DeleteAllContacts/app/src/main/res/layout/activity_main.xml

<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"
    tools:context="me.yoqi.deleteallcontacts.MainActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

    <Button
        android:id="@+id/btn_del_contact"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_alignStart="@+id/btn_del_contact"
        android:layout_below="@+id/textView1"
        android:layout_marginLeft="57dp"
        android:layout_marginStart="57dp"
        android:layout_marginTop="37dp"
        android:text="删除所有联系人" />

    <TextView
        android:id="@+id/tv_contact"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/btn_del_contact"
        android:layout_alignStart="@+id/btn_del_contact"
        android:layout_below="@+id/btn_del_contact"
        android:layout_marginTop="91dp"
        android:scrollbars="vertical"
        android:text="TextView" />

</RelativeLayout>