GridItemDecoration.java

package support.ui.widget;

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

public class GridItemDecoration extends RecyclerView.ItemDecoration {

  private int spanCount;
  private int spacing;
  private boolean includeEdge;

  public GridItemDecoration(int spanCount, int spacing, boolean includeEdge) {
    this.spanCount = spanCount;
    this.spacing = spacing;
    this.includeEdge = includeEdge;
  }

  @Override
  public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
                             RecyclerView.State state) {
    int position = parent.getChildAdapterPosition(view); // item position
    int column = position % spanCount; // item column

    if (includeEdge) {
      outRect.left = spacing - column * spacing / spanCount; // spacing - column * ((1f / spanCount) * spacing)
      outRect.right = (column + 1) * spacing / spanCount; // (column + 1) * ((1f / spanCount) * spacing)

      if (position < spanCount) { // top edge
        outRect.top = spacing;
      }
      outRect.bottom = spacing; // item bottom
    } else {
      outRect.left = column * spacing / spanCount; // column * ((1f / spanCount) * spacing)
      outRect.right = spacing - (column + 1) * spacing / spanCount; // spacing - (column + 1) * ((1f /    spanCount) * spacing)
      if (position >= spanCount) {
        outRect.top = spacing; // item top
      }
    }
  }
}