android 父布局是什么意思,怎么让LinearLayout占据父布局的所有剩余空间

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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=".MainActivity" >

android:id="@+id/main"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:text="@string/hello_world"

android:gravity="left|top" />

android:id="@+id/test_area"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="vertical"

android:visibility="gone"

android:layout_below="@id/main" />