Question

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/frameLayout1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="@drawable/background_gradient" >

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:gravity="center" >

        <ImageButton
            android:id="@+id/buttonLog"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/log"
            android:onClick="log" />

    </RelativeLayout>

</FrameLayout>

I was expecting my button to appear in the center of the screen. However, it appears on the TOP center of the screen (that is, the button is center horizontally, but not vertically).

It seems to me that the RelativeLayout is behaving like it was defined with “wrap_content” instead of “fill_parent”.

The funny thing is that, if I give an actual value to my RelativeLayout height property (android:layout_height), like:

<RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="100dp"
        android:gravity="center" >

Then the button behaves correctly (i.e. the button is also centred vertically). But I don`t want to use actual values. I want to use fill_parent! Why doesn’t it work with “fill_parent” ??

Does anybody know what’s going on?

Thank you in advance!

Answer 1

You specified fill_parent for both the layout_width and layout_height of your RelativeLayout, therefore it fills up it’s parent view.
By default, a relative layout arranges it’s children to the top-left corner, regardless you use fill_parent for the size.

You should achieve the desired aspect by taking advantage of the RelativeLayout's own attribute set, which helps you arrange the child views relatively to each other or to their parent:

<ImageButton
    android:id="@+id/buttonLog"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:background="@drawable/log"
    android:onClick="log" />

Using the android:layout_centerInParent you can achieve this. This attribute if set true, centers this child horizontally and vertically within its parent.