为什么使用surfaceview画图时线条显示不出来？

想做出心电图类似的效果，看了好多代码整合了一下，写好之后发现画不出来，检查也检查不出来问题。

package com.example.wivedraw;



import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.util.AttributeSet;
import android.view.SurfaceHolder;
import android.view.SurfaceView;


class MySurfaceView extends SurfaceView implements SurfaceHolder.Callback, Runnable {
    private SurfaceHolder mSurfaceHolder;
    //绘图的Canvas
    private Canvas mCanvas;
    //子线程标志位
    private boolean mIsDrawing;
    private int[] x = new int[1440];
    private int[] y = new int[1440];
    public int i=0;
    private Paint mPaint;
    private Path mPath;
    public MySurfaceView(Context context) {
        this(context, null);
    }

    public MySurfaceView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

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

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        mIsDrawing = true;
        x[0]=999;
        y[0]=getHeight()/2;
        new Thread(this).start();
    }

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

    }

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

    @Override
    public void run() {

        while (mIsDrawing){
            drawWive();
            try {
                Thread.sleep(1);
            }catch (Exception e){
                e.printStackTrace();
            }
        }

    }

    private void drawWive() {
        //获得canvas对象
        mCanvas = mSurfaceHolder.lockCanvas();
        //绘制背景
        mCanvas.drawColor(Color.WHITE);

        int width =getWidth() ;
        int height=getHeight();
        mPaint = new Paint();
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setAntiAlias(true);//抗锯齿
        mPaint.setStrokeWidth(5);
        mPath = new Path();
        //路径起始点(0, 100)
        mPath.moveTo(x[0],y[0]);

        for (int j = 0; j < i; j++) {
            x[j] -= 1;
        }

        x[i] = 1440;
        y[i] = (int) (100 * Math.sin(2 * i * Math.PI / 180) + height / 2);

        for (int j = 0; j <= i; j++) {
            mPath.lineTo(x[j], y[j]);
        }

        i+=1;

        if (mCanvas != null){
           //释放canvas对象并提交画布
                mSurfaceHolder.unlockCanvasAndPost(mCanvas);
            }
    }

    /**
     * 初始化View
     */
    private void initView(){
        mSurfaceHolder = getHolder();
        mSurfaceHolder.addCallback(this);
        setFocusable(true);
        setKeepScreenOn(true);
        setFocusableInTouchMode(true);
    }
}