为什么我不能在片段的onCreateView中调用方法？(Why can't I call methods in fragment's onCreateView?)

这是我第一次处理片段，所以我不知道如何操作它们。我有一个片段，我有onCreateView方法。在onCreateView方法中，我有从SQLite数据库填充自定义列表视图的代码：

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_library, container, false); Log.i("TAG", "onCreateView"); // Variables lView = (ListView) v.findViewById(R.id.listView); components = new ArrayList<RecordDetails>(); db = new DatabaseHandler(getActivity()); Cursor c = db.getAllRecordedFiles(); if(c.moveToFirst()){ element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); while (c.moveToNext()) { element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); } lView.setAdapter(new CustomListViewAdapter(components, getActivity())); } return v; }

这很好用。现在假设我想将用于填充listview的代码放入方法中，然后在onCreateView中调用该方法，如下所示：

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_library, container, false); Log.i("TAG", "onCreateView"); // Variables lView = (ListView) v.findViewById(R.id.listView); components = new ArrayList<RecordDetails>(); db = new DatabaseHandler(getActivity()); Cursor c = db.getAllRecordedFiles(); populateListView(); return v; } public void populateListView(){ if(c.moveToFirst()){ element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); while (c.moveToNext()) { element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); } lView.setAdapter(new CustomListViewAdapter(components, getActivity())); } }

当我这样做时，应用程序崩溃与以下LogCat输出：

02-20 20:57:17.505: E/AndroidRuntime(4675): FATAL EXCEPTION: main 02-20 20:57:17.505: E/AndroidRuntime(4675): java.lang.NullPointerException 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.matejhacin.beautifulvoicerecorder.LibraryFragment.populateListView(LibraryFragment.java:70) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.matejhacin.beautifulvoicerecorder.LibraryFragment.onCreateView(LibraryFragment.java:64) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:927) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1467) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:472) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.populate(ViewPager.java:1068) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.populate(ViewPager.java:914) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2451) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1905) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1104) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1284) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer.doCallbacks(Choreographer.java:562) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer.doFrame(Choreographer.java:532) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Handler.handleCallback(Handler.java:730) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Handler.dispatchMessage(Handler.java:92) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Looper.loop(Looper.java:137) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.app.ActivityThread.main(ActivityThread.java:5289) 02-20 20:57:17.505: E/AndroidRuntime(4675): at java.lang.reflect.Method.invokeNative(Native Method) 02-20 20:57:17.505: E/AndroidRuntime(4675): at java.lang.reflect.Method.invoke(Method.java:525) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555) 02-20 20:57:17.505: E/AndroidRuntime(4675): at dalvik.system.NativeStart.main(Native Method)

所以我猜测片段在调用方法时表现不同。我该怎么办？

This is my first time dealing with fragments so I don't know how to operate with them that well yet. I have a fragment in which I have onCreateView method. In that onCreateView method I have code that populates custom listview from SQLite database:

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_library, container, false); Log.i("TAG", "onCreateView"); // Variables lView = (ListView) v.findViewById(R.id.listView); components = new ArrayList<RecordDetails>(); db = new DatabaseHandler(getActivity()); Cursor c = db.getAllRecordedFiles(); if(c.moveToFirst()){ element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); while (c.moveToNext()) { element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); } lView.setAdapter(new CustomListViewAdapter(components, getActivity())); } return v; }

This works fine. Now let's say I want to put the code for populating listview into method and then call that method in onCreateView like this:

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_library, container, false); Log.i("TAG", "onCreateView"); // Variables lView = (ListView) v.findViewById(R.id.listView); components = new ArrayList<RecordDetails>(); db = new DatabaseHandler(getActivity()); Cursor c = db.getAllRecordedFiles(); populateListView(); return v; } public void populateListView(){ if(c.moveToFirst()){ element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); while (c.moveToNext()) { element = new RecordDetails(); element.setTitle(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_NAME))); element.setDate(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DATE))); element.setDuration(c.getString(c.getColumnIndex(SQLiteHelper.MUSIC_DURATION))); components.add(element); } lView.setAdapter(new CustomListViewAdapter(components, getActivity())); } }

And when I do this, the app crashes with the following LogCat output:

02-20 20:57:17.505: E/AndroidRuntime(4675): FATAL EXCEPTION: main 02-20 20:57:17.505: E/AndroidRuntime(4675): java.lang.NullPointerException 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.matejhacin.beautifulvoicerecorder.LibraryFragment.populateListView(LibraryFragment.java:70) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.matejhacin.beautifulvoicerecorder.LibraryFragment.onCreateView(LibraryFragment.java:64) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:927) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1467) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:472) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.populate(ViewPager.java:1068) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.populate(ViewPager.java:914) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2451) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.View.measure(View.java:15855) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1905) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1104) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1284) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer.doCallbacks(Choreographer.java:562) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer.doFrame(Choreographer.java:532) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Handler.handleCallback(Handler.java:730) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Handler.dispatchMessage(Handler.java:92) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.os.Looper.loop(Looper.java:137) 02-20 20:57:17.505: E/AndroidRuntime(4675): at android.app.ActivityThread.main(ActivityThread.java:5289) 02-20 20:57:17.505: E/AndroidRuntime(4675): at java.lang.reflect.Method.invokeNative(Native Method) 02-20 20:57:17.505: E/AndroidRuntime(4675): at java.lang.reflect.Method.invoke(Method.java:525) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) 02-20 20:57:17.505: E/AndroidRuntime(4675): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555) 02-20 20:57:17.505: E/AndroidRuntime(4675): at dalvik.system.NativeStart.main(Native Method)

So I'm guessing that Fragments act differently when calling methods. What should I do?

最满意答案

您在onCreateView中创建的Cursor c类成员和本地成员是不同的对象，并重新声明您正在“隐藏”类成员的局部变量。

你可以改变

Cursor c = db.getAllRecordedFiles();

同

c = db.getAllRecordedFiles();

或者你可以添加一个参数给populateListView传递Cursor对象

public void populateListView(Cursor c){ }

并从onCreateView中调用它

populateListView(db.getAllRecordedFiles());

the Cursor c class member and the local one you create inside onCreateView are different objects, and redeclaring a local variable you are "hiding" the class member.

you can change

Cursor c = db.getAllRecordedFiles();

with

c = db.getAllRecordedFiles();

or you can add a parameter to populateListView passing the Cursor object

public void populateListView(Cursor c){ }

and call it from onCreateView like

populateListView(db.getAllRecordedFiles());

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