Addressing Objects from another Activity

Question

I would like to do a pretty simple task:

I got an activity A and an activity B.

Activity A contains an object that I want to change from activity B.

I tried to do so with following code:

Intent intent = new Intent(this, A.class); 
FragmentManager fragmentManager = getSupportFragmentManager(); 
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();     
fragmentTransaction.replace(R.id.fragment_container, new LogFragment()); 
fragmentTransaction.commit(); 
startActivity(intent);

Explanation:

fragment_container is the object I would like to replace by LogFragment (thats a fragment, speaking names for the win ^^ ).

This code doesn't work because he tells me that he can't find "fragment_container".

I guess thats because I am still in activity B while the "fragment_container" is found in activity A.

Is there a way how I can still address "fragment_container" while beeing in activity B? Another possibility could be that I would overwrite the startActivity method to something like startActivity(intent, fragment).

Is this possible and if yes, which methods do I need to overwrite? (I got basic java knowlegde but sadly no experience with android programming.)

Thanks for any help.

Answer 1

I am not sure what exactly you want to do. That you try to perform a FragmentTransaction like that seems weird. If you explain more about what you want to achieve I can give you a more accurate answer.

Anyway you can pass data between Activities by adding it to the Intent. For example here I add the class of the Fragment you want to be displayed in Activity A to the Intent. The key "fragmentClass" is used to get the data back from the Intent in Activity A.

Intent intent = new Intent(this, A.class); 
intent.putExtra("fragmentClass", LogFragment.class);
startActivity(intent);

In Activity As onCreate():

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

    // savedInstanceState is null on the first Start of the Activity, as in when it was started with an Intent
    if(savedInstanceState == null) {

        // We get the Intent that started the Activity
        Intent intent = getIntent();

        // And retrieve the Class object we attached to the Intent with the key "fragmentClass".
        Class<?> fragmentClass = (Class<?>) intent.getSerializableExtra("fragmentClass");

        // Now we can instantiate the Fragment and perform the FragmentTransaction
        Fragment fragment = Fragment.instantiate(this, fragmentClass.getName());

        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.commit();
    }
}

So by using this code you could start the Activity A with any Fragment you like, you just have to add the class of the Fragment you want to be displayed to the Intent as I showed above.

Answer 2

I would not use Fragments for this task (unless you have a very good reason to do so).

What You can do to change the object in activity A from Activity B is to start the activity B from the activity A using the startActivityforResult method:

  //this is in activity A
  protected void startActivityB() {
       Intent i = new Intent(this, ActivityB.class);
       startActivityForResult(i, ACTIVITY_B);//ACTIVITY_B is a constant to identify B
  }

In the activity B, when you have done your stuff and you want to go back to activity A passing the Object:

  //this is in Activity B
  protected void goBackToActivityA() {
          Object myObjectToBePassedBack = ...;//whatever you have done/ or do to it
          Intent returnIntent = new Intent();
          returnIntent.putExtra("obj", myObjectToBePassedBack);
          setResult(RESULT_OK, returnIntent);
          finish();
  }

Then in activity A:

  protected void onActivityResult(int requestCode, int resultCode, Intent data) {
     if (requestCode == ACTIVITY_B) {
        if (resultCode == RESULT_OK) {
            Object receivedBack = data.getExtra("obj");
            //Do what you want here with the object passed back
        }
     }
  }

Notice that the object must be serialisable to do this.

Answer 3

If you want all activities to have access to shared state then extend Application (see Extending Application to share variables globally)

Answer 4

Wait, so do you need to send objects between two Fragments, or two Activities? The way it's done is different.

If you're trying to send from activity to activity, you need to use the Extras section of the Intent. To place something in the Extras, it must implement the Parcelable interface.

Just throw your POJO class into this site, and it will have everything for Parcelling. http://www.parcelabler.com/

However, if you want to send it from one Fragment to another, then you should use the fragment.setBundle(bundle); function, and place the Parcellable into that.

I am not entirely sure what you're trying to do with the Fragments and Activities. I think you want to use 1 activity and 2 fragments in this case, rather than 2 activities and... one fragment that's shared between them, I think? I really am not sure.