被分割线造成的痛苦面具

问题描述：

了解到自定义分割线要在自定义一个RecyclerView.ItemDecoration类，但是在网上的解答和里面用的方法都让我一脸懵逼，实在是痛苦面具带来好几天都搞不懂这些代码到底怎样绘制的分割线，已经浏览过很多类似的帖子了下面是找了相对简单的一个

源码在此：https://blog.csdn.net/liuxingrong666/article/details/83178213

例如这里LayoutParams代表的是什么含义（虽然已经查过但还是觉得云里雾里）；二是left right top bottom是绘制的分割线的长度吗，它们这些值真的看不懂；还有最后这第一行画顶部分割线又是什么意思呢

这边最后一列不是应该不需要绘制左边吗？

​最后一行不是应该不需要绘制顶部吗？真的看了好几天代码好像一个蹦都看不懂

相关代码：

public class GridItemDivider extends RecyclerView.ItemDecoration {

    private String TAG = getClass().getSimpleName();
    private static final int[] ATTRS = new int[]{android.R.attr.listDivider};
    private Drawable divider;

    public GridItemDivider(Context context) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        divider = a.getDrawable(0);
        a.recycle();
    }

    public GridItemDivider(Drawable drawable) {
        divider = drawable;
    }

    public GridItemDivider(int height, int color) {
        GradientDrawable shapeDrawable = new GradientDrawable();
        shapeDrawable.setColor(color);
        shapeDrawable.setShape(GradientDrawable.RECTANGLE);
        shapeDrawable.setSize(height, height);
        divider = shapeDrawable;
    }


    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {

        drawHorizontal(c, parent);
        drawVertical(c, parent);

    }

    private int getSpanCount(RecyclerView parent) {
        // 列数
        int spanCount = -1;
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {

            spanCount = ((GridLayoutManager) layoutManager).getSpanCount();
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            spanCount = ((StaggeredGridLayoutManager) layoutManager)
                    .getSpanCount();
        }
        return spanCount;
    }

    public void drawHorizontal(Canvas c, RecyclerView parent) {
        int childCount = parent.getChildCount(); //获取可见item的数量
        int spanCount = getSpanCount(parent);
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                    .getLayoutParams();
            final int left = child.getLeft() - params.leftMargin;
            final int right = child.getRight() + params.rightMargin
                    + divider.getIntrinsicWidth();
            final int top = child.getBottom() + params.bottomMargin;
            final int bottom = top + divider.getIntrinsicHeight();
            divider.setBounds(left, top, right, bottom);
            divider.draw(c);
            if (i < spanCount) { //画第一行顶部的分割线
                drawHorizontalForFirstRow(c, child);
            }
        }
    }

    private void drawHorizontalForFirstRow(Canvas c, View child) {
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                .getLayoutParams();
        int left = child.getLeft() - params.leftMargin - divider.getIntrinsicWidth();
        int top = child.getTop() - params.topMargin - divider.getIntrinsicHeight();
        int right = child.getRight() + params.rightMargin + divider.getIntrinsicWidth();
        int bottom = top + divider.getIntrinsicHeight();
        divider.setBounds(left, top, right, bottom);
        divider.draw(c);
    }

    private void drawVerticalForFirstColum(Canvas c, View child) {
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                .getLayoutParams();
        int left = child.getLeft() - params.leftMargin - divider.getIntrinsicWidth();
        int top = child.getTop() - params.topMargin;
        int right = child.getLeft() - params.leftMargin;
        int bottom = top + child.getHeight() + divider.getIntrinsicHeight();
        divider.setBounds(left, top, right, bottom);
        divider.draw(c);
    }

    public void drawVertical(Canvas c, RecyclerView parent) {
        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);

            final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                    .getLayoutParams();
            final int top = child.getTop() - params.topMargin;
            final int bottom = child.getBottom() + params.bottomMargin;
            final int left = child.getRight() + params.rightMargin;
            final int right = left + divider.getIntrinsicWidth();
            divider.setBounds(left, top, right, bottom);
            divider.draw(c);
            if (isFirstColum(parent, i, getSpanCount(parent))) { //画第一列左边分割线
                drawVerticalForFirstColum(c, child);
            }
        }
    }

    private boolean isLastColum(RecyclerView parent, int pos, int spanCount,
                                int childCount) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            if ((pos + 1) % spanCount == 0)// 如果是最后一列，则不需要绘制右边
            {
                return true;
            }
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            int orientation = ((StaggeredGridLayoutManager) layoutManager)
                    .getOrientation();
            if (orientation == StaggeredGridLayoutManager.VERTICAL) {
                if ((pos + 1) % spanCount == 0)// 如果是最后一列，则不需要绘制右边
                {
                    return true;
                }
            } else {
                childCount = childCount - childCount % spanCount;
                if (pos >= childCount)// 如果是最后一列，则不需要绘制右边
                    return true;
            }
        }
        return false;
    }

    private boolean isLastRaw(RecyclerView parent, int pos, int spanCount,
                              int childCount) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            childCount = childCount - childCount % spanCount;
            if (pos >= childCount)// 如果是最后一行，则不需要绘制底部
                return true;
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            int orientation = ((StaggeredGridLayoutManager) layoutManager)
                    .getOrientation();
            if (orientation == StaggeredGridLayoutManager.VERTICAL) {
                childCount = childCount - childCount % spanCount;
                // 如果是最后一行，则不需要绘制底部
                if (pos >= childCount)
                    return true;
            } else
            {
                // 如果是最后一行，则不需要绘制底部
                if ((pos + 1) % spanCount == 0) {
                    return true;
                }
            }
        }
        return false;
    }

    //是否为第一列
    private boolean isFirstColum(RecyclerView parent, int pos, int spanCount) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) { //网格布局
            if ((pos + 1) % spanCount == 1) {
                return true;
            }
        }
        return false;
    }

    //是否为第一行
    private boolean isFirstRaw(int pos, int spanCount) {
        if (pos < spanCount) {
            return true;
        }
        return false;
    }

    @Override
    public void getItemOffsets(Rect outRect, int itemPosition,
                               RecyclerView parent) {
        int spanCount = getSpanCount(parent); //列数

        if (itemPosition == 0) { //第一行第一个，四边都画
            outRect.set(divider.getIntrinsicWidth(), divider.getIntrinsicHeight(),
                    divider.getIntrinsicWidth(), divider.getIntrinsicHeight());
        } else if (isFirstRaw(itemPosition, spanCount)) { //第一行，画上下右三边
            outRect.set(0, divider.getIntrinsicHeight(), divider.getIntrinsicWidth(), divider.getIntrinsicHeight());
        } else if (isFirstColum(parent, itemPosition, spanCount)) { //第一列，画左右下三边
            outRect.set(divider.getIntrinsicWidth(), 0, divider.getIntrinsicWidth(), divider.getIntrinsicHeight());
        } else { //其他，画右下两边
            outRect.set(0, 0, divider.getIntrinsicWidth(), divider.getIntrinsicHeight());
        }

    }

}