Android 如何在RecyclerView项目装饰中在画布上绘制CardView阴影？

我最近在使用具有多种视图类型的RecyclerView和PagedListAdapter时遇到了各种问题。不同视图类型的原因本质上是为了添加节标题。切换到使用物品装饰似乎更稳定和更好的模式

所以我想我应该对另一个RecyclerView做同样的事情，尝试消除多个视图类型，这样RecyclerView中的每一行都对应于基础PagedList中的一个项目。问题是，这次它不是一个简单的节头文本视图。这是一张卡片

我在获得正确的宽度时遇到了一点困难（这个CardView是用来匹配父母的）。我想我知道了，但我还有一个问题。CardView正在绘制，但没有背景阴影。我从StackOverflow的问题中看到，其他人也有同样的问题。似乎没有使用常规布局/测量/绘制功能绘制立面阴影

如何在项目装饰中获得我的CardView阴影？有办法吗

这就是我目前拥有的：

class CardItemDecoration(val adapter: ReservationAdapter) : RecyclerView.ItemDecoration() {
    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        super.getItemOffsets(outRect, view, parent, state)

        val position = parent.getChildAdapterPosition(view)

        if (adapter.hasCard && position == 0) {
            outRect.top = getcardView(parent.context, parent).measuredHeight
        }
    }

    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDraw(c, parent, state)

        val left = parent.paddingLeft
        val right = parent.width - parent.paddingRight

        if (adapter.hascard && parent.childCount > 0) {
            val child = parent.getChildAt(0)

            val layout = getCardView(parent.context, parent)

            // Draw in the space made by getItemOffsets()
            layout.layout(left, 0, right, layout.measuredHeight)
            c.save()
            // Adjust Y coordinates, as they'll be different for each row
            val top = child.top - layout.measuredHeight
            c.translate(0f, top.toFloat())
            layout.draw(c)
            c.restore()
        }
    }

    private lateinit var cardView: ViewGroup

    private fun getCardView(context: Context, parent: RecyclerView): View {
        if (!::cardView.isInitialized) {
            cardView = LinearLayout(context)
            LayoutInflater.from(context).inflate(R.layout.call_out_bis_profile, cardView, true)

            cardView.apply {
                findViewById<TextView>(R.id.infoTextView).text = context.getString(R.string.card_description)

            }

            val width = parent.width - parent.paddingLeft - parent.paddingRight
            cardView.measure(View.MeasureSpec.makeMeasureSpec(width, View.MeasureSpec.EXACTLY),
                    View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED))
        }
        return cardView
    }
}

类CardItemDecoration（val适配器：ReservationAdapter）：RecyclerView.ItemDecoration（）{ 覆盖有趣的GetItemOffset（outRect:Rect，view:view，父对象：RecyclerView，状态：RecyclerView.state）{ super.getItemOffset（outRect、view、parent、state） val position=parent.getChildAdapterPosition（视图） if（adapter.hasCard&&position==0）{ outRect.top=getcardView（parent.context，parent）.measuredHeight } } 重写onDraw（c:Canvas，父对象：RecyclerView，状态：RecyclerView.state）{ super.onDraw（c、父级、状态） val left=parent.paddingLeft val right=parent.width-parent.paddingRight if（adapter.hascard&&parent.childCount>0）{ val child=parent.getChildAt（0） val layout=getCardView（parent.context，parent） //在getItemOffsets（）生成的空间中绘制 布局。布局（左、0、右、布局。测量高度） c、 保存（） //调整Y坐标，因为每行的坐标都不同 val top=child.top-layout.measuredHeight c、 转换（0f，top.toFloat（）） 布局图（c） c、 还原（） } } 私有lateinit变量cardView:ViewGroup private fun getCardView（上下文：上下文，父：RecyclerView）：视图{ 如果（！：：cardView.i初始化）{ cardView=LinearLayout（上下文） LayoutFlater.from（上下文）。充气（R.layout.call_out_bis_profile，cardView，true） cardwiew.apply{ findviewbyd（R.id.infoTextView）.text=context.getString（R.string.card\u说明） } val width=parent.width-parent.paddingLeft-parent.paddingRight cardView.measure（视图.MeasureSpec.makeMeasureSpec（宽度，视图.MeasureSpec.精确）， View.MeasureSpec.makeMeasureSpec（0，View.MeasureSpec.UNSPECIFIED）） } 返回卡视图 } }

