Presenter Abstraction Layer in Open Event Organizer Android App

Open Event Organizer App design follows Model View Presenter (MVP) architecture which enables heavy unit testing. MVP is a trending architecture design followed these days. If you are not aware of MVP architecture, then please refer any of the tutorial (few links are given at the end of this blog) about it before reading this. In the design, the code becomes little repetitive as the application size increases due to so many presenters and views, which degrades the code readability. So to avoid this and keep the functionality code clean in the App, we have created a Presenter Abstraction Layer which contains the repetitive code and the layer is extended wherever required in the app. I will be talking about the Presenter Abstraction Layer implementation through the App in this blog.

First of all, create a base interface. The base interface contains methods which every presenter will have. The base interface for presenter in the App looks like:

public interface IBasePresenter {
   void start();
   void detach();


In the method start, presenter loads all the required data from the model and sends it to the view. And releases all the resources in detach. These two methods are required in all the presenters. This interface is extended by other two interfaces which will be actually used by the views. The relevant code is:

public interface IPresenter<V> extends IBasePresenter {
   void attach(V view);

public interface IDetailPresenter<K, V> extends IBasePresenter {
   void attach(K key, V view);


Method attach is used to attach view and the data id (if required) to the presenter. In the app, most of the presenters require an extra data which is used in loading data from the model. Hence two interfaces are extended from the base interface. Now comes the implementation part.

public abstract class BasePresenter<V> implements IPresenter<V> {
   private V view;
   private CompositeDisposable compositeDisposable;

   public void attach(V view) {
       this.view = view;
       this.compositeDisposable = new CompositeDisposable();

   public void detach() {
       view = null;

   protected V getView() {
       return view;

   protected CompositeDisposable getDisposable() {
       return compositeDisposable;


The App uses ReactiveX Observables for async operations which contain fragment/activity context hence these need to be disposed at some lifecycle of fragment/activity. detach nulls the view and disposes the compositeDisposable. This method is called at the onStop lifecycle of fragment/activity. The observable subscriptions are one of the major reasons for memory leaks if not disposed at correct lifecycle in Android. So the detach method is called at onStop lifecycle when activity goes into background or fragment is switched by FragmentTransaction. Another base presenter class looks like:

public  abstract class BaseDetailPresenter<K, V> extends BasePresenter<V> implements IDetailPresenter<K, V> {
   private K id;

   public void attach(K id, V view) {
       super.attach(view); = id;

   protected K getId() {
       return id;


This class extends the previous one except for the attach method. As the presenters extending this, require an extra data id which is passed through this method. So the id can be used in the presenter extending this class using getId. The presenters in the app extend one of these two classes. This helps in making a firm app structure and the development process easier. Abstraction layer should be used wherever same code is repeated. This increases code readability and decreases the chances of creating bugs especially when a team is working on the same project.

1. MVP for Android: how to organize the presentation layer, by Antonio Leiva
2. Android Code That Scales, With MVP, by Nathan Barraille
3. Ted Mosby – Software Architect, by Hannes Dorfmann

Unifying Data from Different Scrapers of loklak server using Post

Loklak Server project is a software that scrapes data from different websites through different endpoints. It is difficult to create a single endpoint. For a single endpoint, there is a need of a decent design for using multiple scrapers. For such a task, multiple changes are needed. That is why one of the changes I introduced was Post class that acts as both wrapper and an interface for data objects of search scrapers (though implementation in scrapers is in progress).

Post is a subclass of JSONObject that helps in working with JSON data in Java. In other words, Post is a JSONObject with an identity (we call it postId) and and a timestamp of the data scraped. It is used to capture data fetched by the web-scrapers. Benefit of JSONObject as superclass is that it provides methods to capture and access data efficiently.

Why Post?

At present there is a Class MessageEntry which is the superclass of TwitterTweet (data object of TwitterScraper). It has numerous methods that can be used by data objects to clean and analyse data. But it has a disadvantage, it is a specialized for social websites like Twitter, but will become redundant for different types websites like Quora, Github, etc.

Whereas Post object is a small but powerful and flexible object with its ability to deal with data like JSONObject. It contains getter and setter methods, identity members used to provide each Post object a unique identity. It doesn’t have any methods for analysis and cleaning of data, but MessageEntry class’ methods can be used for this purpose.

Uses of Post Object

When I started working on Post Object, it could be used as marker interface for data objects. Following are the advantages I came up with it:

1) Accessing the data object of any scraper using its variable. And yes, this is the primary reason it is an interface.

2) But in addition to accessing the data objects, one can also directly use it to fetch, modify or use data without knowing the scraper it belongs. This feature is useful in Timeline iterator.

This is an example how Post interface is used to append two lists of Posts (maybe carrying different type of data) into one.

public void mergePost(PostTimeline list) {
    for (Post post: list) {


Post as a wrapper object

While working on Post object, I converted it into a class to also use it as a wrapper. But why a wrapper? Wrapper can be used to wrap a list of Post objects into one object. It doesn’t have any identity or timestamp. It is just a utility to dump a pack of data objects with homogeneous attributes.

This is an example implementation of Post object as wrapper. typeArray is a wrapper which is used to store 2 arrays of data objects in it. These data object arrays are timeline objects that are saved as JSONArray objects in the Post wrapper.

    Post typeArray = new Post(true);
    switch(type) {
        case "users":
            typeArray.put("users", scrapeProfile(br, url).toArray());
        case "question":
            typeArray.put("question", scrapeQues(br, url).toArray());