Question

I have two Android projects, a ‘library project’ containing a custom layout, and an ‘application project’ containing an application which uses the layout.

Everything seems to build and execute fine, except that the visual layout editor throws a ClassNotFoundException (which I assume is a bug in the plug-in), but when I try to start to make use of the attributes I defined for the custom layout in the xml, I can no longer build. That is; this works:

<?xml version="1.0" encoding="utf-8"?>
<se.fnord.android.layout.PredicateLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"> 
  <TextView
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="asdfasdf"
    />
</se.fnord.android.layout.PredicateLayout>

Whereas this does not:

<?xml version="1.0" encoding="utf-8"?>
<se.fnord.android.layout.PredicateLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:fnord="http://schemas.android.com/apk/res/se.fnord.android"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"> 
  <TextView
    fnord:layout_horizontalSpacing="1px"
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="asdfasdf"
    />
</se.fnord.android.layout.PredicateLayout>

The build fails with a message from aapt:

ERROR No resource identifier found for attribute ‘layout_horizontalSpacing’ in package ‘se.fnord.android’

The resource identifier does exist in the R-file and attrs.xml contained the library project, and if I put the layout code and resources directly in the application project everything works fine. The layout_horizontalSpacing attribute (and layout_verticalSpacing) is a custom attribute used in the PredicateLayout.LayoutParam class to specify the distance to the next widget.

So far I’ve tried the standard eclipse ways by specifying project references and build path project dependencies. I was also told to try the tag in the application manifest, which did not help.

So, what do I need to do for the references in the xml-file to work?

I don’t know if it’s relevant, but the ‘library’ manifest looks like this:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="se.fnord.android"
      android:versionCode="1" android:versionName="1.0.0">
</manifest>

The ‘application’ manifest like this:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="se.fnord.appname"
      android:versionCode="1" android:versionName="1.0.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".AppName"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

(The ‘PredicateLayout’, btw, is a cleaned-up version of this).

Answer 1

The earliest versions of Android sdk did not support sharing at the source code level in a nice way. You could jar up your .class files and then add that into the lib/ folder, but this solution did not allow direct sharing of source code and just as importantly did not support the sharing of resources or aidl files.

Then in May 2010, Android introduced the Library Project mechanism. A Library Project is structured very similar to a normal Android project, but rather than being used to produce an apk, it serves only to provide code and resources to other projects. Like an ordinary project, a Library Project usually contains src and res folders, along with an AndroidManifest.xml file; however the manifest should be mostly empty with the exception of the manifest element and the package attribute (no longer true – you can now declare Activities and other components in the manifest of your Library Project). In addition, the project.properties file for a Library Project needs to contain the property:

"android.library=true"

To make a reference from an ordinary (apk-producing) project to a Library Project, you need to add a “android.library.reference.N” line into the project.properties file of the ordinary project. For example, if my main project wants to point to two Library Projects, my project.properties file for the main project would include the following:

android.library.reference.1=../LibraryA
android.library.reference.2=../../LibraryB

where the ../ and the ../../ are the respective paths from the main project to the Library Projects (this is just an example). Note this list of references is 1-based and it should not contain gaps or duplicates. Google is well aware that this is not a perfect system but it was a reasonable solution that was compatible with Ant and Eclipse. Generally speaking, your IDE will attempt to maintain these files for you, but sometimes you may need to edit them by hand.

At first Library Projects did not support the following:

1. Library Projects pointing to other Library Projects
2. Library Projects containing aidl files
3. Library Projects containing assets folder

However subsequent sdk releases solved all of these problems.

For more information on Library Projects, see the official documentation.