---

这是一个卡视图装饰示例。这是我的参考：

Paint；
静态RoundRectHelper sRoundRectHelper；
油漆mCornerShadowPaint；
油漆，油漆，油漆；
最终RectF mPreShadowBounds；
漂浮麦考内拉迪乌斯；
路径mCornerShadowPath；
浮动mShadowSize；
私有布尔mDirty=true；
私人最终int mShadowStartColor；
私有最终int mShadowEndColor；
私人浮动支付；
公共CardViewDecoration（资源、int背景色、浮动半径）{
mShadowStartColor=resources.getColor（R.color.cardwiew\u shadow\u start\u color）；
mShadowEndColor=resources.getColor（R.color.cardwiew\u shadow\u end\u color）；
mShadowSize=resources.getDimension（R.dimen.cardview\u shadow\u size）*shadow\u乘数；
mPaint=新油漆（油漆.防伪别名标志|油漆.抖动标志）；
mPaint.setColor（背景色）；
mCornerShadowPaint=新油漆（油漆.防伪别名标志|油漆.抖动标志）；
mCornerShadowPaint.setStyle（Paint.Style.FILL）；
mCornerShadowPaint.setDither（真）；
mCornerRadius=半径；
mPreShadowBounds=new RectF（）；
mEdgeShadowPaint=新油漆（mCornerShadowPaint）；
构建阴影角（）；
}
@凌驾
公共void onDrawOver（画布c、RecyclerView父级、RecyclerView.State）{
Rect bounds=new Rect（）；
浮动edgeShadowTop=-mCornerRadius-mShadowSize；
RecycleView.LayoutManager lm=parent.getLayoutManager（）；
浮子尺寸16DP=16f；
int padding16dp=（int）TypedValue.applyDimension（TypedValue.COMPLEX_UNIT_DIP，size16dp，parent.getContext（）.getResources（）.getDisplayMetrics（））；
对于（int i=0；iPaint mPaint;

static RoundRectHelper sRoundRectHelper;

Paint mCornerShadowPaint;

Paint mEdgeShadowPaint;

final RectF mPreShadowBounds;

float mCornerRadius;

Path mCornerShadowPath;

float mShadowSize;

private boolean mDirty = true;

private final int mShadowStartColor;

private final int mShadowEndColor;
private float mPadding;


public CardViewDecoration(Resources resources, int backgroundColor, float radius) {
    mShadowStartColor = resources.getColor(R.color.cardview_shadow_start_color);
    mShadowEndColor = resources.getColor(R.color.cardview_shadow_end_color);
    mShadowSize = resources.getDimension(R.dimen.cardview_shadow_size) * SHADOW_MULTIPLIER;

    mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
    mPaint.setColor(backgroundColor);
    mCornerShadowPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
    mCornerShadowPaint.setStyle(Paint.Style.FILL);
    mCornerShadowPaint.setDither(true);
    mCornerRadius = radius;
    mPreShadowBounds = new RectF();
    mEdgeShadowPaint = new Paint(mCornerShadowPaint);

    buildShadowCorners();
}

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
    Rect bounds = new Rect();
    float edgeShadowTop = -mCornerRadius - mShadowSize;

    RecyclerView.LayoutManager lm = parent.getLayoutManager();
    float size16dp = 16f;
    int padding16dp = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, size16dp, parent.getContext().getResources().getDisplayMetrics());

    for (int i = 0; i < parent.getChildCount(); i++) {
        int save = c.save();

        // using decorated values, remove what we set before
        View child = parent.getChildAt(i);
        bounds.set(lm.getDecoratedLeft(child) + padding16dp - (int) mPadding,
                lm.getDecoratedTop(child),
                lm.getDecoratedRight(child) - padding16dp + (int) mPadding,
                lm.getDecoratedBottom(child));

        RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();
        int position = params.getViewAdapterPosition();
        int viewType = parent.getAdapter().getItemViewType(position);


        if (viewType == HeaderItemTestAdapter.HEADER) {
            bounds.top = (int) (bounds.top + padding16dp - mPadding);

            // LT
            c.translate(bounds.left + mCornerRadius, bounds.top + mCornerRadius);
            c.drawPath(mCornerShadowPath, mCornerShadowPaint);
            c.drawRect(0, edgeShadowTop, bounds.width() - 2 * mCornerRadius, -mCornerRadius, mEdgeShadowPaint);

            // RT
            c.rotate(90f);
            c.translate(0, -bounds.width() + 2 * mCornerRadius);
            c.drawPath(mCornerShadowPath, mCornerShadowPaint);
            c.drawRect(0, edgeShadowTop, bounds.height() - mCornerRadius, -mCornerRadius, mEdgeShadowPaint);

