RecyclerView grid layout divider. Partly from https://github.com/bignerdranch/simple-item-decoration

GridDividerItemDecoration.java

import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Adds interior dividers to a RecyclerView with a GridLayoutManager.
 */
public class GridDividerItemDecoration extends RecyclerView.ItemDecoration {

    private Drawable mHorizontalDivider;
    private Drawable mVerticalDivider;
    private int mNumColumns;

    public GridDividerItemDecoration(Drawable horizontalDivider, Drawable verticalDivider, int numColumns) {
        mHorizontalDivider = horizontalDivider;
        mVerticalDivider = verticalDivider;
        mNumColumns = numColumns;
    }

   
    @Override
    public void onDraw(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        drawHorizontalDividers(canvas, parent, state);
        drawVerticalDividers(canvas, parent, state);

    }

   
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);

        boolean childIsInLeftmostColumn = (parent.getChildAdapterPosition(view) % mNumColumns) == 0;
        if (childIsInLeftmostColumn) {
            outRect.left = mHorizontalDivider.getIntrinsicWidth();
            outRect.right = mHorizontalDivider.getIntrinsicWidth();
        } else {
            outRect.right = mHorizontalDivider.getIntrinsicWidth();

        }

        boolean childIsInFirstRow = (parent.getChildAdapterPosition(view)) < mNumColumns;

        if (childIsInFirstRow) {
            outRect.top = mVerticalDivider.getIntrinsicHeight();
            outRect.bottom = mVerticalDivider.getIntrinsicHeight();//+outRect.height();
        } else {
            outRect.bottom = mVerticalDivider.getIntrinsicHeight();//+outRect.height();

        }

    }

    private void drawHorizontalDividers(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int rowCount = childCount % mNumColumns == 0 ? childCount / mNumColumns : (childCount / mNumColumns + 1);
        int lastRowChildCount = childCount % mNumColumns == 0 ? mNumColumns : childCount % mNumColumns;

        if (childCount > 0) {
            for (int i = 0; i <= mNumColumns; i++) {

                int firstRowChildIndex = i;
                int lastRowChildIndex;
                if (i < lastRowChildCount) {
                    lastRowChildIndex = i + (rowCount - 1) * mNumColumns;
                } else {
                    lastRowChildIndex = i + (rowCount - 2) * mNumColumns;
                }

                if (firstRowChildIndex < childCount) {
                    View firstRowChild = parent.getChildAt(firstRowChildIndex);
                    View lastRowChild = parent.getChildAt(lastRowChildIndex);
                    if (i == 0) {
                        {
                            int dividerTop = firstRowChild.getTop();
                            int dividerRight = firstRowChild.getLeft();
                            int dividerLeft = dividerRight - mHorizontalDivider.getIntrinsicWidth();
                            int dividerBottom = lastRowChild.getBottom();

                            mHorizontalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                            mHorizontalDivider.draw(canvas);
                        }
                        {
                            int dividerTop = firstRowChild.getTop();
                            int dividerRight = firstRowChild.getRight() + mHorizontalDivider.getIntrinsicWidth();
                            int dividerLeft = firstRowChild.getRight();
                            int dividerBottom = lastRowChild.getBottom();

                            mHorizontalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                            mHorizontalDivider.draw(canvas);
                        }
                    } else {
                        int dividerTop = firstRowChild.getTop();
                        int dividerRight = firstRowChild.getRight() + mHorizontalDivider.getIntrinsicWidth();
                        int dividerLeft = firstRowChild.getRight();
                        int dividerBottom = lastRowChild.getBottom();

                        mHorizontalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                        mHorizontalDivider.draw(canvas);
                    }
                }
            }
        }
    }

 
    private void drawVerticalDividers(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int rowCount = childCount % mNumColumns == 0 ? childCount / mNumColumns : (childCount / mNumColumns + 1);
        int rightmostChildIndex;
        int leftmostChildIndex;
        for (int i = 0; i <= rowCount; i++) {
            if (i == rowCount) {
                leftmostChildIndex = (i - 1) * mNumColumns;
                rightmostChildIndex = parent.getChildCount() - 1;
            } else {
                leftmostChildIndex = i * mNumColumns;
                rightmostChildIndex = (i * mNumColumns) + mNumColumns - 1;
                if (rightmostChildIndex >= childCount) {
                    rightmostChildIndex = childCount - 1;
                }
            }

            View leftmostChild = parent.getChildAt(leftmostChildIndex);
            View rightmostChild = parent.getChildAt(rightmostChildIndex);


            if (i == 0) {
                {
                    int dividerLeft = leftmostChild.getLeft() - mVerticalDivider.getIntrinsicWidth();
                    int dividerBottom = leftmostChild.getTop();
                    int dividerTop = leftmostChild.getTop() - mVerticalDivider.getIntrinsicHeight();
                    int dividerRight = rightmostChild.getRight() + mVerticalDivider.getIntrinsicWidth();

                    mVerticalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                    mVerticalDivider.draw(canvas);
                }
                {
                    int dividerLeft = leftmostChild.getLeft() - mVerticalDivider.getIntrinsicWidth();
                    int dividerBottom = leftmostChild.getBottom() + mVerticalDivider.getIntrinsicHeight();
                    int dividerTop = leftmostChild.getBottom();
                    int dividerRight = rightmostChild.getRight() + mVerticalDivider.getIntrinsicWidth();

                    mVerticalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                    mVerticalDivider.draw(canvas);
                }

            } else {
                int dividerLeft = leftmostChild.getLeft() - mVerticalDivider.getIntrinsicWidth();
                int dividerBottom = leftmostChild.getBottom() + mVerticalDivider.getIntrinsicHeight();
                int dividerTop = leftmostChild.getBottom();
                int dividerRight = rightmostChild.getRight() + mVerticalDivider.getIntrinsicWidth();

                mVerticalDivider.setBounds(dividerLeft, dividerTop, dividerRight, dividerBottom);
                mVerticalDivider.draw(canvas);
            }

        }
    }
}