Composition over inheritance for Android components like Activity or Fragment
Java Kotlin
Latest commit 5ae76a2 Dec 14, 2016 @passsy committed on GitHub Merge pull request #40 from passsy/feature/support_25.1.0
Feature/support 25.1.0



Composition over inheritance

Allows to add functionality into an Android Activity. Just because we all have a BaseActivity in our projects containing too much unused stuff. When it grows, it get unmaintainable.

Possible Usecases

  • Plugin for the GooglePlayApiClient handling all the edgecases
  • Wrap your layout in a predefined container by overriding setContentView()
  • a Plugin showing a loading spinner
  • a Plugin for requesting permissions and automatically handling all response codes
  • gradually add libraries like Mosby (without extending from a MvpActivity) or Flow to your Activities when you need it
  • and so much more...

State of the Art

Given you have an Activity showing a list of tweets (TweetStreamActivity) and you want add view tracking.


You could do it with inheritance and use TrackedTweetStreamActivity from now on:

public class TrackedTweetStreamActivity extends TweetStreamActivity {

    protected void onResume() {

more likely you would create a TrackedActivity and extend the TweetStreamActivity from it:

public abstract class TrackedActivity extends AppCompatActivity {

    public abstract String getTrackingName();

    protected void onResume() {
public class TrackedTweetStreamActivity extends TweetStreamActivity {

    public String getTrackingName() {
        return "stream";

Both solutions work but don't scale well. You'll most likely end up with big inheritance structures:

class MvpActivity extends AppCompatActivity { ... }

class BaseActivity extends AppCompatActivity { ... }

class BaseMvpActivity extends MvpActivity { ... }

class WizardUiActivity extends BaseActivity { ... }

class TrackedWizardUiActivity extends WizardUiActivity { ... }

class TrackedBaseActivity extends BaseActivity { ... }

class TrackedMvpBaseActivity extends BaseMvpActivity { ... }


Some libraries out there provide both, a specialized Activity extending AppCompatActivity and a delegate with a documentation when to call which function of the delegate in your Activity.

public class TrackingDelegate {

     * usage:
     * <pre>{@code
     * @Override
     * protected void onResume() {
     *     super.onResume();
     *     mTrackingDelegate.onResume();
     * }
     * } </pre>
    public void onResume() {

public class TweetStreamActivity extends AppCompatActivity {

    private final TrackingDelegate mTrackingDelegate = new TrackingDelegate();

    protected void onResume() {

This is an elegant solution but breaks when updating such a library and the delegate call position has changed. Or when the delegate added new callbacks which don't get automatically implemented by increasing the version number in the build.gradle.

Composite Android solution

CompositeAndroid let's you add delegates to your Activity without adding calls to the correct location. Such delegates are called Plugins. A Plugin is able to inject code at every position in the Activity lifecycle. It is able to override every method.

Get Started Download

CompositeAndroid is available via jcenter

dependencies {
    // it's very important to use the same version as the support library
    def supportLibraryVersion = "25.0.0"

    // contains CompositeActivity
    compile "com.pascalwelsch.compositeandroid:activity:$supportLibraryVersion"

    // contains CompositeFragment and CompositeDialogFragment
    compile "com.pascalwelsch.compositeandroid:fragment:$supportLibraryVersion"

    // core module (not required, only abstract classes and utils)
    compile "com.pascalwelsch.compositeandroid:core:$supportLibraryVersion"

Extend from a composite implementation

Extend from one of the composite implementations when you want to add plugins. This is the only inheritance you have to make.

- public class MyActivity extends AppCompatActivity {
+ public class MyActivity extends CompositeActivity {
- public class MyFragment extends Fragment { // v4 support library
+ public class MyFragment extends CompositeFragment {

Add a plugins

Use the constructor to add plugins. Do not add plugins in #onCreate(). That's too late. Many Activity methods are called before #onCreate() which could be important for a plugin to work.

public class MainActivity extends CompositeActivity {

    final LoadingIndicatorPlugin loadingPlugin = new LoadingIndicatorPlugin();

    public MainActivity() {
        addPlugin(new ViewTrackingPlugin("Main"));

    public void onCreate(Bundle savedInstanceState) {
        // ...

        // example usage of the LoadingIndicatorPlugin

Read more about the ordering of the Plugins here

Write a plugin

This is the strength of CompositeAndroid. You don't really have to learn something new. It works like you'd extend you Activity to add functionality. Let's change the TrackedActivity from above and create a ViewTrackingPlugin.

Here the original

public abstract class TrackedActivity extends AppCompatActivity {

    public abstract String getTrackingName();

    protected void onResume() {

As plugin:

public class ViewTrackingPlugin extends ActivityPlugin {

    private final String mViewName;

    protected TrackedPlugin(final String viewName) {
        mViewName = viewName;

    public void onResume() {

The implementation inside of onResume() hasn't changed!

Plugin features

Here some information about plugins. The Activity example is used but it works the same for other classes, too.

  • it's possible to override every Activity method from a Plugin
  • execute code before calling super executes code before super of Activity
  • explicitly not calling super is allowed and results in not calling super of the Activity. (The activity will tell if the super call was required)
  • execute code after calling super executes code after super of Activity


Not everything works exactly like you'd use inheritance. Here is a small list of minor things you have to know:

Important for all Plugin authors

  • you can't call an Activity method of a Plugin such as onResume() or getResources(). Otherwise the call order of the added plugins is not guaranteed. Instead call those methods on the real Activity with getActivity.onResume() or getActivity.getResources().


  • CompositeActivity#onRetainCustomNonConfigurationInstance() is final and required for internal usage, use CompositeActivity#onRetainCompositeCustomNonConfigurationInstance() instead
  • CompositeActivity#getLastCustomNonConfigurationInstance() is final and required for internal usage, use CompositeActivity#getLastCompositeCustomNonConfigurationInstance() instead
  • Saving a NonConfigurationInstance inside of a Plugin works by overriding onRetainNonConfigurationInstance() and returning an instance of CompositeNonConfigurationInstance(key, object). Get the data again with getLastNonConfigurationInstance(key) and make sure you use the correct key.

Project stage

Early alpha. Nothing breaks when you extend CompositeActivity. Getting simple callbacks is safe. Overriding methods and changing parameters is tested. Sound good? Well, I already found edgecases (getLastNonConfigurationInstance() and onRetainCustomNonConfigurationInstance()) and I'm sure there are more I haven't discovered yet.

I successfully replaced a BaseActivity in a big grown project with a CompositeActivity. It works without problems, so far. This means it's kind of stable if you don't try to break it.

The documentation is currently not existing. I'm sorry, but right now it's more like a proof of concept which works well.

Inspiration and other Android Composition Libraries

  • Navi of course, but
    • it doesn't support all methods (only methods without return value)
    • it does only support code execution before or after calling super, not very flexible
    • no plugin API
  • Lightcycle
    • supports only basic lifecycle methods.
  • Decorator
    • works only in scope of your own project. It doesn't allow including libraries providing plugins because the is no global Activity implementation.
    • Every "DecoratedActivity" is generated for a specific usecase based on a blueprint you have to create every time


Copyright 2016 Pascal Welsch

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