android画笔错位问题的解决

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下面的画画板的代码：

public class MainActivity extends Activity {private ImageView iv;
private Bitmap baseBitmap;
private Canvas canvas;
private Paint paint;@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);iv = (ImageView)findViewById(R.id.iv);paint = new Paint();
paint.setStrokeWidth(5);//画笔的像素
paint.setColor(Color.BLACK);//画笔的颜色//此时ImageView iv还为被创建，所以为0，报错
//一般情况下直接指定
baseBitmap = Bitmap.createBitmap(320,
350, Config.RGB_565);
canvas = new Canvas(baseBitmap);
canvas.drawColor(Color.RED);//用户在屏幕上移动的轨迹
iv.setOnTouchListener(new OnTouchListener() {
//定义手指开始位置的坐标
int startX;
int startY;@Override
public boolean onTouch(View v, MotionEvent event) {switch(event.getAction()){
case MotionEvent.ACTION_DOWN://手指第一次接触屏幕
//创建一个可以被修改的Bitmap
System.out.println("手指按下");System.out.println(iv.getWidth());
System.out.println(iv.getHeight());startX = (int) event.getX();
startY = (int) event.getY();
break;case MotionEvent.ACTION_MOVE://手指在屏幕上滑动
System.out.println("手指移动");
int newX = (int) event.getX();
int newY = (int) event.getY();canvas.drawLine(startX, startY, newX, newY, paint);//实时更新起始点
startX = (int) event.getX();
startY = (int) event.getY();
iv.setImageBitmap(baseBitmap);
break;case MotionEvent.ACTION_UP://手指离开屏幕
System.out.println("手指抬起");
break;
}return true;//持续响应触摸时间，true
}
});
}public void save(View view){}}