GridSpacingItemDecoration.java

package com.example.widget;

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


public class GridSpacingItemDecoration extends RecyclerView.ItemDecoration
{
	private int mSpacing;


	public GridSpacingItemDecoration(int spacingPixelSize)
	{
		mSpacing = spacingPixelSize;
	}


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

		int position = recyclerView.getChildLayoutPosition(view);
		int spanCount = getSpanCount(recyclerView);
		int itemCount = recyclerView.getAdapter().getItemCount();

		// top offset
		if(position < spanCount)
		{
			outRect.top = mSpacing;
		}
		else
		{
			outRect.top = mSpacing / 2;
		}

		// bottom offset
		if(itemCount % spanCount == 0 && position >= itemCount - spanCount)
		{
			outRect.bottom = mSpacing;
		}
		else if(itemCount % spanCount != 0 && position >= itemCount - itemCount % spanCount)
		{
			outRect.bottom = mSpacing;
		}
		else
		{
			outRect.bottom = mSpacing / 2;
		}

		// left offset
		if(position % spanCount == 0)
		{
			outRect.left = mSpacing;
		}
		else
		{
			outRect.left = mSpacing / 2;
		}

		// right offset
		if(position % spanCount == spanCount - 1)
		{
			outRect.right = mSpacing;
		}
		else
		{
			outRect.right = mSpacing / 2;
		}
	}


	private int getSpanCount(RecyclerView recyclerView)
	{
		if(recyclerView.getLayoutManager() instanceof GridLayoutManager)
		{
			GridLayoutManager gridLayoutManager = (GridLayoutManager) recyclerView.getLayoutManager();
			return gridLayoutManager.getSpanCount();
		}
		else
		{
			throw new IllegalStateException(this.getClass().getSimpleName() + " can only be used with a " + GridLayoutManager.class.getSimpleName());
		}
	}
}