问安卓RelativeLayout stretch背景

EN

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:background="@mipmap/backgroung"
    tools:context=".MainScreen">


    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="30pt"
        android:text="@string/level1"
        android:onClick="@string/level1"
        android:id="@+id/level1"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="60pt"
        android:text="@string/level2"
        android:id="@+id/level2"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="90pt"
        android:text="@string/level3"
        android:id="@+id/level3"/>