java - 共享首选项空指针异常

Question

我正在努力将共享首选项添加到我的笔记 Android 应用程序中，但我遇到了一个特殊的空指针异常。我目前正在我的应用程序中测试字体大小和字体的设置，并且已经能够成功保存两者的共享首选项，但在检索部分时遇到问题。当设置更改时，他们在应用程序打开的剩余时间内成功更改了 fragment 上的字体和字体大小，但如果重新启动应用程序，我无法恢复它们。

第一个奇怪的空指针位于 onCreate 中。

创建时:

    //create a new note fragment if one has not been created yet
    mNoteFragment = (NoteFragment) getFragmentManager().findFragmentById(R.id.container);
    if (mNoteFragment == null) {
        mNoteFragment = new NoteFragment();
        getFragmentManager().beginTransaction()
                .replace(R.id.container, mNoteFragment).commit();
    }

    //restore SharedPreferences
    SharedPreferences sharedPrefs = getPreferences(0);
    int stylePref = sharedPrefs.getInt(SharedPreferanceConstants.PREF_FONT_SIZE, 2);
    String fontPref = sharedPrefs.getString(SharedPreferanceConstants.PREF_TYPEFACE, "");
    Log.e("STYLEID", String.valueOf(stylePref));
    Log.e("FONTTYPE", fontPref);
    onStyleChange(null , stylePref); //NULL POINTER EXCEPTION HERE
    onFontChange(null, fontPref);

日志输出“3”和“Impact”，它们是大小和字体的正确值，表明 stylePref 和 fontPref 不为空。

下一个空指针如下:

    @Override
    public void onStyleChange(CustomStyleDialogFragment dialog, int styleId) {
        Log.d("NOTEFRAGMENT", String.valueOf(mNoteFragment));
        mNoteFragment.setCustomStyle(styleId); //NULL POINTER EXCEPTION HERE
    }

    @Override
    public void onFontChange(CustomStyleDialogFragment dialog, String fontName) {
        mNoteFragment.setCustomFont(fontName);
    }

我已经测试了记录 styleId 的值并得到“3”，所以这似乎不是问题。根据日志，mNoteFragment 也不为 null。

这是 NoteFragment.java 中的第三个 NP 异常。这是我最终将 EditText View 设置为所需设置的地方。在没有共享首选项的情况下，我可以轻松更改字体和大小，因此我不确定这里的问题。

    public void setCustomFont(String fontName) {
        if(fontName.equals("Helvetica")) {
            mEditText.setTypeface(mHelvetica);
        }
        else if(fontName.equals("Helvetica-Neue")) {
            mEditText.setTypeface(mHelveticaNeue);
        }
        else if(fontName.equals("Impact")) {
            mEditText.setTypeface(mImpact);
        }
        else {
            mEditText.setTypeface(Typeface.DEFAULT);
        }
    }

    public void setCustomStyle(int styleId) {
        if(styleId == 1) {
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Small);
        }
        else if(styleId == 2) {
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Medium);
        }
        else if(styleId == 3) {
            //NULL POINTER EXCEPTION HERE
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Large);
        }
}

当然，这是 logcat。预先感谢!

