gridlayoutmanager水平居中

要让 `GridLayoutManager` 水平居中，可以使用自定义的 `CenterGridLayoutManager` 类，如下所示：

public class CenterGridLayoutManager extends GridLayoutManager {

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

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

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

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        super.onLayoutChildren(recycler, state);

        if (getItemCount() == 0) {
            return;
        }

        int viewWidth = getWidth();
        int viewHeight = getHeight();

        int childWidth = getDecoratedMeasuredWidth(getChildAt(0));
        int childHeight = getDecoratedMeasuredHeight(getChildAt(0));

        int spanCount = getSpanCount();
        int rowCount = getItemCount() / spanCount + (getItemCount() % spanCount == 0 ? 0 : 1);

        int horizontalSpace = getDecoratedWidth(getChildAt(0)) * (spanCount - 1);
        int verticalSpace = getDecoratedHeight(getChildAt(0)) * (rowCount - 1);

        int totalWidth = childWidth * spanCount + horizontalSpace;
        int totalHeight = childHeight * rowCount + verticalSpace;

        int left = (viewWidth - totalWidth) / 2;
        int top = (viewHeight - totalHeight) / 2;

        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            int position = getPosition(child);

            int row = position / spanCount;
            int col = position % spanCount;

            int leftPos = left + col * childWidth;
            int topPos = top + row * childHeight;

            layoutDecorated(child, leftPos, topPos, leftPos + childWidth, topPos + childHeight);
        }
    }
}

该类继承了 `GridLayoutManager`，并在 `onLayoutChildren` 方法中实现了居中布局的逻辑。在 `onLayoutChildren` 方法中，先计算出所有子项的总宽度和总高度，然后计算出左边和上边的偏移量，最后遍历所有子项，分别设置子项的位置。设置子项位置时，根据子项所在的行列计算出左上角和右下角的坐标，然后调用 `layoutDecorated` 方法设置子项的位置。