Android 切换卡（TabWidget）

TabWidget继承自TabActivity类，并实现setOnTabChangedListener的onTabChanged方法来监听Tab的改变：
布局文件：
<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:Android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/tabhost"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
        <TabWidget
            android:id="@android:id/tabs"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content" />
        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent">
            <TextView
                android:id="@+id/textview1"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:text="this is a tab" />
            <TextView
                android:id="@+id/textview2"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:text="this is another tab" />
            <TextView
                android:id="@+id/textview3"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:text="this is a third tab" />
        </FrameLayout>
    </LinearLayout>
</TabHost>