Android - Activity onCreate and Fragment onCreate

Question

I'm still new to Android programming, so this question is rather basic. I see lots of code samples on the Internet where UI components, like a TextView, are being initialised and accessed in the onCreate() method of an Activity.

When I use Android Studio to create a new project - FirstApp - with default settings, a blank Activity called MainActivity bringing along with it activity_main and fragment_main, I can immediately compile this into an APK, deploy it on my device, and I get a screen with a header "FirstApp" and a TextView in the top left showing "Hello world!".

If I give the TextView an ID of textview1, and create a member variable, TextView myTextView;, then I can reference it in the onCreate() method of the Activity, like so (no compiler errors, of course):

    @Override
    protected void onCreate (Bundle savedInstanceState)
    {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_main);
        myTextView = (TextView) findViewById (R.id.textview1);
        myTextView.setText ("Hello tablet!");

        if (savedInstanceState == null)
        {
            getSupportFragmentManager().beginTransaction().add (R.id.container,new PlaceholderFragment()).commit();
        }
    }

However, if I compile and run the APK it results in an "Unfortunately, FirstApp has stopped." message.

I have previously got around this issue by moving startup code that accesses UI components into the onStart() method of the Activity, like so:

    @Override
    protected void onStart()
    {
        super.onStart();
        myTextView = (TextView) findViewById (R.id.textview1);
        myTextView.setText ("Hello tablet!");
    }

Which would result in a working APK with a single TextView in the top left showing "Hello tablet!" - my simple questions are two-fold...

1. If the project uses Fragments then should I fully expect that the UI components cannot be accessed in the onCreate() method of the Activity, as I see happening with lots of sample code on the Internet, probably because they've not been created yet?
2. Is it acceptable for me to be accessing UI components within the onStart() method of the Activity (which does work) - or should I be doing something else? Previously I have also used the onCreateView method of the Fragment, but is the best place to access UI components inside a Fragment actually in the onCreate() method of the Fragment, which I have not yet tried?

I also note that the onCreate() method of the default Fragment Android Studio creates for you when you create a new project does not have a stub provided... but onCreateView does, and the lifecycle documentation implies (to me, anyway) that this might be the best place to be doing things like this.

Any guidance on this is appreciated.

Answer 1

The fragment transaction commit command only puts the transaction in Que - the transaction will be processed some time in the future. This is why you couldnt use it straight from on create.

I suggest you to use fragments to encapsulate their Ui behavior - I wouldn't change a fragment's Ui elements explicitly from the activity.

OnCreateView inside a fragment is a good place to init the fragment's Ui elements since it is called when the fragment view is created.

Working with fragments is quite tricky and painful at start but From my experiance they do help to create a much more modular code.

I suggest you read more about fragments in the docs before starting a serious project with them. http://developer.android.com/guide/components/fragments.html

Answer 2

@chipopo is right in the diagnostics and in the encapsulation recommendations.

But in case you need you can call FarmentManager.executePendingTransactions() after commiting the transaction. This ensures that the operations on fragments queue are execute synchronously. I used that sometimes in non UI retainedInstance fragments.