RecyclerView 四周圆角显示控制，最大高度扩展

• 1、依赖引用CardView 方式
• • • 1.1 添加CardView依赖
    • 1.2 常用属性
    • 1.3 Tips
    • 1.4 组件使用
• 2、[RecyclerView添加ItemDecoration](https://github.com/wordplat/RecyclerViewCornerRadius).
• 3、MaxHeightRecyclerView [RecyclerView扩展最大高度 ](https://blog.csdn.net/z226688/article/details/83722638).
• • • 扩展
    • 在style.xml文件中加入
    • 在layout.xml直接应用
• 4、九宫格分割线 无最右边与最下边线

	implementation 'androidx.cardview:cardview:1.0.0'

	// 阴影的大小
	app:cardElevation 
	// 阴影最大高度
	app:cardMaxElevation 
	// 卡片的背景色
	app:cardBackgroundColor 
	// 卡片的圆角大小
	app:cardCornerRadius 
	// 卡片内容于边距的间隔
	app:contentPadding 
	// 设置content内边距
	app:contentPaddingBottom
	app:contentPaddingTop
	app:contentPaddingLeft
	app:contentPaddingRight
	app:contentPaddingStart
	app:contentPaddingEnd
	// 设置内边距，V21+的版本和之前的版本仍旧具有一样的计算方式
	app:cardUseCompatPadding 
	// 在V20和之前的版本中添加内边距，这个属性为了防止内容和边角的
	app:cardPreventConrerOverlap

	// 设置圆角
	app:cardCornerRadius
	
	// 设置背景颜色,这里设置背景不是setBackground！！！
	app:cardBackgroundColor
	
	// 添加padding
	app:cardUseCompatPadding
	
	// 解决CardView四个角有灰色阴影的问题
	app:cardElevation="0dp"
	
	// CardView 5.0以下出现内边距问题,去掉自动添加的padding
	cardPreventCornerOverlap = "false"

	public class MaxHeightRecyclerView extends RecyclerView {
	    private int mMaxHeight;
	
	    public MaxHeightRecyclerView(Context context) {
	        super(context);
	    }
	
	    public MaxHeightRecyclerView(Context context, AttributeSet attrs) {
	        super(context, attrs);
	        initialize(context, attrs);
	    }
	
	    public MaxHeightRecyclerView(Context context, AttributeSet attrs, int defStyleAttr) {
	        super(context, attrs, defStyleAttr);
	        initialize(context, attrs);
	    }
	
	    private void initialize(Context context, AttributeSet attrs) {
	        TypedArray arr = context.obtainStyledAttributes(attrs, R.styleable.MaxHeightRecyclerView);
	        mMaxHeight = arr.getLayoutDimension(R.styleable.MaxHeightRecyclerView_maxHeight, mMaxHeight);
	        arr.recycle();
	    }
	
	    @Override
	    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	        if (mMaxHeight > 0) {
	            heightMeasureSpec = MeasureSpec.makeMeasureSpec(mMaxHeight, MeasureSpec.AT_MOST);
	        }
	        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
	    }
	}

	app:maxHeight="400dp"

public class GridDividerItemDecoration extends RecyclerView.ItemDecoration {
    private static final int[] ATTRS = new int[]{android.R.attr.listDivider};
    private Drawable mDivider;

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

    @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();
        }
        return spanCount;
    }

    public void drawHorizontal(Canvas c, RecyclerView parent) {
        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 left = child.getLeft() - params.leftMargin;
            final int right = child.getRight() + params.rightMargin
                    + mDivider.getIntrinsicWidth();
            final int top = child.getBottom() + params.bottomMargin;
            final int bottom = top + mDivider.getIntrinsicHeight();
            mDivider.setBounds(left, top, right, bottom);
            mDivider.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 + mDivider.getIntrinsicWidth();

            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(c);
        }
    }

    
    private boolean isLastRow(int itemPosition, RecyclerView parent) {
        int spanCount = getSpanCount(parent);
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        //有多少列
        if (layoutManager instanceof GridLayoutManager) {
            int childCount = parent.getAdapter().getItemCount();

            double count = Math.ceil((double) childCount / (double) spanCount);//总行数
            double currentCount = Math.ceil((double) (itemPosition + 1) / spanCount);//当前行数

            //最后当前数量小于总的
            return !(currentCount < count);
        }
        return true;
    }


    
    private boolean isLastColumn(int itemPosition, RecyclerView parent) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        //有多少列
        if (layoutManager instanceof GridLayoutManager) {
            int spanCount = getSpanCount(parent);
            //因为是从0可以所以要将ItemPosition先加1
            return (itemPosition + 1) % spanCount == 0;
        }
        return false;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State
            state) {
        super.getItemOffsets(outRect, view, parent, state);
        if (isLastRow(parent.getChildLayoutPosition(view), parent))// 如果是最后一行，则不需要绘制底部
        {
            outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
        }

        if (isLastColumn(parent.getChildLayoutPosition(view), parent))// 如果是最后一列，则不需要绘制右边
        {
            outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
        }
    }
}