在android的xml布局中，几百上千行的xml文件是常事。布局文件的增大一定程度上增加了视图编写的复杂度，而且如果视图的嵌套越深android在加载视图时的性能也会越差，甚至会出现一些低端设备内存不足而崩溃等奇异情况。
对性能要求比较高或适配终端机型广泛的应用，通过编写自定视图控件来优化应用是常用的方式。今天我实现一个类似appstore的更新应用个数提醒的实例，来探讨自画视图的应用场合。 
5个底部tab通过RadioGroup结合RadioButton来实现，如果您不清楚怎么实现该视图请参阅《》。红圆和数字"8"通过继承RadioGroup的类MyRadioGroup 重载dispatchDraw方法绘制，再用MyRadioGroup 替换布局里的RadioGroup。为什么重载的不是onDraw而是dispatchDraw呢？下面先来看看dispatchDraw的描述
Called by draw to draw the child views. This may be overridden by derived classes to gain control just before its children are drawn (but after its own view has been drawn).
在onDraw绘制您画的元素在原布局元素的下面，而我们是想把自己画的元素覆盖在原布局元素的上面，所以得用dispatchDraw。好了一些思路讲完了上代码
import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Rect;import android.graphics.drawable.Drawable;import android.util.AttributeSet;import android.widget.RadioGroup;/** * 转载请注明 http://hemowolf.iteye.com */public class MyRadioGroup extends RadioGroup { Drawable mBg; Paint mPaintText; @Override protected void dispatchDraw(Canvas canvas) { super.dispatchDraw(canvas); if (mBg == null) mBg = getResources().getDrawable(R.drawable.tab_unread_bg); if(mPaintText==null){ mPaintText=new Paint(); mPaintText.setTextSize(18f); mPaintText.setColor(Color.WHITE); mPaintText.setFakeBoldText(true); } //获取字体所占宽度和高度 String text="8"; Rect rect= new Rect(); mPaintText.getTextBounds(text, 0, text.length(), rect); int textWidth = rect.width(), textHeight = rect.height(); int bgWidth = textWidth+30 > mBg.getIntrinsicWidth() ? textWidth +30: mBg.getIntrinsicWidth() , bgHeight = textHeight > mBg.getIntrinsicHeight() ? textHeight : mBg.getIntrinsicHeight(); int bgX = this.getWidth() + this.getPaddingLeft() - bgWidth ,bgY = this.getPaddingTop(); mBg.setBounds(bgX, bgY, bgX + bgWidth, bgY + bgHeight); mBg.draw(canvas); int x = bgX + (bgWidth - textWidth) / 2 - rect.left, y = bgY + (bgHeight - textHeight) / 2 - rect.top; canvas.drawText(text, x, y, mPaintText); } public MyRadioGroup(Context context) { super(context); } public MyRadioGroup(Context context, AttributeSet attrs) { super(context, attrs); }} 详细的实现请参考实例。