问题
I am working with Android architecture components.
What i want is when user type "0" in Edittext and click on Button to replace Fragment with new one , and if type anything else post Toast error message. In Problem is when i back from new Fragment(BlankFragment) and click on button again and type "0" again and click, onchange() is called multiple times so Fragment is get created multiple times
FragmentExample.class:
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
manager = getActivity().getSupportFragmentManager();
viewmModel = ViewModelProviders.of(getActivity(), viewModelFactory)
.get(VModel.class);
View v = inflater.inflate(R.layout.fragment_list, container, false);
b = (Button) v.findViewById(R.id.b);
et = (EditText) v.findViewById(R.id.et);
viewmModel.observeData().observe(getActivity(), new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
if(s.equals("0")) {
BlankFragment fragment = (BlankFragment) manager.findFragmentByTag(DETAIL_FRAG);
if (fragment == null) {
fragment = BlankFragment.newInstance();
}
addFragmentToActivity(manager,
fragment,
R.id.root_activity_detail,
DETAIL_FRAG
);
} else {
Toast.makeText(getContext(), "Wrong text", Toast.LENGTH_SHORT).show();
}
}
});
b.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
viewmModel.setData(et.getText().toString());
}
});
return v;
}
private void addFragmentToActivity(FragmentManager fragmentManager, BlankFragment fragment, int root_activity_detail, String detailFrag) {
android.support.v4.app.FragmentTransaction transaction = fragmentManager.beginTransaction();
transaction.replace(root_activity_detail, fragment, detailFrag).addToBackStack(detailFrag);
transaction.commit();
}
Repository class:
public class Repository {
MutableLiveData<String> dataLive = new MutableLiveData<>();
public Repository() {
}
public void setListData(String data) {
dataLive.setValue(data);
}
public MutableLiveData<String> getData() {
return dataLive;
}
}
BlankFragment.class:
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
listItemViewModel = ViewModelProviders.of(this, viewModelFactory)
.get(VModel.class);
listItemViewModel.setData("");
return inflater.inflate(R.layout.fragment_blank, container, false);
}
回答1:
The problem here is that when you dettach the fragment from the acitivity, both fragment and its viewmodel are not destroyed. When you come back, you add a new observer to the livedata when the old observer is still there in the same fragment （If you add the observer in onCreateView()）.
There is an article (Even a SO thread in fact) talking about it (with solution).
The easy way to fix it (also in the article) is that remove any observer from the livedata before you add observer to it.
Update: In the support lib v28, a new LifeCycleOwner called ViewLifeCycleOwner should fix that more info in here
回答2:
Instead of using getActivity as LifecycleOwner, you should use fragment.
Change
viewmModel.observeData().observe(getActivity(), new Observer<String>() {
to
viewmModel.observeData().removeObservers(this);
viewmModel.observeData().observe(this, new Observer<String>() {
回答3:
here is what you are doing wrong...
viewmModel.observeData().observe(getActivity(), new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
if(s.equals("0")) {
BlankFragment fragment = (BlankFragment) manager.findFragmentByTag(DETAIL_FRAG);
if (fragment == null) {
fragment = BlankFragment.newInstance();
}
addFragmentToActivity(manager,
fragment,
R.id.root_activity_detail,
DETAIL_FRAG
);
} else {
Toast.makeText(getContext(), "Wrong text", Toast.LENGTH_SHORT).show();
}
}
});
in above code instead of "getActivity()" either you can use "this" or "viewLifecycleOwner".
Because as you are passing the getActivity() in observe method, whenever you open your fragment you are attaching the new instance of the observer with the Activity not with the fragment. So observer will keep alive even if you kill your fragment. So when livedata postvalue, it will send data to all the observers, as there are too many observers observing livedata, then all will get notified. Because of this, your observer gets called too many times. so you have to observe live data in fragment something like this.
viewmModel.observeData().observe(this, new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
if(s.equals("0")) {
BlankFragment fragment = (BlankFragment) manager.findFragmentByTag(DETAIL_FRAG);
if (fragment == null) {
fragment = BlankFragment.newInstance();
}
addFragmentToActivity(manager,
fragment,
R.id.root_activity_detail,
DETAIL_FRAG
);
} else {
Toast.makeText(getContext(), "Wrong text", Toast.LENGTH_SHORT).show();
}
}
});
But still your onchanged method will get called two times.
