Android自定义Button并设置不同背景图片的方法

public class MyButton extends Button {
//This constructormust be 
public MyButton(Context context, AttributeSet attrs) {
 super(context, attrs);
 }
 public MyButton(Context context) {
 super(context);
 }
 private Paint mPaint = null;
 private String mText;
 private int mX, mY;
 public void onSetText(String text, int nLeft, int nBottom, int nTextSize,
  int nTextColor) {
 mPaint = new Paint();
 mPaint.setTextSize(nTextSize);
 mPaint.setColor(nTextColor);
 this.mText = text;
 this.mX = nLeft;
 this.mY = nBottom;
 }
 private int mDownBmpId, mUpBmpId;
 public void onSetBmp(int nDownID, int nUpID) {
 this.mDownBmpId = nDownID;
 this.mUpBmpId = nUpID;
 }
 @Override
 public void onDraw(Canvas canvas) {
 if (mPaint != null)
  canvas.drawText(mText, mX, mY, mPaint);
 super.onDraw(canvas);
 }
 @Override
 public boolean onTouchEvent(MotionEvent event) {
 if (event.getAction() == MotionEvent.ACTION_DOWN) {
  super.setBackgroundResource(mDownBmpId);
 } else if (event.getAction() == MotionEvent.ACTION_UP) {
  super.setBackgroundResource(mUpBmpId);
 }
 return super.onTouchEvent(event);
 }
}

<com.MyButton
  android:id="@+id/test_btn" android:layout_width="120px"
  android:layout_height="fill_parent" android:text="Test"
  android:background="@drawable/btn_u" />

MyButton btn = (MyButton)findViewById(R.id.test_btn);
btn.onSetBmp(R.drawable.btn_d, R.drawable.btn_u);