Assign width to half available screen width declaratively

If your widget is a Button:

<LinearLayout android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:weightSum="2"
    android:orientation="horizontal">
    <Button android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="somebutton"/>

    <TextView android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>
</LinearLayout>

I'm assuming you want your widget to take up one half, and another widget to take up the other half. The trick is using a LinearLayout, setting layout_width="fill_parent" on both widgets, and setting layout_weight to the same value on both widgets as well. If there are two widgets, both with the same weight, the LinearLayout will split the width between the two widgets.