检测应用程序是否从另一个活动返回MainActivity

Question

Scenario:

• my application refreshes its contents in the OnStart() method of the MainActivity by downloading some stuff from the internet. I use the OnStart() as I'd like the contents to be refreshed whenever the user opens the application, not just at the OnCreate()
• The application has a tablayout and the contents of the different tabs are called as fragments from a ViewPager
• The fragments contain custom ListViews. Clicking to a listitem calls a new activity displaying more information about the given item. The OnClickListener is called from the fragment with the following code:

:

myListView.setOnItemClickListener(
        new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Intent myNewIntent = new Intent(parent.getContext(), activity_details.class);
                startActivity(myNewIntent);
            }
        });

The problem I have is that when the user returns from this newly opened activity by pressing the back button of his phone then my app returns to the MainActivity, the OnStart() method is executed again, so the app download stuff again from the internet, which is undesired. (I need a refresh only when the user opens the app)

This means I need some sort of a mechanism to signal that we are returning from a different Activity. I have couple ideas:

1. Using sharedpreferences.
2. Using some sort of a global variable (not a big friend of this idea)
3. Database (ehh... I dont think so)
4. Using the onActivityResult in the MainActivity. This sounds ok for the first sight but as I call the new activity from a fragment it separates the actual activity call from where I process the results, therefore complicates the code

Help me out with sharing your ideas.

Answer 1

Why don't you use an interface, and override onBackPressed so it sends to MainActivity that "hey i am returning , so don't download anything" via the interface

fragment

public class itemFragment extends Fragment {

    private OnFragmentInteractionListener mListener;

    public itemFragment() {
        // Required empty public constructor
    }


    public static itemFragment newInstance() {
        return new itemFragment();
    }

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

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_item, container, false);
    }

    // TODO: Rename method, update argument and hook method into UI event


    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnFragmentInteractionListener) {
            mListener = (OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        //send true to Main Activity before detaching
        mListener.onFragmentInteraction(true);
        mListener = null;
    }


    public interface OnFragmentInteractionListener {

        void onFragmentInteraction(Boolean iamBack);
    }
}

MainActivity

public class MainActivity extends AppCompatActivity implements itemFragment.OnFragmentInteractionListener {
    boolean backPress = false;

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

    @Override
    protected void onStart() {
        super.onStart();
        //Download whatever you want if backPress is false
        if (!backPress) {

        }
    }

    @Override
    public void onFragmentInteraction(Boolean backPress) {
        this.backPress = backPress;
    }
}

Note : I still didn't try it, so tell me if you face any problem.

Answer 2

I think using SharedPreferences would be a good solution. You could set a value in the onBackPressed() method in the activity you don't want to trigger the sync code in and then check for it in the onStart() method of your MainActivity, and reset the value if it indicates you are returning from a state that you don't want to do a sync. You may find other states (maybe configuration changes) you would not want to sync on returning from as well and use the same setting.

Answer 3

it reads detecting return from another activity , which hints for startActivityForResult() ; this does not complicate anything, while dispatching the event, so that the Fragment will get notified:

for the MainActivity :

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    this.currentFragment.onActivityResult(requestCode, resultCode, data);
}

of course, the current Fragment also would need to implement onActivityResult() :

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    ...
}

then one can return data from the DetailActivity :

Intent data = new Intent();
this.setResult(AppCompatActivity.RESULT_OK, data);
this.finish();

Answer 4

The easiest solution is an activity variable. Even if you don't like this kind of solution. I don't know what is your architecture but with viewModel it's totally clean. Even without it, it's understandable and easy to maintain.

Like this:

private var syncToPerformed: Boolean = true

override fun onKeyDown(keyCode: Int, event: KeyEvent?): Boolean {
    if (keyCode == KeyEvent.KEYCODE_HOME) {
        syncToPerformed = true
    }
    return super.onKeyDown(keyCode, event)
}

override fun onCreate() {
    if (syncToPerformed) {
        // DO YOUR SYNCHRONISATION
       syncToPerformed = false
    }
}

Also it's not useful to use onStart with this solution.