11-18 10:31:53.030  18966-18966/com.richluick.blocnotes I/Process﹕ Sending signal. PID: 18966 SIG: 9
11-18 10:35:32.838  19248-19248/com.richluick.blocnotes D/STYLEID﹕ 3
11-18 10:35:32.838  19248-19248/com.richluick.blocnotes D/FONTTYPE﹕ Impact
11-18 10:35:32.839  19248-19248/com.richluick.blocnotes D/ERROR﹕ NoteFragment{422972f8 id=0x7f090001}
11-18 10:35:32.840  19248-19248/com.richluick.blocnotes D/AndroidRuntime﹕ Shutting down VM
11-18 10:35:32.840  19248-19248/com.richluick.blocnotes W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x4197dd40)
11-18 10:35:32.842  19248-19248/com.richluick.blocnotes E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.richluick.blocnotes, PID: 19248
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.richluick.blocnotes/com.richluick.blocnotes.BlocNotes}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
            at android.app.ActivityThread.access$800(ActivityThread.java:139)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5097)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.richluick.blocnotes.NoteFragment.setCustomStyle(NoteFragment.java:86)
            at com.richluick.blocnotes.BlocNotes.onStyleChange(BlocNotes.java:139)
            at com.richluick.blocnotes.BlocNotes.onCreate(BlocNotes.java:61)
            at android.app.Activity.performCreate(Activity.java:5248)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
            at android.app.ActivityThread.access$800(ActivityThread.java:139)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5097)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

NoteFragment.java

public class NoteFragment extends Fragment {

    public EditText mEditText;
    private Typeface mHelvetica;
    private Typeface mHelveticaNeue;
    private Typeface mImpact;

    private static final String TEXT = "text";

    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putString(TEXT, mEditText.getText().toString());
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_note, container, false);
        setHasOptionsMenu(true);

        //restore state of app when activity is destroyed and restarted
        mEditText = (EditText) rootView.findViewById(R.id.editText);
        if (savedInstanceState != null) {
            mEditText.setText(savedInstanceState.getString(TEXT));
        }

        //Store the font assets as variables
        mHelvetica = Typeface.createFromAsset(getActivity().getAssets(), "fonts/Helvetica_Reg.ttf");
        mHelveticaNeue = Typeface.createFromAsset(getActivity().getAssets(), "fonts/HelveticaNeue_Lt.ttf");
        mImpact = Typeface.createFromAsset(getActivity().getAssets(), "fonts/impact.ttf");

        return rootView;
    }

    /**
     * This is a setter method for setting the font the user has selected from the spinner
     *
     * param fontName the name of the font the user selected
     * @return void
     * */
    public void setCustomFont(String fontName) {
        if(fontName.equals("Helvetica")) {
            mEditText.setTypeface(mHelvetica);
        }
        else if(fontName.equals("Helvetica-Neue")) {
            mEditText.setTypeface(mHelveticaNeue);
        }
        else if(fontName.equals("Impact")) {
            mEditText.setTypeface(mImpact);
        }
        else {
            mEditText.setTypeface(Typeface.DEFAULT);
        }
    }

    /**
     * This is a setter method for setting the font style the user has selected from custom menu
     *
     * param styleId the integer id of the font stlye selected (SMALL, MEDIUM, LARGE)
     * @return void
     * */
    public void setCustomStyle(int styleId) {
        if(styleId == 1) {
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Small);
        }
        else if(styleId == 2) {
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Medium);
        }
        else if(styleId == 3) {
            Log.d("EDITTEXT", String.valueOf(mEditText));
            mEditText.setTextAppearance(getActivity(), android.R.style.TextAppearance_Large);
        }
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Inflate the menu; this adds items to the action bar if it is present.
        super.onCreateOptionsMenu(menu, inflater);

        inflater = getActivity().getMenuInflater();
        inflater.inflate(R.menu.bloc_notes, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_erase) {
            mEditText.setText("");
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

在您的 SomeActivity.onCreate() 中，您正在创建 NoteFragment 的实例，但即使您通过事务提交它，它也不会被膨胀和创建马上。这基本上意味着，NoteFragment.onCreateView() 尚未执行，并且 fragment UI 尚未创建。但是，您的 Activity.onCreate() 假定 mFragment.mEditText 已设置，但事实并非如此，因此您会遇到 NullPointerException。

看一下 FragmentTransation.commit 方法:http://developer.android.com/reference/android/support/v4/app/FragmentTransaction.html#commit()

Schedules a commit of this transaction. The commit does not happen immediately; it will 
be scheduled as work on the main thread to be done the next time that thread is ready.