主持人是否知道MVP模式中的活动/上下文是个坏主意？

Question

I've been playing around with the MVP pattern for a few weeks now and I've come to the point where I need context to start a service and access Shared Preferences .

I've read that the purpose of MVP is to decouple the view from the logic and having context within a Presenter may defeat that purpose (correct me if I'm wrong on this).

Currently, I have a LoginActivity that looks something like this:

LoginActivity.java

public class LoginActivity extends Activity implements ILoginView {

    private final String LOG_TAG = "LOGIN_ACTIVITY";

    @Inject
    ILoginPresenter mPresenter;
    @Bind(R.id.edit_login_password)
    EditText editLoginPassword;
    @Bind(R.id.edit_login_username)
    EditText editLoginUsername;
    @Bind(R.id.progress)
    ProgressBar mProgressBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        MyApplication.getObjectGraphPresenters().inject(this);
        mPresenter.setLoginView(this, getApplicationContext());
    }

    @Override
    public void onStart() {
        mPresenter.onStart();
        ButterKnife.bind(this);
        super.onStart();
    }

    @Override
    public void onResume() {
        mPresenter.onResume();
        super.onResume();
    }

    @Override
    public void onPause() {
        mPresenter.onPause();
        super.onPause();
    }

    @Override
    public void onStop() {
        mPresenter.onStop();
        super.onStop();
    }

    @Override
    public void onDestroy() {
        ButterKnife.unbind(this);
        super.onDestroy();
    }

    @OnClick(R.id.button_login)
    public void onClickLogin(View view) {
        mPresenter.validateCredentials(editLoginUsername.getText().toString(),
                editLoginPassword.getText().toString());
    }

    @Override public void showProgress() { mProgressBar.setVisibility(View.VISIBLE); }

    @Override public void hideProgress() {
        mProgressBar.setVisibility(View.GONE);
    }

    @Override public void setUsernameError() { editLoginUsername.setError("Username Error"); }

    @Override public void setPasswordError() { editLoginPassword.setError("Password Error"); }

    @Override public void navigateToHome() {
        startActivity(new Intent(this, HomeActivity.class));
        finish();
    }
}

Presenter Interface ILoginPresenter.java

public interface ILoginPresenter {
    public void validateCredentials(String username, String password);


    public void onUsernameError();

    public void onPasswordError();

    public void onSuccess(LoginEvent event);

    public void setLoginView(ILoginView loginView, Context context);

    public void onResume();

    public void onPause();

    public void onStart();

    public void onStop();
}

Lastly, my Presenter:

LoginPresenterImpl.java

public class LoginPresenterImpl implements ILoginPresenter {

    @Inject
    Bus bus;

    private final String LOG_TAG = "LOGIN_PRESENTER";
    private ILoginView loginView;
    private Context context;
    private LoginInteractorImpl loginInteractor;

    public LoginPresenterImpl() {
        MyApplication.getObjectGraph().inject(this);
        this.loginInteractor = new LoginInteractorImpl();
    }

    /**
     * This method is set by the activity so that way we have context of the interface
     * for the activity while being able to inject this presenter into the activity.
     *
     * @param loginView
     */
    @Override
    public void setLoginView(ILoginView loginView, Context context) {
        this.loginView = loginView;
        this.context = context;

        if(SessionUtil.isLoggedIn(this.context)) {
            Log.i(LOG_TAG, "User logged in already");
            this.loginView.navigateToHome();
        }
    }

    @Override
    public void validateCredentials(String username, String password) {
        loginView.showProgress();
        loginInteractor.login(username, password, this);
    }

    @Override
    public void onUsernameError() {
        loginView.setUsernameError();
        loginView.hideProgress();
    }

    @Override
    public void onPasswordError() {
        loginView.setPasswordError();
        loginView.hideProgress();
    }

    @Subscribe
    @Override
    public void onSuccess(LoginEvent event) {
        if (event.getIsSuccess()) {
            SharedPreferences.Editor editor =
                    context.getSharedPreferences(SharedPrefs.LOGIN_PREFERENCES
                            .isLoggedIn, 0).edit();
            editor.putString("logged_in", "true");
            editor.commit();

            loginView.navigateToHome();
            loginView.hideProgress();
        }
    }

    @Override
    public void onStart() {
        bus.register(this);
    }

    @Override
    public void onStop() {
        bus.unregister(this);

    }

    @Override
    public void onPause() {

    }

    @Override
    public void onResume() {
    }
}

As you can see, I passed the context from the Activity into my Presenter just so I can access the Shared Preferences . I'm quite worried about passing the context into my presenter. Is this an okay thing to do? Or should I be doing it some other way?

