GridHeaderSpacingItemDecoration.java

package net.derohimat.basemvp.ui.view;

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

public class GridHeaderSpacingItemDecoration extends RecyclerView.ItemDecoration {
    private int spanCount;
    private int spacing;
    private boolean includeEdge;
    private boolean includeHeader;
    public GridHeaderSpacingItemDecoration(int spanCount, int spacing, boolean includeEdge, boolean includeHeader) {
        this.spanCount = spanCount;
        this.spacing = spacing;
        this.includeEdge = includeEdge;
        this.includeHeader = includeHeader;
    }
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        int position = parent.getChildAdapterPosition(view); // item position
        if (includeHeader && position == 0)
            return;
        if (includeHeader && spanCount > 2) {
            position = position - (spanCount - 1);
        }
        int column = position % spanCount; // item column
        if (includeEdge) {
            outRect.left = spacing - column * spacing / spanCount; // spacing - column * ((1f / spanCount) * spacing)
//            if (includeHeader && position == 0) {
//                outRect.right = column * spacing / spanCount; // (column + 1) * ((1f / spanCount) * spacing)
//            } else {
            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
            }
        }
    }
}