Accessing app R class from my Android library

Issue

I'm developing a library that needs to access layout items of the app implementing it. The only way I know how to do it is with reflection. In other words, if I create a constructor to my Library API like this:

public MyLibraryAPI(String packageName) {
    Class appR = Class.forName(String.format("%s.R", packageName));
    ...
}

And the developer would instantiate the library with his package name as the parameter in the constructor.

What I ultimately need is to let my inner classes know the Android Views used in the developers layout (.xml files) - both the id and the type. Is there a way to achieve this without reflection and escape the performance overhead? I'm certain it cannot be done, but asking in case there's an expert that sees what I fail to notice.

EDIT: Additionally, proguard, by default, obfuscates the code for protection but, as a consequence, fails to provide JVM with means to achieve reflection at runtime, so if I use reflection I would have to ask the developer to turn off proguard obfuscation for his or her R class which is a bummer.

Solution

Reflection on Android is extremely costly. Some well-meaning and popular libraries like Roboguice have fallen over partly because of the performance cost of reflection.

I suspect some kind of code generation is the correct solution here. Dagger 2, Butter Knife and the Data Binding Library are successful examples of Android libraries that employ code generation. Since the data binding library performs inspections on the XML, it must be available to code generation libraries at that stage in the build and you may be able to base your implementation on that: here's a link to the source jars at Maven Central.

Apart from that, yes it seems there will be some compromise between ease of use and difficulty-to-implement. If you force your consumers to annotate their classes with your annotations, it becomes harder to use but probably much easier for you to implement. If you restrict yourself to inspecting XML and the generated R file and generating code from just that then I think your job will be a lot more difficult. On the other hand, using annotations has become rather commonplace and it may not be such an issue with your users.

Good luck!

Answered By - David Rawson