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Add column spacing in RecyclerView with GridLayoutManager

Android Recyclerview GridLayoutManager column spacing Here is the question, the first answer does not work well in my project,and it makes the spacing bigger between item and item. the second answer is quite perfect.But if RecyclerView has headers,it does not work well. Then I fixed it.

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

/**
 * Created by Android Studio
 * User: Ailurus(ailurus@foxmail.com)
 * Date: 2015-10-28
 * Time: 15:20
 */
public class GridSpacingItemDecoration extends RecyclerView.ItemDecoration {

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

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

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

        if (position >= 0) {
            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
                }
            }
        } else {
            outRect.left = 0;
            outRect.right = 0;
            outRect.top = 0;
            outRect.bottom = 0;
        }
    }
}

If you want to use it, just do as the following.

int spacingInPixels = getResources().getDimensionPixelSize(R.dimen.grid_layout_margin);
recyclerView.addItemDecoration(new GridSpacingItemDecoration(2, spacingInPixels, true, 0));
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