            // LBorder
            c.rotate(180f);
            c.translate(-bounds.height(), -bounds.width() + 2 * mCornerRadius);
            c.drawRect(mCornerRadius, edgeShadowTop, bounds.height(), -mCornerRadius, mEdgeShadowPaint);


        } else {
            if (parent.getAdapter().getItemViewType(position + 1) == HeaderItemTestAdapter.HEADER) {
                bounds.bottom = (int) (bounds.bottom - padding16dp + mPadding);

                // last item before next header
                c.rotate(180f);
                c.translate(-bounds.left - bounds.width() + mCornerRadius, -bounds.top - bounds.height() + mCornerRadius);

                c.drawPath(mCornerShadowPath, mCornerShadowPaint);
                c.drawRect(0, edgeShadowTop, bounds.width() - 2 * mCornerRadius, -mCornerRadius, mEdgeShadowPaint);

                // RT / Right border
                c.rotate(90f);
                c.translate(0, -bounds.width() + 2 * mCornerRadius);
                c.drawPath(mCornerShadowPath, mCornerShadowPaint);
                c.drawRect(0, edgeShadowTop, bounds.height() - mCornerRadius, -mCornerRadius, mEdgeShadowPaint);

                // Left border
                c.rotate(180f);
                c.translate(-bounds.height(), -bounds.width() + 2 * mCornerRadius);
                c.drawRect(mCornerRadius, edgeShadowTop, bounds.height(), -mCornerRadius, mEdgeShadowPaint);
            } else {
                // Right border
                c.translate(bounds.left, bounds.top);
                c.rotate(90f);
                c.translate(0, -bounds.width() + mCornerRadius);
                c.drawRect(0, edgeShadowTop, bounds.height(), -mCornerRadius, mEdgeShadowPaint);

                // Left border
                c.rotate(180f);
                c.translate(-bounds.height(), -bounds.width() + 2 * mCornerRadius);
                c.drawRect(0, edgeShadowTop, bounds.height(), -mCornerRadius, mEdgeShadowPaint);
            }
        }
        c.restoreToCount(save);
    }
}

private void buildShadowCorners() {

    mPadding = 0f;

    RectF innerBounds = new RectF(-mCornerRadius, -mCornerRadius, mCornerRadius, mCornerRadius);
    RectF outerBounds = new RectF(innerBounds);
    outerBounds.inset(-mShadowSize, -mShadowSize);

    if (mCornerShadowPath == null) {
        mCornerShadowPath = new Path();
    } else {
        mCornerShadowPath.reset();
    }
    mCornerShadowPath.setFillType(Path.FillType.EVEN_ODD);
    mCornerShadowPath.moveTo(-mCornerRadius, 0);
    mCornerShadowPath.rLineTo(-mShadowSize, 0);
    // outer arc
    mCornerShadowPath.arcTo(outerBounds, 180f, 90f, false);
    // inner arc
    mCornerShadowPath.arcTo(innerBounds, 270f, -90f, false);
    mCornerShadowPath.close();

    float startRatio = mCornerRadius / (mCornerRadius + mShadowSize);
    mCornerShadowPaint.setShader(new RadialGradient(0, 0, mCornerRadius + mShadowSize, new int[]{
            mShadowStartColor, mShadowStartColor, mShadowEndColor}, new float[]{0f, startRatio, 1f},
            Shader.TileMode.CLAMP));

    // we offset the content shadowSize/2 pixels up to make it more realistic.
    // this is why edge shadow shader has some extra space
    // When drawing bottom edge shadow, we use that extra space.
    mEdgeShadowPaint.setShader(new LinearGradient(0, -mCornerRadius + mShadowSize, 0, -mCornerRadius - mShadowSize,
            new int[]{mShadowStartColor, mShadowStartColor, mShadowEndColor}, new float[]{0f, .5f, 1f},
            Shader.TileMode.CLAMP));
}

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);
    Resources resources = parent.getContext().getResources();

    float size16dp = 16f;
    int padding16dp = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, size16dp, resources.getDisplayMetrics());

    RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();
    int position = params.getViewAdapterPosition();
    int viewType = parent.getAdapter().getItemViewType(position);

    if (viewType == HeaderItemTestAdapter.HEADER) {
        // header
        outRect.set(0, (int) (padding16dp), 0, 0);
    } else {
        if (parent.getAdapter().getItemViewType(position + 1) == HeaderItemTestAdapter.HEADER) {
            // last item before next header
            outRect.set(0, 0, 0, (int) (padding16dp));
        }
    }

    outRect.left = (int) padding16dp;
    outRect.right = (int) padding16dp;
}
}