPageIfNeeded(recyclerView.getChildAdapterPosition(view))
        return view
    }

    override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager?, velocityX: Int, velocityY: Int): Int {
        val position = super.findTargetSnapPosition(layoutManager, velocityX, velocityY)
        notifyNewPageIfNeeded(position)
        return position
    }

    protected fun notifyNewPageIfNeeded(page: Int) {
        if (page != lastPage) {
            this.externalListener.onPageSelected(page)
            lastPage = page
        }
    }
}

open class PagerSnapOnScrollListener(val recyclerView: RecyclerView, val externalListener: RVPagerStateListener, maxPages: Int) : RecyclerView.OnScrollListener() {
    var pageStates: MutableList<VisiblePageState> = ArrayList(maxPages)
    var pageStatesPool = List(maxPages, { _ -> VisiblePageState(0, recyclerView, 0, 0, 0f) })

    init {
        recyclerView.addOnScrollListener(this)
    }

    override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
        val layoutManager = recyclerView!!.layoutManager as LinearLayoutManager

        val firstPos = layoutManager.findFirstVisibleItemPosition()
        val lastPos = layoutManager.findLastVisibleItemPosition()

        val screenEndX = recyclerView.context.resources.displayMetrics.widthPixels
        val midScreen = (screenEndX / 2)

        for (position in firstPos..lastPos) {
            val view = layoutManager.findViewByPosition(position)
            val viewWidth = view.measuredWidth
            val viewStartX = view.x
            val viewEndX = viewStartX + viewWidth
            if (viewEndX >= 0 && viewStartX <= screenEndX) {
                val viewHalfWidth = view.measuredWidth / 2f

                val pageState = pageStatesPool[position - firstPos]
                pageState.index = position
                pageState.view = view
                pageState.viewCenterX = (viewStartX + viewWidth / 2f).toInt()
                pageState.distanceToSettledPixels = (pageState.viewCenterX - midScreen)
                pageState.distanceToSettled = (pageState.viewCenterX + viewHalfWidth) / (midScreen + viewHalfWidth)
                pageStates.add(pageState)
            }
        }
        externalListener.onPageScroll(pageStates)

        // Clear this in advance so as to avoid holding refs to views.
        pageStates.clear()
    }

    override fun onScrollStateChanged(recyclerView: RecyclerView?, newState: Int) {
        externalListener.onScrollStateChanged(newState)
    }
}