Question

我有一个Activity QuestScreen，它在创建时包含Fragment FragmentQuest 。如果我按下按钮，则触发屏幕活动的 next（）方法，导致用 FragmentAction 替换 FragmentQuest 。

到目前为止替换工作。问题是，在替换之后我想在FragmentAction中设置一个TextView，以获得在 QuestScreen活动。

我的第一次尝试是在 FragmentAction 的 onCreatView 之外设置 TextView 。这会导致Nullpointer异常处理TextView。

到目前为止，我已查看过：This，That，This和That

下面的尝试导致代码中标记了NullpointerException。

在QuestScreen类中

 private void next(){

    //The needed value for the TextView is stored in nextTransition
    Transition nextTransition;
    //Check if an entry is in the list
    if(quest.getTransitionsAbleToFire().size() > 0){
        nextTransition = quest.getTransitionsAbleToFire().get(0);

        fragmentAction = (FragmentAction) fm.findFragmentByTag(TAG_FRAGMENT_ACTION);

        // create the fragment and data the first time
        if (fragmentAction == null) {
            Log.d("QuestScreen onCreate" , "create fAction for first time");
            // add the fragment
            fragmentAction = (FragmentAction) Fragment.instantiate(this, FragmentAction.class.getName(), null);

            fm.beginTransaction().add(fragmentAction, TAG_FRAGMENT_ACTION).commit();
            FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.flFragmentContainer, fragmentAction);
            fragmentTransaction.commit();
        }

        //Triggers the corresponding activity in fragmentAction, to set the TextView to a value which is stored in nextTransition
        fragmentAction.nextAction(nextTransition);
        nextTransition.fire();

    }
    else{
        CharSequence text = "Quest END";
        int duration = Toast.LENGTH_SHORT;
        Toast toast = Toast.makeText(getApplicationContext(), text, duration);
        toast.show();
    }
}

在FragmentAction类中

public class FragmentAction extends Fragment {

private TextView txtYourQuest;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRetainInstance(true);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // CREATE VIEWS HERE

    View layout = inflater.inflate(R.layout.content_action_fragment, container, false);
    txtYourQuest = (TextView) layout.findViewById(R.id.txtV_Your_Quest);
    return layout;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
}

public void nextAction(Transition prmTransition){

    // --> Here in the enxt Line comes the NPE :  java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference

    FrameLayout container = (FrameLayout) getActivity().findViewById(R.id.flFragmentContainer);
    LayoutInflater.from(getActivity())
            .inflate(R.layout.content_action_fragment, container, false);

    txtYourQuest.setText("Your " + prmTransition.getAction().getPosition() + ". is to:\n "
    + prmTransition.getAction().getActionName());
}

Answer 1

对象引用
上的Activity.findViewById（int）'

这意味着您的getActivity()会以某种方式返回null。要找出答案，为什么会发生这种情况，谷歌就像“getActivity()返回null”。 Here 就是这种关于stackoverflow的问题的一个很好的例子。

这是摆脱崩溃的解决方法。

为您的片段添加活动字段。

活动活动;

覆盖onAttach()方法并将值设置为activity字段。

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    this.activity = activity;
}

将getActivity()方法替换为activity变量。

FrameLayout container =（FrameLayout）activity.findViewById（R.id.flFragmentContainer）;

使用另一个片段（NPE）中生成的值设置片段的TextView

1 个答案: