Button laid out using layout_toLeftOf in a RelativeLayout does not show up

I am trying to layout a Button to the left of a TextView that is centered on the screen. My layout looks like this:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:gravity="center">

 <TextView  
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="foo"
     android:id="@+id/center"
     />

    <Button android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="Left button"
     android:layout_toLeftOf="@id/center" />
</RelativeLayout>

Unfortunately, the button just does not appear. I get the following result:

As you see, the button does开发者_开发百科n't show up. It works if I use layout_toRightOf, then the button appears to the right of the TextView, just as expected.

Any ideas what I am doing wrong here?

1. You have not provided any rules for positioning the TextView. If you want the TextView centered, say so (e.g., android:layout_centerInParent="true")
2. You have not provided any rules for vertically positioning the Button.
3. Use hierarchyviewer to inspect your layout to figure out where things are being positioned.

Try

<Button android:id="@+id/button"
     android:layout_alignParentLeft="true"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="Left button">
</Button>
<TextView  
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="foo"
     android:layout_toRightOf="@id/button"
     android:id="@+id/center">
</TextView>

You can also pad your text to move it where you want...