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Android - onCreate and Fragment onCreate Activity

I'm still new to Android programming, so this question is pretty simple. I see a lot of code examples on the Internet where user interface components like TextView are initialized and accessible in the onCreate() method of Activity.

When I use Android Studio to create a new project - FirstApp - with default settings, an empty action called MainActivity , bringing activity_main and fragment_main along with it, I can immediately compile it into an APK, deploy it on my device, and I get a screen with The "FirstApp" heading and the TextView in the upper left corner showing "Hello world!"

If I give TextView identifier textview1 and create a member variable, TextView myTextView; , then I can refer to it in the onCreate() Method of Activity, for example (without compiler errors, 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, this results in "Unfortunately, FirstApp has stopped." message.

Earlier, I came across this problem by moving the startup code that accesses the user interface components to the onStart() Activity method, for example:

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

This will cause the working APK with one TextView to display “Hello tablet!” In the upper left. - my simple questions doubled ...

  • If the project uses fragments, I should fully expect that the user interface components will not be available in the onCreate() Activity method, how do I see what happens with a lot of code examples on the Internet, perhaps because they have not been created yet?
  • Is access to the user interface components acceptable in the onStart() method of Activity (which works) - or should I do something else? I also used the onCreateView method for Fragment before, but is it best to access the user interface components inside the fragment actually in the onCreate() method for the fragment that I haven't tried yet?

I also note that the onCreate() method of Fragment Android Studio creates for you when you create a new project, there is no stub provided ... but onCreateView does, and the life-cycle documentation implies (for me, anyway), that this may be the best place for such things.

Any recommendations on this subject are welcome.

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2 answers

The fragment transaction commit command only transfers the transaction to Que - the transaction will be processed for some time in the future. That is why you could not use it directly from create.

I suggest you use fragments to encapsulate their Ui behavior - I would not change the Ui elements of a fragment explicitly from activity.

An OnCreateView inside a fragment is a good place to initialize the elements of a fragment of a fragment, because it is called when a fragment view is created.

Working with fragments is rather confusing and painful in the beginning, but from my experience they help to create much more modular code.

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

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@chipopo is right to diagnose and recommend encapsulation.

But if necessary, you can call FarmentManager.executePendingTransactions() after the transaction. This ensures that operations in the fragment queue are performed synchronously. I used this sometimes in fragments that were not saved in the user interface.

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