How can I use the negative of an Android attribute in XML?

Question

I have a simple relative layout containing an ImageView for an icon and a TextView for a title:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                style="@style/Test.NavBar.Heading"
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_gravity="center_horizontal"
                android:gravity="center_horizontal">
    <ImageView
            style="@style/Test.NavBar.Heading.Icon"
            android:id="@+id/test_logo"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/ic_launcher"/>
    <TextView
            style="@style/Test.NavBar.Heading.Text.Large"
            android:id="@+id/test_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@+id/test_logo"
            android:layout_toEndOf="@+id/test_logo"
            android:layout_alignTop="@id/test_logo"
            android:layout_alignBottom="@id/test_logo"
            android:gravity="center_vertical"
            android:text="My Title"/>
</RelativeLayout>

This is embedded in another view, which centres my view horizontally. All well and good except that I need the layout to be centred on the TextView rather than that RelativeLayout .

The only thing I have access to is an attribute which gives me the width of the item in the ImageView , called iconWidth . Normally you can offset the layout using a margin attribute, for example above I could add

android:layout_marginLeft="?attr/iconWidth"

but that pushes the layout to the right not the left. Instead I need to do something like:

android:layout_marginLeft="-?attr/iconWidth"

except of course that isn't valid syntax.

I have to do this in a single layout and have to do it in XML and can't ask for additional attributes to be added. Given these constraints, how can I offset the contents of the RelativeLayout as required?

Answer 1

After various attempts to find a solution to this the best way that I found was to simple add another icon after the text but to make it invisible:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                style="@style/Test.NavBar.Heading"
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_gravity="center_horizontal"
                android:gravity="center_horizontal">
    <ImageView
            style="@style/Test.NavBar.Heading.Icon"
            android:id="@+id/test_logo"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/ic_launcher"/>
    <TextView
            style="@style/Test.NavBar.Heading.Text.Large"
            android:id="@+id/test_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@+id/test_logo"
            android:layout_toEndOf="@+id/test_logo"
            android:layout_alignTop="@id/test_logo"
            android:layout_alignBottom="@id/test_logo"
            android:gravity="center_vertical"
            android:text="My Title"/>
    <ImageView
            style="@style/Test.NavBar.Heading.Icon"
            android:id="@+id/test_blank"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@+id/test_title"
            android:layout_toEndOf="@+id/test_title"
            android:src="@drawable/ic_launcher"
            android:visibility="invisible"/>
</RelativeLayout>

With this additional but invisible icon the TextView is now centered in the middle of the RelativeLayout and this solves the issue.