You can stop this by checking one condition inside your onchanged method..
dash_viewModel.getDashLiveData().observe(viewLifecycleOwner, object : Observer<AsyncResponse> {
override fun onChanged(t: AsyncResponse?) {
if(viewLifecycleOwner.lifecycle.currentState==Lifecycle.State.RESUMED){
setData(t)
}
}
})
from my research, I have found out that, if fragment using the ViewModel of its corresponding activity, So when even you start observing the livedata, it will first send you the most recently emitted item. even if you didn't call it from your fragment.
so onChange method got called two times
When the fragment is on start state - to receive the most recently emitted item
When the fragment is on Resumed state - to receive the call made by fragment either for api.
so on changed I always check the state of the fragment with the help of viewLifecycleOwner like this
if(viewLifecycleOwner.lifecycle.currentState==Lifecycle.State.RESUMED){
// if the fragment in resumed state then only start observing data
}
viewlifecycleowner is provided by both Fragments and Activity as Google implemented this solution directly in support library 28.0.0 and androidx with getViewLifecycleOwner() method. viewlifecycleowner contains info about the lifecycle of the component.
in java you can use getViewLifecycleOwner() intead of viewlifecycleowner .
回答4:
You shouldn't create your viewmModel in onCreateView but rather in onCreate so you don't add a listener to your data each time view is created.
回答5:
Here is an example how i solve this problem .[TESTED AND WORKING]
viewModel.getLoginResponse().observe(getViewLifecycleOwner(), new Observer<String>() {
@Override
public void onChanged(String response) {
if(getViewLifecycleOwner().getLifecycle().getCurrentState()== Lifecycle.State.RESUMED){
// your code here ...
}
}
});
回答6:
viewmModel = ViewModelProviders.of(getActivity(), viewModelFactory)
.get(VModel.class);
As your viewmModel's LifecycleOwner is activity, so the observer will only be automatically removed when the state of lifecycle is Lifecycle.State.DESTROYED.
In your situation, the observer will not be automatically removed.So you have to remove the previous observer manually or pass the same instance of observer every time.
回答7:
Just declare your Observer as a field variable so you don't create a new observer every time the lifecycle calls that part of your code. ;)
i.e. with kotlin:
YourFragment: Fragment() {
private val dataObserver = Observer<Data> { data ->
manageData(data)
}
...
//now you should subscribe your data after you instantiate your viewModel either in onCreate, onCreateView, onViewCreated, depends on your case..
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel.liveData.observe(this, dataObserver)
}
...
}
回答8:
Adding some useful information after @Samuel-Eminet,
it's true that onCreate(Bundle?) is called only once on the Fragment creation and when you press back, the view is recreated but not the fragment (hence why the ViewModel is the same. If you subscribe in any method of the lifecycle that affects the view, it will resubscribe again and again.
Observers would have been gone and you won't be able to tell even if you ask for liveData.hasObservers().
The best thing to do is subscribing when onCreate(Bundle?) but many of us are using the binding, and the view isn't created at this time, so this is the best way to do it:
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
lifecycleScope.launchWhenStarted {
subscribeUI()
}
}
now you're telling the Fragment's lifecycle to do something when it starts the Fragment and it will call it only once.
回答9:
Observe the livedata only once in a fragment. For that call the observe method in onCreate() rather than onCreateView(). When we press back button the onCreateView() method is called which makes the viewmodel to observe data again.
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mPatientViewModel.getGetCaseDetailLiveData().observe(this, jsonObjectResponse -> parseViewSentResponse(jsonObjectResponse));
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// TODO: inflate a fragment view
View rootView = super.onCreateView(inflater, container, savedInstanceState);
return rootView;
}
来源：https://stackoverflow.com/questions/48914311/viewmodel-onchange-gets-called-multiple-times-when-back-from-fragment