如何在自定义ImageView安卓上清除之前绘制的画布？

public class CustomImageView extends ImageView {
    private Bitmap mBitmap;
    private Canvas mCanvas;

    public CustomImageView(Context context) {
        super(context);
    }

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

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

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if (mBitmap == null) {
            mBitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
            mCanvas = new Canvas(mBitmap);
        }

        // 清除之前绘制的画布
        mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);

        // 在新的画布上绘制需要显示的内容
        // ...

        // 将新的Bitmap对象绘制到ImageView上
        canvas.drawBitmap(mBitmap, 0, 0, null);
    }
}