EDIT Implemented Jahnold's 3rd preference

So let's ignore the interface and implementation because it's pretty much the entire thing. So now I'm injecting the interface for the Sharedpreference into my presenter. Here's my code for the AppModule

AppModule.java

@Module(library = true,
    injects = {
            LoginInteractorImpl.class,
            LoginPresenterImpl.class,
            HomeInteractorImpl.class,
            HomePresenterImpl.class,

    }
)
public class AppModule {

    private MyApplication application;

    public AppModule(MyApplication application) {
        this.application = application;
    }

    @Provides
    @Singleton
    public RestClient getRestClient() {
        return new RestClient();
    }

    @Provides
    @Singleton
    public Bus getBus() {
        return new Bus(ThreadEnforcer.ANY);
    }

    @Provides
    @Singleton
    public ISharedPreferencesRepository getSharedPreferenceRepository() { return new SharedPreferencesRepositoryImpl(application.getBaseContext()); }

    }
}

The way I get the context is from MyApplication.java

When the application begins, I make sure to create this Object graph with this line of code:

objectGraph = ObjectGraph.create(new AppModule(this));

Is this okay? I mean I now don't have to pass the context from the activity into my presenter, but I still have context of the application.

Answer 1

It has been some time since you asked this question but I thought it would be useful to provide an answer anyway. I would strongly suggest that the presenter should have no concept of the Android Context (or any other Android classes). By completely separating your Presenter code from the Android system code you are able to test it on the JVM without the complication of mocking system components.

To achieve this I think you have three options.

Access SharedPreferences from the View

This is my least favourite of the three as accessing SharedPreferences is not a view action. However it does keep the Android system code in the Activity away from the Presenter. In your view interface have a method:

boolean isLoggedIn();

which can be called from the presenter.

Inject SharedPreferences Using Dagger

As you are already using Dagger to inject the event bus you could add SharedPreferences to your ObjectGraph and as such would get a SharedPreferences instance which has been constructed using the ApplicationContext. This was you get the them without having to pass a Context into your presenter.

The downside of this approach is that you are still passing in an Android system class (SharedPreferences) and would have to mock it when you wanted to test the Presenter.

Create a SharePreferencesRepository Interface

This is my preferred method for accessing SharedPreferences data from within a Presenter. Basically you treat SharedPreferences as a model and have a repository interface for it.

Your interface would be similar to:

public interface SharedPreferencesRepository {

    boolean isLoggedIn();
}

You can then have a concrete implementation of this:

public class SharedPreferencesRepositoryImpl implements SharedPreferencesRepository {

    private SharedPreferences prefs;

    public SharedPreferencesRepositoryImpl(Context context) {

        prefs = PreferenceManager.getDefaultSharedPreferences(context);
    }

    @Override
    public boolean isLoggedIn() {

        return prefs.getBoolean(Constants.IS_LOGGED_IN, false);
    }

}

It is the SharedPreferencesRepository interface that you then inject with Dagger into your Presenter. This way a very simple mock can be provided at runtime during tests. During normal operation the concrete implementation is provided.

Answer 2

This question was answered some time ago, and, assuming that the definition of MVP is what OP used in his code, the answer by @Jahnold is really good.

However, it should be pointed out that MVP is a high level concept, and there can be many implementations following MVP principles - there is more than one way to skin the cat.

There is another implementation of MVP , which is based on the idea that Activities in Android are not UI Elements , which designates Activity and Fragment as MVP presenters. In this configuration, MVP presenters have a direct access to Context .

By the way, even in the aforementioned implementation of MVP, I wouldn't use Context in order to get access to SharedPreferences in presenter - I would still define a wrapper class for SharedPreferences and inject it into presenter.

Answer 3

Most of the domain elements, like DB or network, needs Context to be built. Thay cannot be created in View because View cannot have any knowledge about Model. They must be then created in Presenter. They can be injected by Dagger, but is it also using Context. So Context is used in Presenter xP

The hack is that if we want to avoid Context in Presenter then we can just make the constructor that is creating all these Model objects from Context and not saving it. But in my opinion, it is stupid. New JUnit in Android has access to Context.

Another hack is to make Context nullable, and in domain objects there should be mechanism to provide testing instance in case of null in context. I also don't like this hack.