Set the width of relativelayout 1/3 the width of the screen?

Question

I have a layout like below. Now, I don't want to set the width of relative layout to fixed 240 dp. I want to set the width of the relative layout to 1/3 the width of the screen. Is it possible to do that in the xml file. If it is impossible, how can I achieve that using java code ?

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/fullscreen"
        style="@style/translucent">
           <RelativeLayout
            android:layout_width="240dp"
            android:layout_height="fill_parent"
            android:layout_gravity="right"
            android:gravity="center"
            android:background="#88000000"
            android:id="@+id/sidebar">

           </RelativeLayout>

    </FrameLayout>

Answer 1

use weightsum="3" in the parent and layout_weight=1 in the child. Take a look a this reference

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:id="@+id/fullscreen"
    style="@style/translucent"
    android:orientation="horizontal"
    android:weightSum="3">

    <RelativeLayout
        android:layout_width="0dp"
        android:layout_height="fill_parent"
        android:layout_gravity="right"
        android:gravity="center"
        android:background="#88000000"
        android:id="@+id/sidebar"
        android:layout_weight="1" 
        >

    </RelativeLayout>

    <!-- other views, with a total layout_weight of 2 -->

</LinearLayout>

Answer 2

You have to use a LinearLayout to get the width of a view to be a third of its parentview.

Something like:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <RelativeLayout
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="center"
        android:background="#88000000">
    </RelativeLayout>
    <ViewGroup
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="2">
    </ViewGroup>
</LinearLayout>

The key bit is the ratio of the layout_weights. The documentation is pretty good.