how to access fragments elements in MainActivity()?

问题

In my project, I want to set visibility of fragments buttons from MainActivity. But the problem is, it gives NullPointerException(). I also maked listBtn & gridBtn as static. I used below code :

 FirstFragment fragment = (FirstFragment)getSupportFragmentManager().findFragmentById(R.id.    <frameLayout Id>);

  main_page_fragment.listBtn.setVisibility(View.GONE);
  main_page_fragment.gridBtn.setVisibility(View.GONE);

回答1:

You cannot access to your fragment view from Activity class because activity uses its own view (ex: R.layout.activity_main). Rather you can set visibility in your corresponding fragment class which will do the same job.

public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.details, container, false);
        Button listBtn = (Button)view.findviewById(R.id.listBrn);
        Button gridBtn = (Button)view.findviewById(R.id.gridBrn);
        listBtn.setVisibility(View.GONE);
        gridBtn.setVisibility(View.GONE);
        return view;
    }

回答2:

Fragment onCreateView callback is called after onCreate method of activity, so i think you have tried to get access from it. That views will be accessible only after onResumeFragments callback is called, you should perform your actions with fragments there.

Another tip is that you strongly should not call views of fragments directly like you did or via static reference to views that's the worst. You should avoid such dependencies on fragments inner implementation. Instead of it, better is create some method like setInitialState (the name depends on your business logic) and just call it from activity.

So result code: In activity:

private FirstFragment fragment;

protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //init fragment here
}

@Override
protected void onResumeFragments() {
    super.onResumeFragments();
    fragment.setInitialState();
}

In fragment:

//this will be called on fragment #onResume step, so views will be ready here.
public void setInitialState() {
   listBtn.setVisibility(View.GONE);
   gridBtn.setVisibility(View.GONE);
}

回答3:

If you add your fragments dynamically from MainActivity like so:

YourFragment fragment = new YourFragment();
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.fragmentContainer, fragment, YOUR_TAG)
            .commit();

Then you can define method in your fragment like so:

public void hideButtons()
{
    yourBtn.setVisibility(View.GONE);
}

And call it from activity:

fragment.hideButtons();

回答4:

Although it is not recommended, you can achieve what you want by doing this:

Declare your views as static fields in the Fragment. After fragment has gone through its onCreateView and onActivityCreated, set the value of the static view which you want to access from main activity (findViewById(), etc). Then you can contact your static view from Main Activity. BTW whenever you wanted to invoke a method on the static view from main activity, check if it's null or not.

来源：https://stackoverflow.com/questions/41331109/how-to-access-fragments-elements-in-mainactivity

标签

android

nullpointerexception

fragment