Android Java:片段在onCreateView()中返回空视图 [英] Android Java: Fragments returning null view in onCreateView()
问题描述
我目前正在研究使用MVC设计模式和android java中的片段的程序.我已经弄清楚了我的一个片段并使它工作，但是当我复制其他片段以遵循相同的代码结构(具有特殊功能)时，在onCreateView方法中出现了空指针异常.
I am currently working on a program that uses an MVC design pattern and fragments in android java. I have figured out one of my fragments and gotten it working but when I copied the other fragments to follow the same code structure (with specialized functionality), I get a null pointer exception in the onCreateView method.
我现在在我的垃圾笔记本电脑上，它似乎无法处理android仿真，因此我明天可以发布确切的错误代码.虽然我有我的源代码，但我已经被撞墙了很长时间，以至于知道它在哪里破裂.
I'm on my junk laptop right now and it can't seem to handle android emulation so I can post the exact error code tomorrow. I have my source code though and I have been hitting my head against the wall long enough to know where it is breaking.
我看到了我的问题.我正在通过从每个片段的View.java类中调用一个方法来测试我的代码.此方法更新视图中的表.由于视图尚未显示在屏幕上，因此尚未为其调用onCreateView().由于尚未调用onCreateView()，因此尝试访问视图将导致空指针.有什么好方法可以为MainActivity中的每个片段调用onCreateView()，以便我可以尽早初始化视图?
I see my problem. I'm testing the my code by calling a method from within the View.java class from each fragment. This method updates a table in the view. Since the views haven't been displayed on screen yet, onCreateView() hasn't been called for them. Since onCreateView() hasn't been called, trying to access the view results in a null pointer. Is there any good way to call onCreateView() for each fragment from my MainActivity just so I can initialize the views early?
(工作片段的一部分):
(Part of the working fragment):
public class DispatchView extends Fragment {
private final List<DispatchModel> models = new ArrayList<DispatchModel>();
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.dispatchfragment, container,
false);
return view;
}
除DispatchView之外的所有片段都会在返回视图时中断.他们返回的是null而不是实际的对象. 破碎片段之一的一部分:
All fragments, except for DispatchView, break upon returning view. They are returning null rather than an actual object. Part of one of the broken fragments:
public class ConnectionsLogView extends Fragment {
private final List<ConnectionsLogModel> models = new ArrayList<ConnectionsLogModel>();
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.connectionslogfragment,
container, false);
return view;
}
对片段进行声明和初始化.我尝试将新的数据条目(类)推入它们后，它们(除Dispatch MVC之外的任何一个)都断开了. 在我的MainActivity.java中:
The fragments are declared and initialized. They (any of them except the Dispatch MVC) break after I try to push a new data entry (class) into them. In my MainActivity.java:
public class MainActivity extends Activity {
// Declare Tab Variables and fragment objects
private mDMI app;
ActionBar.Tab Tab1, Tab2, Tab3, Tab4;
Fragment dispatchTab = new DispatchView();
Fragment dispatchLogTab = new DispatchLogView();
Fragment activeConnectionsTab = new ConnectionsView();
Fragment connectionLogTab = new ConnectionsLogView();
DispatchModel dispatchModel;
DispatchController dispatchController;
DispatchLogModel dispatchLogModel;
DispatchLogController dispatchLogController;
ConnectionsModel connectionsModel;
ConnectionsController connectionsController;
ConnectionsLogModel connectionsLogModel;
ConnectionsLogController connectionsLogController;
public MainActivity() {
super();
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
app = (mDMI) getApplication();
dispatchModel = app.getDispatchModel();
dispatchController = new DispatchController(dispatchTab, dispatchModel);
dispatchLogModel = app.getDispatchLogModel();
dispatchLogController = new DispatchLogController(dispatchLogTab,
dispatchLogModel);
connectionsModel = app.getConnectionsModel();
connectionsController = new ConnectionsController(activeConnectionsTab,
connectionsModel);
connectionsLogModel = app.getConnLogModel();
connectionsLogController = new ConnectionsLogController(
connectionLogTab, connectionsLogModel);
setContentView(R.layout.activity_main);
已识别xml字符串 在我的R.java中:
The xml strings are identified In my R.java:
public static final class layout {
public static final int activity_login=0x7f030000;
public static final int activity_main=0x7f030001;
public static final int connectionsfragment=0x7f030002;
public static final int connectionslogfragment=0x7f030003;
public static final int dispatchfragment=0x7f030004;
public static final int dispatchlogfragment=0x7f030005;
}
推荐答案
请勿以这种方式创建片段.而是使用标准的Android模式:
Don't create your fragments that way. Instead use a standard Android pattern:
public class FeedFragment extends Fragment {
public FeedFragment() {
super();
}
public static FeedFragment newInstance(Context context) {
if (context != null) {
sContext = context.getApplicationContext();
} else {
sContext = YourApp.getInstance().getApplicationContext();
}
return new FeedFragment();
}
}
然后不要在您的活动中像那样创建它们……
Then don't create them like that in your activity…
请在您的活动"中使用标准的Android模式:
instead use a standard Android pattern in your Activity:
@Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentManager fm = getSupportFragmentManager();
Fragment fragment = fm.findFragmentById(R.id.content_frame);
if ( fragment == null ) {
fragment = FeedFragment.newInstance(this);
fm.beginTransaction()
.add(R.id.content_frame, dispatchTab, "DISPATCH_FRAG")
.commit();
}
}
要稍后再获取片段，您可以...
To retrieve the fragment later you can do…
final FragmentManager fm = getSupportFragmentManager();
Fragment fragment = fm.findFragmentByTag("DISPATCH_FRAG);
if (fragment != null) {
// cast and use your fragment
}
如果以后需要，甚至可以存储参考.
You can even store a reference if you need it later.
关于您的null问题，如果没有确切的崩溃/日志，很难说出来.
Regarding your null problem, it's really hard to tell without the exact crash/log.
但是您的代码有点混乱，这可能是原因.片段生命周期非常棘手.
But your code is a little bit messy and that may be the cause. Fragment LifeCycle is tricky.
这篇关于Android Java:片段在onCreateView()中返回空视图的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
