Android Fragments 从另一个活动返回后更新片段

在我的应用中，我有我的主活动，在这个活动中有一个片段，显示以下内容：“Hello，（yourname）"。您可以在首选项Activity中更改“yourname”。以下是在片段中更改名称的代码：

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是名称的Textview的id，“example_text”是名称更改器首选项的首选项键的id，到目前为止听起来不错，对吗？
好吧，现在的问题是：当在preference activity中更改名称，从而更改“example_text”的首选项值时，我按下back，frontpage（我的MainActivity）出现。但是，名称不会立即更改，我必须在我的任务管理器中关闭应用程序，或者转到“about this app”并终止进程。之后，也只有这样，如果我再次打开应用程序，新名称才会显示在我的片段中。
好吧，我想我需要在回到我的主活动时更新这个片段。可能使用onBackPressed。但问题是，怎么做？
编辑：切换到横向时出错：

4-07 19:51:37.825    7831-7831/com.package.appname E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.package.appname, PID: 7831
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.package.appname/com.package.appname.Frontpage}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.package.appname.Frontpage$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.package.appname.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.package.appname.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$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
 Caused by: java.lang.InstantiationException: class com.package.appname.Frontpage$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.package.appname.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.package.appname.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)

创建如下界面

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

将以下代码添加到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;
        }
    }

}

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

Android Fragments 从另一个活动返回后更新片段

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