CompositeAndroid

License

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

State of the Art

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

Inheritance

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

public class TrackedTweetStreamActivity extends TweetStreamActivity {

    @Override
    protected void onResume() {
        super.onResume();
        Analytics.trackView("stream");
    }
}

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

public abstract class TrackedActivity extends AppCompatActivity {

    public abstract String getTrackingName();

    @Override
    protected void onResume() {
        super.onResume();
        Analytics.trackView(getTrackingName());
    }
}
public class TrackedTweetStreamActivity extends TrackedActivity {

    @Override
    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 { ... }

Delegation

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() {
        Analytics.trackView("<viewName>");
    }

}
public class TweetStreamActivity extends AppCompatActivity {

    private final TrackingDelegate mTrackingDelegate = new TrackingDelegate();

    @Override
    protected void onResume() {
        super.onResume();
        mTrackingDelegate.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
    implementation "com.pascalwelsch.compositeandroid:activity:$supportLibraryVersion"

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

    // core module (not required, only abstract classes and utils)
    implementation "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"));
        addPlugin(loadingPlugin);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // ...

        // example usage of the LoadingIndicatorPlugin
        loadingPlugin.showLoadingIndicator();
    }
}

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();

    @Override
    protected void onResume() {
        super.onResume();
        Analytics.trackView(getTrackingName());
    }
}

As plugin:

public class ViewTrackingPlugin extends ActivityPlugin {

    private final String mViewName;

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

    @Override
    public void onResume() {
        super.onResume();
        Analytics.trackView(mViewName);
    }
}

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.

restrictions

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

onRetainNonConfigurationInstace

Project stage

CompositeAndroid gets used in productions without major problems. There could be more performance related improvements but it works reliably right now.

Minor problems are:

It was a proof of conecpt and it turned out to work great. So great I haven't touched it a lot after the initial draft. Things like the documentation are still missing. I'm still keeping this project up to date but I don't invest much time in performance improvements. I don't need it, it works at it is for me.

Inspiration and other Android Composition Libraries

License

Copyright 2016 Pascal Welsch

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.