Need Context in Model in MVP

Question

I need to use the Context of activity in the model while using MVP in android to get the list of all the installed application.what is the correct way to access the context or any alternative to achieve the same while following the MVP pattern.

Here are the classes:

Main Activity.java

public class MainActivity extends BaseActivity
    implements MainView,View.OnClickListener {

    private MainPresenter mPresenter;
    private Button sendButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       init();
       createPresenter();
    }

    private void init(){
       sendButton= (Button) findViewById(R.id.button_send);
       sendButton.setOnClickListener(this);
    }

    private void createPresenter() {
       mPresenter=new MainPresenter();
       mPresenter.addView(this);
    }

    @Override
    public void onClick(View view) {
       switch (view.getId()){
          case R.id.button_send:
             mPresenter.onSendButtonClick();
             break;
        }
    }

   @Override
   public void openOptionsActivity() {
        Intent intent=new Intent(this,OptionsActivity.class);
        startActivity(intent);
   }
}

Main Presenter.java

public class MainPresenter extends BasePresenter {

    MainModel model;

    public void onSendButtonClick() {
       model.getListOfAllApps();
    }

    @Override
    public void addView(MainView view) {
        super.addView(view);
        model = new MainModel();
    }

}

Main Model.java

public class MainModel {

    public void getListOfAllApps(){
        final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);
        mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);
        final List pkgAppsList = getPackageManager().queryIntentActivities(mainIntent, 0);
  
    }

}

Having issue in getPackageManager().queryIntentActivities(mainIntent, 0) .how to do it as not having any context here.

Solution

I answered a similar question here which you may want to have a look at too. I'll give the breakdown of how I think you could solve this particular problem though.

Use a static context from the Application class

This method would work but I'm not fond of it. It makes testing harder and couples your code together.

public class App extends Application {

    private static Context context;

    public static Context getContext() {
        return context;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        context = getApplicationContext();
    }
}

Then in your MainModel:

public class MainModel {

    public List<String> getListOfAllApps(){

        final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);
        mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);
        final List<ResolveInfo> pkgAppsList = App.getContext().getPackageManager().queryIntentActivities(mainIntent, 0);

        List<String> results = new ArrayList<>();
        for (ResolveInfo app : pkgAppsList) {
            results.add(app.resolvePackageName);
        }
        return results;
    }
}

Now we've got that out the way, let's look at some better options.

Do it in the Activity

So your Activity implements your View. It's probably doing a few Anrdoidy things too such as onActivityResult. There's an argument for keeping Android code in the Activity and just accessing it through the View interface:

public interface MainView {

    List<String> getListOfAllApps();
}

The Activity:

public class MainActivity extends BaseActivity implements MainView {

    //..

    @Override
    public List<String> getListOfAllApps(){

        final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);
        mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);
        final List<ResolveInfo> pkgAppsList = getPackageManager().queryIntentActivities(mainIntent, 0);

        List<String> results = new ArrayList<>();
        for (ResolveInfo app : pkgAppsList) {
            results.add(app.resolvePackageName);
        }
        return results;
    }

    //..
}

And the Presenter:

public class MainPresenter extends BasePresenter {

    public void onSendButtonClick(){

        view.getListOfAllApps();
    }
}

Abstract the details in a separate class

Whilst the last option doesn't break the rules of MVP it doesn't feel quite right as getting a list of packages isn't really a View operation. My preferred option is to hide the use of Context behind an interface/class.

Create a class PackageModel (or whatever name takes your fancy):

public class PackageModel {

    private Context context;

    public PackageModel(Context context) {
        this.context = context;
    }

    public List<String> getListOfAllApps(){

        final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);
        mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);
        final List<ResolveInfo> pkgAppsList = context.getPackageManager().queryIntentActivities(mainIntent, 0);

        List<String> results = new ArrayList<>();
        for (ResolveInfo app : pkgAppsList) {
            results.add(app.resolvePackageName);
        }
        return results;
    }
} 

Now have your Presenter require this as a constructor parameter:

public class MainPresenter extends BasePresenter {

    private PackageModel packageModel;

    public MainPresenter(PackageModel packageModel) {
        this.packageModel = packageModel;
    }

    public void onSendButtonClick(){

        packageModel.getListOfAllApps();
    }
}

Finally in your Activity:

public class MainActivity extends BaseActivity implements MainView {

    private MainPresenter presenter;

    private void createPresenter() {

        PackageModel packageModel = new PackageModel(this);
        presenter = new MainPresenter(packageModel);
        presenter.addView(this);
    }
}

Now the use of Context is hidden from the Presenter and it can carry on without any knowledge of Android. This is known as constructor injection. If you're using a dependency injection framework it can build all the dependencies for you.

If you wanted to you could make an interface for PackageModel but I don't think it's really necessary as a mocking framework like Mockito can create a stub without using an interface.

Comments

[Sufian]

As this comment said, it is only hiding the dependency, which is worse. The answer on the same thread is very good as it makes dependencies very clear. I found this video great on singletons and hidden dependencies.

[Nicolás Carrasco-Stevenson]

I would just make a small adjustment to point 3 and make PackageModel an interface and make getListOfAllApps() its single method.

[Amir Ziarati]

your first way is awful. just delete the fist way. its absolutely wrong. the presenter and model should be in pure java. the interface is the true way.

[Amir Ziarati]

you should not use context in presenter and model. this code wont be testable.

[Jahnold]

@AmirZiarati Indeed it is not the best way to do it, but I mention it to show what people might consider before moving onto better options.

[Thracian]

@AmirZiarati, i need to get local strings context.getString(R.string...) in model. How can i achieve this without using context as a parameter of model's constructor or using context at all?

[Amir Ziarati]

@Thracian just fetch it from the resource using the context in view layer and send it to model layer as a string dependency.

[Newaj]

@Jahnold In your 3rd solution, do you consider PackageModel as MVP model? If so, then creating a PackageModel instance in MainActivity creates link between VIEW & MODEL, am I right? But as far as I know, there shouldn't be any link between VIEW & MODEL in MVP pattern. I am new in MVP topic, I haven't got this point.

[Newaj]

hi @Jahnold , I haven't got a point in the 3rd solution. You have created a PackageModel instance in the View & called packageModel.getListOfAllApps() in presenter. So, is PackageModel working as MVP model here? If not, then where is the MODEL part for this task here? If it is considered as Model, then creating an instance of PackageModel creates link between MVP View & Model, isn't it? Also PackageModel has android term(Context), but Model should not have android term. I am new in MVP and not getting the point here. I have a separate question with bounty, you may consider it as well.