android 现行布局

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="bottom|center_vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/bn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bn1" />
    <Button
        android:id="@+id/bn2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bn2" />
    <Button
        android:id="@+id/bn3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bn3" />
    <Button
        android:id="@+id/bn4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bn4" />

</RelativeLayout>