This is actually a deceptively "simple" question, but in reality a complicated problem in the context of Android development.
Activities are the "process entry point", meaning that any Activity you see can act as the "first point of entry to your application on start-up". People think that only the Activity that has the MAIN/LAUNCHER intent filter can be launched at start-up, but this is false.
Any Activity can act as the "first Activity", because Android can restart it from any point with the current active navigation stack.
Anyways, with that in mind, an Activity can show a View, and people often use the Activity to hold each screen of their app (instead of using it as an entry point, and swapping out view controllers in it ~ fragments).
So if you have multiple Activities, then you need to share data between them in such a way, that you take it into consideration that both activities can be started up at any time as the first Activity of the app.
For this, what you need to do is not "set the text view's text directly from another class", but you need to modify observable shared data.
The newly released official Android Architecture Components provide the LiveData<T> class, which has a subclass called MutableLiveData<T>.
To update the data from one class to another Activity, what you must do is have a global data exposed as a LiveData
public class MyApplication extends Application {
private static MyApplication INSTANCE;
DataRepository dataRepository; // this is YOUR class
@Override
public void onCreate() {
super.onCreate();
INSTANCE = this;
dataRepository = new DataRepository();
}
public static MyApplication get() {
return INSTANCE;
}
}
The DataRepository should expose LiveData:
public class DataRepository {
private final MutableLiveData<MyData> data = new MutableLiveData<>();
public LiveData<MyData> getMyData() {
return data;
}
public void updateText(String text) {
MyData newData = data.getValue()
.toBuilder() // immutable new copy
.setText(text)
.build();
data.setValue(newData);
}
}
Where the Activity subscribes to this:
public class MyActivity extends BaseActivity {
DataRepository dataRepository;
TextView textView;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
MyApplication app = (MyApplication)getApplicationContext();
dataRepository = app.getDataRepository();
setContentView(R.layout.main_activity);
textView = findViewById(R.id.textview);
dataRepository.getMyData().observe(this, new Observer() {
@Override
public void onChange(MyObject myObject) {
textView.setText(myObject.getText());
}
}
}
So to update this text, you need to get the DataRepository class, and call updateText on it:
DataRepository dataRepository = MyApplication.get().dataRepository();
dataRepository.updateText("my new text");
And this will properly update your Activity text view.
Beware that you should also persist the data to onSaveInstanceState(Bundle so that it is not lost (assuming the data is not from disk).
To do that, you need to do the following:
public class BaseActivity extends AppCompatActivity {
DataRepository dataRepository;
private static boolean didRestoreGlobals = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
MyApplication app = (MyApplication)getApplicationContext();
dataRepository = app.getDataRepository();
super.onCreate(savedInstanceState);
if(!didRestoreGlobals) {
didRestoreGlobals = true;
if(savedInstanceState != null) {
dataRepository.restoreState(savedInstanceState.getBundle("dataRepository"));
}
}
}
@Override
protected void onSaveInstanceState(Bundle bundle) {
super.onSaveInstanceState(bundle);
bundle.putBundle("dataRepository", dataRepository.saveState());
}
}
And then add saveState/restoreState methods to DataRepository accordingly.
