LinearLayout、RelativeLayout、FrameLayout居中显示

     <LinearLayout
         android:layout_width="fill_parent"
         android:layout_height="wrap_content"
         android:gravity="center_horizontal"
         android:orientation="vertical">
         <ImageView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:background="@drawable/icon"
             />
     </LinearLayout>
     <FrameLayout
         android:layout_width="fill_parent"
         android:layout_height="wrap_content">
         <ImageView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:background="@drawable/icon"
             android:layout_gravity="center"
             />
     </FrameLayout>
     <RelativeLayout
         android:layout_width="fill_parent"
         android:layout_height="wrap_content">
         <ImageView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:background="@drawable/icon"
             android:layout_centerInParent="true"
             />
     </RelativeLayout>