MiddleDividerItemDecoration, suitable for both LinearLayoutManager and GridLayoutManager.

MiddleDividerItemDecoration.kt

/*
 * Copyright 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


package androidx.recyclerview.widget

import android.content.Context
import android.graphics.Canvas
import android.graphics.Rect
import android.graphics.drawable.Drawable
import android.util.Log
import android.view.View
import android.widget.LinearLayout
import android.graphics.PorterDuff
import android.graphics.PorterDuffColorFilter

/**
 * MiddleDividerItemDecoration is a [RecyclerView.ItemDecoration] that can be used as a divider
 * between items of a [LinearLayoutManager]. It supports both [.HORIZONTAL] and
 * [.VERTICAL] orientations.
 * It can also supports [.ALL], included both the horizontal and vertical. Mainly used for GridLayout.
 * <pre>
 * For normal usage with LinearLayout,
 * val mItemDecoration = MiddleDividerItemDecoration(context!!,DividerItemDecoration.VERTICAL)
 * For GridLayoutManager with inner decorations,
 * val mItemDecoration = MiddleDividerItemDecoration(context!!,MiddleDividerItemDecoration.ALL)
 * recyclerView.addItemDecoration(mItemDecoration);
</pre> *
 */
class MiddleDividerItemDecoration
/**
 * Creates a divider [RecyclerView.ItemDecoration] that can be used with a
 * [LinearLayoutManager].
 *
 * @param context Current context, it will be used to access resources.
 * @param orientation Divider orientation. Should be [.HORIZONTAL] or [.VERTICAL].
 */
    (context: Context, orientation: Int) : RecyclerView.ItemDecoration() {

    private var mDivider: Drawable? = null

    /**
     * Current orientation. Either [.HORIZONTAL] or [.VERTICAL].
     */
    private var mOrientation: Int = 0

    private val mBounds = Rect()

    init {
        val a = context.obtainStyledAttributes(ATTRS)
        mDivider = a.getDrawable(0)
        if (mDivider == null) {
            Log.w(
                TAG,
                "@android:attr/listDivider was not set in the theme used for this " + "DividerItemDecoration. Please set that attribute all call setDrawable()"
            )
        }
        a.recycle()
        setOrientation(orientation)
    }

    fun setDividerColor(color: Int) {
        mDivider!!.colorFilter = PorterDuffColorFilter(color, PorterDuff.Mode.SRC_ATOP)
    }

    /**
     * Sets the orientation for this divider. This should be called if
     * [RecyclerView.LayoutManager] changes orientation.
     *
     * @param orientation [.HORIZONTAL] or [.VERTICAL]
     */
    fun setOrientation(orientation: Int) {
        if (orientation != HORIZONTAL && orientation != VERTICAL && orientation != ALL) {
            throw IllegalArgumentException(
                "Invalid orientation. It should be either HORIZONTAL or VERTICAL"
            )
        }
        mOrientation = orientation
    }

    /**
     * Sets the [Drawable] for this divider.
     *
     * @param drawable Drawable that should be used as a divider.
     */
    fun setDrawable(drawable: Drawable) {
        if (drawable == null) {
            throw IllegalArgumentException("Drawable cannot be null.")
        }
        mDivider = drawable
    }

    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        if (parent.layoutManager == null || mDivider == null) {
            return
        }

        when (mOrientation) {
            ALL -> {
                drawVertical(c, parent)
                drawHorizontal(c, parent)
            }
            VERTICAL -> drawVertical(c, parent)
            else -> drawHorizontal(c, parent)
        }
    }

    private fun drawVertical(canvas: Canvas, parent: RecyclerView) {
        canvas.save()
        val left: Int
        val right: Int

        if (parent.clipToPadding) {
            left = parent.paddingLeft
            right = parent.width - parent.paddingRight
            canvas.clipRect(
                left, parent.paddingTop, right,
                parent.height - parent.paddingBottom
            )
        } else {
            left = 0
            right = parent.width
        }

        var childCount = parent.childCount
        if (parent.layoutManager is GridLayoutManager) {
            var leftItems = childCount % (parent.layoutManager as GridLayoutManager).spanCount
            if (leftItems == 0) {
                leftItems = (parent.layoutManager as GridLayoutManager).spanCount
            }
            //Identify last row, and don't draw border for these items
            childCount -= leftItems
        }

        for (i in 0 until childCount - 1) {
            val child = parent.getChildAt(i) ?: return
            parent.getDecoratedBoundsWithMargins(child, mBounds)
            val bottom = mBounds.bottom + Math.round(child.translationY)
            val top = bottom - mDivider!!.intrinsicHeight
            mDivider!!.setBounds(left, top, right, bottom)
            mDivider!!.draw(canvas)
        }
        canvas.restore()
    }

    private fun drawHorizontal(canvas: Canvas, parent: RecyclerView) {
        canvas.save()
        val top: Int
        val bottom: Int

        if (parent.clipToPadding) {
            top = parent.paddingTop
            bottom = parent.height - parent.paddingBottom
            canvas.clipRect(
                parent.paddingLeft, top,
                parent.width - parent.paddingRight, bottom
            )
        } else {
            top = 0
            bottom = parent.height
        }

        var childCount = parent.childCount
        if (parent.layoutManager is GridLayoutManager) {
            childCount = (parent.layoutManager as GridLayoutManager).spanCount
        }

        for (i in 0 until childCount - 1) {
            val child = parent.getChildAt(i) ?: return
            parent.layoutManager!!.getDecoratedBoundsWithMargins(child, mBounds)
            val right = mBounds.right + Math.round(child.translationX)
            val left = right - mDivider!!.intrinsicWidth
            mDivider!!.setBounds(left, top, right, bottom)
            mDivider!!.draw(canvas)
        }
        canvas.restore()
    }

    override fun getItemOffsets(
        outRect: Rect, view: View, parent: RecyclerView,
        state: RecyclerView.State
    ) {
        if (mDivider == null) {
            outRect.set(0, 0, 0, 0)
            return
        }
        if (mOrientation == VERTICAL) {
            outRect.set(0, 0, 0, mDivider!!.intrinsicHeight)
        } else {
            outRect.set(0, 0, mDivider!!.intrinsicWidth, 0)
        }
    }

    companion object {
        val HORIZONTAL = LinearLayout.HORIZONTAL
        val VERTICAL = LinearLayout.VERTICAL
        //mainly used for GridLayoutManager
        val ALL = 2

        private val TAG = "DividerItem"
        private val ATTRS = intArrayOf(android.R.attr.listDivider)
    }
}

This is a very cool solution. Thanks a lot.

  var childCount = parent.childCount
        if (parent.layoutManager is GridLayoutManager) {
            childCount = (parent.layoutManager as GridLayoutManager).spanCount
        }

        for (i in 0 until childCount - 1) {
              val child = parent.getChildAt(i)

Here will NPE when children count is small than span count, shoud change to

for (i in 0 until (childCount - 1).coerceAtMost(parent.childCount)) {
            val child = parent.getChildAt(i)