How to configure Android library from an app that imports it?

Question

I'm kinda new to Android development so my question might be weird or not even possible. I wouldn't know!

Anyway, I'm building multiple apps that will have a lot of shared elements, so I decided to build a library with those components and use it in all of the apps, rather than stupid copying and pasting code. For example, the library handles the welcome screen and login/signup flow activities, among other things. So here are the problems this approach might cause:

1. While the behavior is the same across the apps, but the logo that I show at the welcome screen is different. Right now I populate it with an image resource from the library resources (R class) which will be the same for all apps and is obviously not correct.
2. The login/signup process is based on Firebase, which will require the app to have a key to be able to use them. Right now I also populate it with a dummy string resource from the library resources.

So my question really boils down to 3 parts:

1. Is there anyway I could pass this info from the app to the library? can I somehow modify the R class of the library? Or can I use the app's R class from the library? I can also call this part of the library as a function passing the parameters I need. But the first solution looks maybe more clean to me?
2. Whatever the answer to Q1 is. Where would I do this and how? The library has the welcome activity itself which is supposed to be the first activity in the app. How and where do I do this once the app starts and before the first activity starts?
3. If what I'm doing is wrong or impossible, is there any other way to achieve it?

Answer 1

1. Is there anyway I could pass this info from the app to the library? can I somehow modify the R class of the library? Or can I use the app's R class from the library? I can also call this part of the library as a function passing the parameters I need. But the first solution looks maybe more clean to me?

You don't need to modify the R class because you can override the resource file by creating a file with the same name. But it's not a clean solution because you constantly need to ensure your project and library resources name are the same.

2. Whatever the answer to Q1 is. Where would I do this and how? The library has the welcome activity itself which is supposed to be the first activity in the app. How and where do I do this once the app starts and before the first activity starts?

Instead of overriding the resources name, you're better to modify your library to receive a configuration as a contract to use the library. Here the sample:

First, create the class for holding the configuration:

public class Configuration {
  private int welcomeImageDrawableId;
  private int logoDrawableId;

  // constructor
  public Configuration(int welcomeImageDrawableId, int logoDrawableId) {
    this.welcomeImageDrawableId = welcomeImageDrawableId;
    this.logoDrawableId = logoDrawableId;
  }

  // setter and getter.
  public int getLogoDrawableId() {
    return logoDrawableId;
  }
}

Second, use the configuration class for the library by creating a Singleton class which will be used internally by the library:

public class MyLibrary {
    private static MyLibrary myLibrary;
    private Configuration configuration;

    private MyLibrary(){}
    private MyLibrary(Configuration configuration) {
      this.configuration = configuration;
    }

    public static MyLibrary getInstance() {
        if(myLibrary == null) {
            throw new RuntimeException("Need call createInstanceWith method first!!");
        }
        return myLibrary;
    }

    public static MyLibrary createInstanceWith(Configuration configuration) {
        if(myLibrary == null) {
            synchronized(MyLibrary.class) {
                if (myLibrary == null) {
                  myLibrary = new MyLibrary(configuration);
                }
            }
        }
        return test;
    }

    public Configuration getConfiguration() {
      return configuration;
    }
}

Third, use the configuration class in your library via the singleton class. something like this:

// assume imvLogo is an existing ImageView
Configuration configuration = MyLibrary.getInstance().getConfiguration();
imvLogo.setImageResource(configuration.getLogoDrawableId());

Last, register the contract when the library is used with:

Configuration configuration = new Configuration(R.drawable.welcome, R.drawable.logo);
MyLibrary.createInstanceWith(configuration);

Note: all the code isn't tested yet, error is to be expected.