android中Path的使用

public class MySurfaceView2 extends SurfaceView implements SurfaceHolder.Callback,
        View.OnTouchListener {
    private Path mPath = new Path();
    private Paint mPaint = new Paint();
    public MySurfaceView2(Context context) {
        super(context);
        init();
    }

    public MySurfaceView2(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }
    private void init(){

        setOnTouchListener(this);
        getHolder().addCallback(this);

    }
    public void draw(){
        Canvas canvas = getHolder().lockCanvas();
        canvas.drawColor(Color.WHITE);
        canvas.drawPath(mPath,mPaint);
        getHolder().unlockCanvasAndPost(canvas);
    }
    //清理画布
    public void clear(){
        mPath.reset();
        draw();
    }

    @Override
    public void surfaceCreated(SurfaceHolder surfaceHolder) {
    }

    @Override
    public void surfaceChanged(SurfaceHolder surfaceHolder, int i, int i1, int i2) {
        draw();
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder surfaceHolder) {

    }

    @Override
    public boolean onTouch(View view, MotionEvent motionEvent) {
        switch (motionEvent.getAction()){
            case MotionEvent.ACTION_DOWN:
                mPaint.setStyle(Paint.Style.STROKE);
                mPaint.setColor(Color.RED);
                mPath.moveTo(motionEvent.getX(),motionEvent.getY());
                break;
            case MotionEvent.ACTION_MOVE:
                //生成线条
                mPath.lineTo(motionEvent.getX(), motionEvent.getY());
                draw();
                break;
        }
        return true;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              xmlns:app="http://schemas.android.com/apk/res-auto"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <com.example.lance.test2.MySurfaceView2
        android:id="@+id/my_surface"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"/>

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="清除"/>

</LinearLayout>

public class MainActivity extends AppCompatActivity {
    private MySurfaceView2 mMySurfaceView2;
    private Button mButton;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.four);

        mMySurfaceView2 = (MySurfaceView2) findViewById(R.id.my_surface);
        mButton = (Button) findViewById(R.id.btn);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mMySurfaceView2.clear();

            }
        });
    }

}