title: imagebutton above a camera preview
body: <p>i want to put an imagebutton on top of camera preview.
  it's simple unless you want to move the button.</p>
  
  <p>the layout :</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:id="@+id/my_linear_layout"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  &gt;
     &lt;FrameLayout
          android:id="@+id/camera_preview"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:layout_weight="1"
      /&gt;
     &lt;LinearLayout
          android:id="@+id/linear"
          android:layout_width="match_parent"
          android:layout_height="fill_parent"
          android:gravity="bottom"
          android:orientation="vertical" &gt;
  
          &lt;ImageButton
              android:id="@+id/btn"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_gravity="center"/&gt;
      &lt;/LinearLayout&gt;
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>If i put the button at the bottom of the screen it works fine. But if i put the button at the center, for exemple, and move it towards the bottom, it disappears because the linearLayout is not fullscreen.</p>
  
  <p>what it's the best way to do what i want? thank's</p>