VerticalGridCardSpacingDecoration.java

package com.airbnb.epoxy.sample;

import android.graphics.Rect;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.GridLayoutManager.SpanSizeLookup;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.RecyclerView.ItemDecoration;
import android.support.v7.widget.RecyclerView.State;
import android.util.DisplayMetrics;
import android.util.TypedValue;
import android.view.View;

import static android.util.TypedValue.COMPLEX_UNIT_DIP;

/**
 * Modifies item spacing in a recycler view so that items are equally spaced no matter where they
 * are on the grid.
 */
public class VerticalGridCardSpacingDecoration extends ItemDecoration {
  private static final int OUTER_PADDING_DP = 16;
  private static final int INNER_PADDING_DP = 8;
  private int outerPadding = -1;
  private int innerPadding = -1;

  @Override
  public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
    if (outerPadding == -1 || innerPadding == -1) {
      DisplayMetrics m = view.getResources().getDisplayMetrics();
      outerPadding = (int) TypedValue.applyDimension(COMPLEX_UNIT_DIP, OUTER_PADDING_DP, m);
      innerPadding = (int) TypedValue.applyDimension(COMPLEX_UNIT_DIP, INNER_PADDING_DP, m);
    }

    int position = parent.getChildAdapterPosition(view);
    if (position == RecyclerView.NO_POSITION) {
      // View is not shown
      return;
    }

    final GridLayoutManager layoutManager = (GridLayoutManager) parent.getLayoutManager();
    final SpanSizeLookup spanSizeLookup = layoutManager.getSpanSizeLookup();

    // Zero everything out for the common case
    outRect.setEmpty();

    int spanSize = spanSizeLookup.getSpanSize(position);
    int spanCount = layoutManager.getSpanCount();
    int spanIndex = spanSizeLookup.getSpanIndex(position, spanCount);

    if (spanSize == spanCount) {
      // Only item in row
      outRect.left = outerPadding;
      outRect.right = outerPadding;
    } else if (spanIndex == 0) {
      // First item in row
      outRect.left = outerPadding;
      outRect.right = innerPadding;
    } else if (spanIndex == spanCount - 1) {
      // Last item in row
      outRect.left = innerPadding;
      outRect.right = outerPadding;
    } else {
      // Inner item (not relevant for less than three columns)
      outRect.left = innerPadding;
      outRect.right = innerPadding;
    }
  }
}