继承Button类设置宽高的问题

第一次重写View，有些问题不知道如何处理。请多指教！
我有个继承了Button类的类MyButton，实现一些自定义的UI效果，暂时只设置了颜色，先试验一下。
但在onDraw时不知道如何设置矩形的坐标才能显示成自适应的大小。我是先设置了一个初始值，并提供了set矩形四个坐标的函数。
我在layout中设置宽高为wrap_content，大小是可以自适应了，但我在MyButton设置的颜色大小还是我onDraw时设置的固定矩形大小。
我在activity的onCreate函数中设置MyButton的right，bottom坐标，但是不起作用。
我是用先去的MyButton的left，width，top，height的方式来计算出right和bottom的；但是去的的MyButton的left，width，top，height全是0.
这是为什么呢？是要在onCreate函数之后才能设置吗？
代码如下：
1.MyButton.java

package com.min.test;



import android.content.Context;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.graphics.RectF;

import android.util.AttributeSet;

import android.util.Log;

import android.view.KeyEvent;

import android.view.MotionEvent;

import android.widget.Button;



public class MyButton extends Button {

	private int mLeft = 0;

	private int mTop = 0;

	private int mRight = 50;

	private int mBottom = 30;



	public MyButton(Context context) {

		super(context);

	}

	

	public MyButton(Context context, int left, int top, int right, int bottom) {

		super(context);

		this.mLeft = left;

		this.mTop = top;

		this.mRight = right;

		this.mBottom = bottom;

	}

	

	public MyButton(Context context, AttributeSet attrs) {

		super(context, attrs);

	}

	

	public MyButton(Context context, AttributeSet attrs, int defStyle) { 

	    super(context, attrs, defStyle); 

	}



	@Override

	protected void onDraw(Canvas canvas) {

		// TODO Auto-generated method stub

		super.onDraw(canvas);

		Paint mPaint = new Paint();   

        mPaint.setColor(Color.RED);   

        mPaint.setAlpha(128);

        canvas.drawRect(new RectF(mLeft, mTop, mRight, mBottom), mPaint); 

        Log.i("MyButton", "======================================");

	}



	@Override

	public boolean onKeyDown(int keyCode, KeyEvent event) {

		// TODO Auto-generated method stub

		return super.onKeyDown(keyCode, event);

	}



	@Override

	public boolean onKeyUp(int keyCode, KeyEvent event) {

		// TODO Auto-generated method stub

		return super.onKeyUp(keyCode, event);

	}



	@Override

	public boolean onTouchEvent(MotionEvent event) {

		// TODO Auto-generated method stub

		return super.onTouchEvent(event);

	}



	public int getMLeft() {

		return mLeft;

	}



	public void setMLeft(int left) {

		mLeft = left;

	}



	public int getMTop() {

		return mTop;

	}



	public void setMTop(int top) {

		mTop = top;

	}



	public int getMRight() {

		return mRight;

	}



	public void setMRight(int right) {

		mRight = right;

	}



	public int getMBottom() {

		return mBottom;

	}



	public void setMBottom(int bottom) {

		mBottom = bottom;

	}





	

	

}

2.AndroidTest.java

public class AndroidTest extends Activity {

	private final String TAG = "AndroidTest";

	private Context context;

	private MyButton mMyBtnSayHello, mMyBtnClickMe;

	

    /** Called when the activity is first created. */

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        context = this;

        setContentView(R.layout.main);   

        

//        GameSurfaceView gameSurfaceView = new GameSurfaceView(this);

//        gameSurfaceView.Draw();

        

        mMyBtnSayHello = (MyButton)findViewById(R.id.btn_say_hello);

        mMyBtnClickMe = (MyButton)findViewById(R.id.btn_click_me);

        

        int right = mMyBtnSayHello.getMLeft() + mMyBtnSayHello.getWidth();

        int height = mMyBtnSayHello.getMTop() + mMyBtnSayHello.getHeight();

        mMyBtnSayHello.setMRight(right);

        mMyBtnSayHello.setMBottom(height);

        mMyBtnSayHello.invalidate();



        mMyBtnClickMe.setMRight(mMyBtnClickMe.getMLeft() + mMyBtnClickMe.getWidth());

        mMyBtnClickMe.setMBottom(mMyBtnClickMe.getMTop() + mMyBtnClickMe.getHeight());

        mMyBtnClickMe.invalidate();

        

        Log.i(TAG, "onCreate end...");

    }

3.main.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:orientation="vertical"

    android:layout_width="fill_parent"

    android:layout_height="fill_parent"

    >

    <com.min.test.MyButton

    	android:id="@+id/btn_say_hello"  

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"  

        android:text="Say Hello"  

    	/>

	

	<com.min.test.MyButton

    	android:id="@+id/btn_click_me"  

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"  

        android:text="Click Me"  

    	/>

    

</LinearLayout>