Question

我正在尝试从MainActivity调用方法来更新片段中的TextView。

但是，我在onCreate中使用MainActivity方法获得的片段的引用具有空控件（buttons和textviews）。

我认为这是因为Fragment在MainActivity之后被初始化而引用了这个片段。如何使用初始化控件从MainActivity引用此片段？

MainActivity：

public class MainActivity extends AppCompatActivity {
TabAdapter adapter;
Toolbar toolbar;
ViewPager pager;
SlidingTabLayout tabs;
StopwatchFragment stopwatchFragment;

CharSequence titles[] = {"Stopwatch","Countdown"};
int numOfTabs=2;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    toolbar = (Toolbar)findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    adapter = new TabAdapter(getSupportFragmentManager(),numOfTabs,titles);

    pager = (ViewPager)findViewById(R.id.view_pager);
    pager.setAdapter(adapter);

    tabs = (SlidingTabLayout)findViewById(R.id.tabs);
    tabs.setDistributeEvenly(true);     
    tabs.setViewPager(pager);

    stopwatchFragment =   StopwatchFragment)adapter.getItem(TabAdapter.STOPWATCH_FRAGMENT_POSITION);

   }
}

StopwatchFragment：

public class StopwatchFragment extends Fragment {
private static String TAG = "StopwatchFragment";

private TextView tv_elapsed_time;
private TextView tv_elapsed_time_millis;
private long elapsedTime;
private long startTime;
private static final int REFRESH_RATE = 60;
private boolean stopped = false;

private Button btn_lap;
private ListView lv_laps;

private LapsAdapter lapsAdapter;
private List<String> lapsList;

private Handler handler = new Handler();

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_stopwatch,container,false);       

    tv_elapsed_time=(TextView)view.findViewById(R.id.elapsed_time);
    tv_elapsed_time_millis=(TextView)view.findViewById(R.id.elapsed_time_millis);
    lv_laps = (ListView)view.findViewById(R.id.lv_Laps);

    lapsList = new ArrayList<String>();
    lapsAdapter = new LapsAdapter(getActivity(),R.layout.laps_row,lapsList);
    lv_laps.setAdapter(lapsAdapter);

    return view;
}

TabAdapter：

public class TabAdapter extends FragmentStatePagerAdapter {

public static final int STOPWATCH_FRAGMENT_POSITION = 0;
public static  final int  LAPS_FRAGMENT_POSITION = 1;

CharSequence titles[];
int numOfTabs; 

public TabAdapter(FragmentManager fm,int numberOfTabs,CharSequence titles[])   {
    super(fm);
    this.numOfTabs =numberOfTabs;
    this.titles = titles;
}
@Override
public Fragment getItem(int position) {
    switch(position)
    {
        case 0:
            StopwatchFragment stopwatchFragment = new StopwatchFragment();
            return stopwatchFragment;
        case 1:
            CountdownFragment countdownFragment = new CountdownFragment();
            return countdownFragment;
        default:return null;

    }
}

@Override
public int getCount() {
    return numOfTabs;
}

@Override
public CharSequence getPageTitle(int position) {
    return titles[position];
}

}

StopwatchFragment.xml：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_horizontal"
android:orientation="vertical">

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="@dimen/activity_horizontal_margin">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true">

        <TextView
            android:id="@+id/elapsed_time"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:fontFamily="sans-serif-thin"
            android:text="00 00"
            android:textColor="@color/colorAccent"
            android:textSize="112sp" />

        <TextView
            android:id="@+id/elapsed_time_millis"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="00"
            android:textColor="@color/colorAccent"
            android:textSize="24sp" />
    </LinearLayout>

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="360dp"
        android:layout_height="360dp"
        android:max="500"
        android:progress="0"
        android:progressDrawable="@drawable/circular" />
</RelativeLayout>    

<ListView
    android:id="@+id/lv_Laps"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:divider="@null"
    android:dividerHeight="0dp" />

Answer 1

在StopwatchFragment中，您需要将初始化控件的代码移至onActivityCreated方法而不是onCreateView。我在下面的代码中做了这些更改：

public class StopwatchFragment extends Fragment {
private static String TAG = "StopwatchFragment";

private Button btn_start,btn_stop;
private TextView tv_elapsed_time;
private TextView tv_elapsed_time_millis;
private long elapsedTime;
private long startTime;
private static final int REFRESH_RATE = 60;
private boolean stopped = false;

private Button btn_lap;
private ListView lv_laps;

private LapsAdapter lapsAdapter;
private List<String> lapsList;


private Handler handler = new Handler();

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_stopwatch,container,false);   
        tv_elapsed_time=(TextView)view.findViewById(R.id.elapsed_time);
        tv_elapsed_time_millis= (TextView)view.findViewById(R.id.elapsed_time_millis);
        lv_laps = (ListView)view.findViewById(R.id.lv_Laps); 
    return view;
}

    /**
     * This method is called after the onCreateView() method when the host activity is created. 
     * Activity and fragment instance have been created as well as the view hierarchy of the activity. 
     * At this point, view can be accessed with the findViewById() method.
     */
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {  
        super.onActivityCreated(savedInstanceState);

        lapsList = new ArrayList<String>();
        lapsAdapter = new LapsAdapter(getActivity(),R.layout.laps_row,lapsList);
        lv_laps.setAdapter(lapsAdapter);

        Log.d(TAG,"Controls initialized");                      

        Log.d(TAG, "onCreateView");

    }

请相应更新您的StopwatchFragment，试一试，如果有帮助请告诉我。

Answer 2

TabAdapter出现了问题。需要更改构造函数：

public TabAdapter(FragmentManager fm,int numberOfTabs,CharSequence titles[]) {
    super(fm);
    this.numOfTabs = numberOfTabs;
    this.titles = titles;
    stopwatchFragment = new StopwatchFragment();
    countdownFragment = new CountdownFragment();
}

方法getItem（）：

@Override
public Fragment getItem(int position) {
    switch(position)
    {
        case 0: 
            return stopwatchFragment;
        case 1:                
            return countdownFragment;
        default:return null;
    }
}

对Activity的Fragment的引用有未初始化的元素（TextView和其他）

2 个答案: