ScrollToPosition bug fixed version of GridLayoutManager

GridLayoutQuirksFixedManager.java

package android.support.v7.widget;

import android.content.Context;
import android.util.AttributeSet;
import android.view.View;

/**
 * Bug fixed version of GridLayoutManager
 * Created by SangsooNam on 20/09/15.
 * <p/>
 * --- Bug ---
 * <br>
 * https://code.google.com/p/android/issues/detail?id=182517
 * <br>
 * After calling `scrollToPosition`, only first grid column is shown and others are shown in
 * the other row. This is due to wrong `anchorInfo.mPosition`. There are two modes depending on the
 * value `anchorInfo.mLayoutFromEnd`. One is to layout from start to end, and the other is to layout
 * from end to start. When to layout from start to end, `ensureAnchorInFirstSpan` is reasonable.
 * However, it doesn't make sense when to layout from end to start. At that time, we need to ensure
 * that anchor is in the last span instead of the first. This class is fixing that issue.
 */
public class GridLayoutQuirksFixedManager extends GridLayoutManager {

	public GridLayoutQuirksFixedManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
		super(context, attrs, defStyleAttr, defStyleRes);
	}

	public GridLayoutQuirksFixedManager(Context context, int spanCount) {
		super(context, spanCount);
	}

	public GridLayoutQuirksFixedManager(Context context, int spanCount, int orientation, boolean reverseLayout) {
		super(context, spanCount, orientation, reverseLayout);
	}

	@Override
	void onAnchorReady(RecyclerView.State state, AnchorInfo anchorInfo) {
		updateMeasurements();
		if (state.getItemCount() > 0 && !state.isPreLayout()) {
			if (anchorInfo.mLayoutFromEnd) {
				ensureAnchorIsInLastSpan(anchorInfo);
			} else {
				ensureAnchorIsInFirstSpan(anchorInfo);
			}
		}
		if (mSet == null || mSet.length != mSpanCount) {
			mSet = new View[mSpanCount];
		}
	}

	private void ensureAnchorIsInLastSpan(AnchorInfo anchorInfo) {
		int spanIndex = mSpanSizeLookup.getCachedSpanIndex(anchorInfo.mPosition, mSpanCount);
		int spanSize = mSpanSizeLookup.getSpanSize(anchorInfo.mPosition);
		while (spanIndex + spanSize < mSpanCount && anchorInfo.mPosition < getItemCount()) {
			anchorInfo.mPosition++;
			spanIndex = mSpanSizeLookup.getCachedSpanIndex(anchorInfo.mPosition, mSpanCount);
			spanSize = mSpanSizeLookup.getSpanSize(anchorInfo.mPosition);
		}
	}

	/**
	 * The same code from GridLayoutManager
	 */
	private void ensureAnchorIsInFirstSpan(AnchorInfo anchorInfo) {
		int span = mSpanSizeLookup.getCachedSpanIndex(anchorInfo.mPosition, mSpanCount);
		while (span > 0 && anchorInfo.mPosition > 0) {
			anchorInfo.mPosition--;
			span = mSpanSizeLookup.getCachedSpanIndex(anchorInfo.mPosition, mSpanCount);
		}
	}

	/**
	 * The same code from GridLayoutManager
	 */
	private void updateMeasurements() {
		int totalSpace;
		if (getOrientation() == VERTICAL) {
			totalSpace = getWidth() - getPaddingRight() - getPaddingLeft();
		} else {
			totalSpace = getHeight() - getPaddingBottom() - getPaddingTop();
		}
		calculateItemBorders(totalSpace);
	}

	/**
	 * The same code from GridLayoutManager
	 */
	private void calculateItemBorders(int totalSpace) {
		if (mCachedBorders == null || mCachedBorders.length != mSpanCount + 1
			|| mCachedBorders[mCachedBorders.length - 1] != totalSpace) {
			mCachedBorders = new int[mSpanCount + 1];
		}
		mCachedBorders[0] = 0;
		int sizePerSpan = totalSpace / mSpanCount;
		int sizePerSpanRemainder = totalSpace % mSpanCount;
		int consumedPixels = 0;
		int additionalSize = 0;
		for (int i = 1; i <= mSpanCount; i++) {
			int itemSize = sizePerSpan;
			additionalSize += sizePerSpanRemainder;
			if (additionalSize > 0 && (mSpanCount - additionalSize) < sizePerSpanRemainder) {
				itemSize += 1;
				additionalSize -= mSpanCount;
			}
			consumedPixels += itemSize;
			mCachedBorders[i] = consumedPixels;
		}
	}

}