自定義控制元件時經常用到Canvas，畫新的東西之前需要先清除畫布內容，人臉識別專案中需要準確畫出當前人臉位置，清空上一幀位置。

關於清除畫布內容網上有兩種非常流行的方法：

方法一：

        mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);

        Paint paint = new Paint();
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        mCanvas.drawPaint(paint);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
 

我的解決方案是在每次設定Path畫線前，清空一下Path，問題迎刃而解。

清除Path中的內容有兩個方法：
reset( )不保留內部資料結構，但會保留FillType.
rewind( )會保留內部的資料結構，但不保留FillType

關於Path其他更多方法解釋詳見這篇部落格。

自定義SurfaceView程式碼如下：

/**
 * Title:
 * Description:
 * Company: 北京****科技有限公司,010-62538800,[email protected]
 

 *
 * @author Created by ylwang on 2018/1/30
 */


public class CustomSurfaceView extends SurfaceView implements SurfaceHolder.Callback, Runnable {
    private SurfaceHolder mHolder;
    private Canvas mCanvas;//繪圖的畫布
    private volatile boolean mIsDrawing;//控制繪畫執行緒的標誌位

    private Paint mPaint;
    private Path mPath;

    public CustomSurfaceView(Context context) {
        super(context);
        initView();
    }

    public CustomSurfaceView(Context context, AttributeSet attrs) {
        super(context, attrs);
        initView();
    }

    public CustomSurfaceView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initView();
    }

    private Gson gson;
    private List<FaceBean.FacesEntity> listFaces;


    public void setFaceInfo(String faceInfo) {
        FaceBean bean = gson.fromJson(faceInfo, FaceBean.class);
        if (null != bean && null != bean.getFaces()) {
            listFaces = bean.getFaces();
        }
    }

    private void initView() {
        mPaint = new Paint();
        mPaint.setColor(Color.GREEN);//畫筆顏色
        mPaint.setAntiAlias(true);//抗鋸齒
        mPaint.setStrokeWidth(6);//畫筆寬度
        mPaint.setStyle(Paint.Style.STROKE);//空心

        mPath = new Path();

        mHolder = getHolder();//獲取SurfaceHolder物件
        //TODO:保證該SurfaceView在最上層,避免兩個SurfaceView疊加，遮擋問題
        mHolder.setFormat(PixelFormat.TRANSPARENT);
        setZOrderOnTop(true);

        mHolder.addCallback(this);//註冊SurfaceHolder的回撥方法
        setFocusable(true);// 能否用鍵盤獲得焦點
        setFocusableInTouchMode(true);//能否通過觸控獲得焦點
        this.setKeepScreenOn(true);//螢幕常亮

        gson = new Gson();
        listFaces = new ArrayList<>();
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        mIsDrawing = true;
        new Thread(this).start();
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        mIsDrawing = false;
    }

    public static final int TIME_IN_FRAME = 20;

    @Override
    public void run() {
        long start;
        while (mIsDrawing) {
            start = SystemClock.uptimeMillis();//開始繪製
            synchronized (this) {
                setPathToCanvas();
            }
            //保證穩定幀率重新整理介面
            while (SystemClock.uptimeMillis() - start <= TIME_IN_FRAME) {
                Thread.yield();//執行緒讓出
            }
        }
    }

    private void setPathToCanvas() {
        mPath.rewind();//TODO:必須重置，否則不能清屏
        //mPath.reset();
        // mPath = new Path();
        for (FaceBean.FacesEntity face : listFaces) {
            int[] ps = getPositions(face.getFace_rectangle());
            mPath.moveTo(ps[0], ps[1]);
            for (int i = 2; i < ps.length - 1; i += 2) {
                mPath.lineTo(ps[i], ps[i + 1]);
            }
            mPath.close();//第一個點連線到最後一個點，形成一個閉合區域
            //mPath.lineTo(ps[0], ps[1]);
            drawSth();
        }
    }

    /**
     * 獲取每個人臉的四點位置座標
     *
     * @param fre
     * @return
     */
    private int[] getPositions(FaceBean.FacesEntity.Face_rectangleEntity fre) {
        int[] faces = new int[8];
        faces[0] = fre.getLeft_top().getX();
        faces[1] = fre.getLeft_top().getY();
        faces[2] = fre.getRight_top().getX();
        faces[3] = fre.getRight_top().getY();
        faces[4] = fre.getRight_bottom().getX();
        faces[5] = fre.getRight_bottom().getY();
        faces[6] = fre.getLeft_bottom().getX();
        faces[7] = fre.getLeft_bottom().getY();
        for (int i = 0; i < faces.length; i++) {
            faces[i] *= 2;
            if (i % 2 == 0) {
                faces[i] = (640 * 2) - faces[i];
            }
        }
        return faces;
    }

    private void drawSth() {
        try {
            mCanvas = mHolder.lockCanvas(); //拿到當前畫布 然後鎖定
            mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
            mCanvas.drawPath(mPath, mPaint);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (mCanvas != null) {
                mHolder.unlockCanvasAndPost(mCanvas);//保證每次都將繪圖的內容提交
            }
        }
    }
}
 

        mHolder = getHolder();//獲取SurfaceHolder物件
        //TODO:保證該SurfaceView在最上層,避免兩個SurfaceView疊加，遮擋問題
        mHolder.setFormat(PixelFormat.TRANSPARENT);
        setZOrderOnTop(true);