从另一个活动返回后更新片段

在我的应用程序中，我有我的MainActivity，在此活动中有一个片段显示以下内容：“Hello，（yourname）”。您可以在首选项活动中更改“您的名字”。下面是在片段更改名称代码：从另一个活动返回后更新片段

public static class PlaceholderFragment extends Fragment { 

     public PlaceholderFragment() { 
     } 

     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, 
           Bundle savedInstanceState) { 

      LayoutInflater lf = getActivity().getLayoutInflater(); 
      View rootView = lf.inflate(R.layout.fragment_frontpage, container, false); 
      SharedPreferences pref= PreferenceManager.getDefaultSharedPreferences(getActivity()); 
      String text = pref.getString("example_text",""); 
      TextView textView = (TextView) rootView.findViewById(R.id.NameView); 
      textView.setText(text); 
      return rootView; 
     } 


    }

NameView是TextView的为名称的ID，“EXAMPLE_TEXT”是名，换偏好的首选项键的ID。听起来不错，对吧？

那么，现在的问题是：当在首选项活动中更改名称并因此改变“example_text”的首选项值时，我会按回并显示首页（我的MainActivity）。但是，名称不会立即更改，我必须在我的任务管理器中杀死该应用程序，或转到“关于此应用程序”并终止该过程。之后，只有这样，如果我再次打开应用程序，新名称将显示在我的片段中。

那么，我想我需要以某种方式更新片段时，返回到我的MainActivity。可能使用onBackPressed。但问题是，如何？切换错误时景观：

编辑

4-07 19:51:37.825 7831-7831/com.dandddeveloper.leibnizvp E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: com.dandddeveloper.leibnizvp, PID: 7831 
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dandddeveloper.leibnizvp/com.dandddeveloper.leibnizvp.Frontpage}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.dandddeveloper.leibnizvp.Frontpage$PlaceholderFragment: make sure class name exists, is public, and has an empty constructor that is public 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) 
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
     at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3912) 
     at android.app.ActivityThread.access$900(ActivityThread.java:144) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:135) 
     at android.app.ActivityThread.main(ActivityThread.java:5221) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:372) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 
Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.dandddeveloper.leibnizvp.Frontpage$PlaceholderFragment: make sure class name exists, is public, and has an empty constructor that is public 
     at android.support.v4.app.Fragment.instantiate(Fragment.java:431) 
     at android.support.v4.app.FragmentState.instantiate(Fragment.java:101) 
     at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1833) 
     at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:264) 
     at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:122) 
     at com.dandddeveloper.leibnizvp.Frontpage.onCreate(Frontpage.java:68) 
     at android.app.Activity.performCreate(Activity.java:5933) 
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
            at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3912) 
            at android.app.ActivityThread.access$900(ActivityThread.java:144) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:135) 
            at android.app.ActivityThread.main(ActivityThread.java:5221) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:372) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 
Caused by: java.lang.InstantiationException: class com.dandddeveloper.leibnizvp.Frontpage$PlaceholderFragment has no zero argument constructor 
     at java.lang.Class.newInstance(Class.java:1563) 
     at android.support.v4.app.Fragment.instantiate(Fragment.java:420) 
            at android.support.v4.app.FragmentState.instantiate(Fragment.java:101) 
            at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1833) 
            at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:264) 
            at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:122) 
            at com.dandddeveloper.leibnizvp.Frontpage.onCreate(Frontpage.java:68) 
            at android.app.Activity.performCreate(Activity.java:5933) 
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
            at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3912) 
            at android.app.ActivityThread.access$900(ActivityThread.java:144) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:135) 
            at android.app.ActivityThread.main(ActivityThread.java:5221) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:372) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 
Caused by: java.lang.NoSuchMethodException: <init> [] 
     at java.lang.Class.getConstructor(Class.java:531) 
     at java.lang.Class.getDeclaredConstructor(Class.java:510) 
     at java.lang.Class.newInstance(Class.java:1561) 
            at android.support.v4.app.Fragment.instantiate(Fragment.java:420) 
            at android.support.v4.app.FragmentState.instantiate(Fragment.java:101) 
            at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1833) 
            at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:264) 
            at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:122) 
            at com.dandddeveloper.leibnizvp.Frontpage.onCreate(Frontpage.java:68) 
            at android.app.Activity.performCreate(Activity.java:5933) 
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
            at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3912) 
            at android.app.ActivityThread.access$900(ActivityThread.java:144) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:135) 
            at android.app.ActivityThread.main(ActivityThread.java:5221) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:372) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

来源

2015-04-07 just_deko

实际上'Fragment'很像'Activity'，它有'onResume'和'onPause'方法。 – Stan

将此代码添加到onResume（） –

什么代码？使textview更改的那个？（textView.setText（）） –

像这样创建

public interface NameChangedListener { 
    void nameChanged(String name); 
}

接口下面的代码添加到您的MainActvity

private WeakHashMap<NameChangedListener, String> nameChangedListenerHashMap; 

    public void addNameChangedListener(
      NameChangedListener nameChangedListener) { 
     if (nameChangedListenerHashMap == null) { 
      nameChangedListenerHashMap = new WeakHashMap<NameChangedListener, String>(); 
     } 
     nameChangedListenerHashMap.put(nameChangedListener, 
       "entry"); 
    } 

    private void fireNameChange() { 
     if (nameChangedListenerHashMap != null) { 
      Iterator<NameChangedListener> iterator = nameChangedListenerHashMap 
        .keySet().iterator(); 
      while (iterator.hasNext()) { 
       NameChangedListener nameChangedListener = iterator.next(); 
       if (nameChangedListener != null) { 
        SharedPreferences pref = PreferenceManager 
          .getDefaultSharedPreferences(this); 
        String name = pref.getString("example_text", ""); 
        nameChangedListener.nameChanged(name); 
       } 
      } 
     } 
    } 

    @Override 
    protected void onResume() { 
     // TODO Auto-generated method stub 
     super.onResume(); 
     fireNameChange(); 
    }

编辑您的片段

public class PlaceholderFragment extends Fragment implements NameChangedListener{ 

    private TextView textView; 

    private String previousName = null; 

    public PlaceholderFragment() { 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 

     LayoutInflater lf = getActivity().getLayoutInflater(); 
     View rootView = lf.inflate(R.layout.fragment_frontpage, container, 
       false); 
     SharedPreferences pref = PreferenceManager 
       .getDefaultSharedPreferences(getActivity()); 
     previousName = pref.getString("example_text", ""); 
     textView = (TextView) rootView.findViewById(R.id.NameView); 
     textView.setText(previousName); 
     return rootView; 
    } 

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

    @Override 
    public void nameChanged(String name) { 
     // TODO Auto-generated method stub 
     if (!name.equals(previousName)) { 
      textView.setText(name); 
      previousName = name; 
     } 
    } 

}

参见：http://simpledeveloper.com/how-to-communicate-between-fragments-and-activities/

来源

2015-04-07 12:05:53

这会在onResume方法的这一行中引发一个错误：'textView.setText（name）;' –

整件事太长了，我只会说错误和原因：'java .lang.RuntimeException：无法恢复活动{com.dandddeveloper.leibnizvp/com.dandddeveloper.leibnizvp.Frontpage}：java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.TextView.setText（java.lang.Class） CharSequence）'对空引用'原因：'引起：java.lang.NullPointerException：尝试调用虚拟方法'无效android.widget.TextView.setText（java.lang.CharSequence）'在空对象引用在COM .dandddeveloper.leibnizvp.Frontpage $ PlaceholderFragment.onResume' –

编辑答案，只需检查它 –

从另一个活动返回后更新片段

回答

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