I have relative layout like container layout for navigation bar and I have positioned all horizontally ok bu I cannot vertically set that all be at aligned at center vertically.
How to align vertically in RelativeLayout ?

```
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/header_background" >

    <Button
        android:id="@+id/btnPrevious"
        android:layout_width="36dip"
        android:layout_height="36dip"
        android:layout_alignParentLeft="true"
        android:background="@drawable/selector_previous_arrow"
        android:paddingLeft="5dp"
        android:paddingTop="10dp"
        android:visibility="gone" />

    <TextView
        android:id="@+id/title"
        style="@style/title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:paddingTop="10dp"
        android:text="Title" />

    <Button
        android:id="@+id/refresh"
        android:layout_width="36dip"
        android:layout_height="36dip"
        android:layout_alignParentRight="true"
        android:background="@drawable/selector_refresh"
        android:paddingLeft="5dp"
        android:paddingTop="10dp"
        android:paddingRight="5dp"
        android:visibility="gone" />

    <Button
        android:id="@+id/filter"
        android:layout_width="36dip"
        android:layout_height="36dip"
        android:layout_toLeftOf="@+id/refresh"
        android:background="@drawable/selector_filter"
        android:paddingLeft="5dp"
        android:paddingRight="5dp"
        android:paddingTop="10dp"
        android:visibility="gone" />

</RelativeLayout>

```