Why do I want to avoid non-default constructors in fragments?
AndroidAndroid FragmentsAndroid Problem Overview
I am creating an app with Fragments
and in one of them, I created a non-default constructor and got this warning:
Avoid non-default constructors in fragments: use a default constructor plus Fragment#setArguments(Bundle) instead
Can someone tell me why this is not a good idea?
Can you also suggest how I would accomplish this:
public static class MenuFragment extends ListFragment {
public ListView listView1;
Categories category;
//this is my "non-default" constructor
public MenuFragment(Categories category){
this.category = category;
}....
Without using the non-default constructor?
Android Solutions
Solution 1 - Android
It seems like none of the answers actually answer "why use bundle for passing parameters rather than non default constructors"
The reason why you should be passing parameters through bundle is because when the system restores a fragment
(e.g on config change), it will automatically restore your bundle
.
The callbacks like onCreate
or onCreateView
should read the parameters from the bundle
- this way you are guaranteed to restore the state of the fragment
correctly to the same state the fragment
was initialised with (note this state can be different from the onSaveInstanceState bundle
that is passed to the onCreate/onCreateView
)
The recommendation of using the static newInstance()
method is just a recommendation. You can use a non default constructor but make sure you populate the initialisation parameters in the bundle
inside the body of that constructor. And read those parameters in the onCreate()
or onCreateView()
methods.
Solution 2 - Android
Make a bundle object and insert your data (in this example your Category
object). Be careful, you can't pass this object directly into the bundle, unless it's serializable.
I think it's better to build your object in the fragment, and put only an id or something else into bundle. This is the code to create and attach a bundle:
Bundle args = new Bundle();
args.putLong("key", value);
yourFragment.setArguments(args);
After that, in your fragment access data:
Type value = getArguments().getType("key");
That's all.
Solution 3 - Android
Your Fragment
shouldn't have constructors because of how the FragmentManager
instantiates it.
You should have a newInstance()
static method defined with the parameters you need, then bundle them and set them as the arguments of the fragment, which you can later access with the Bundle
parameter.
For example:
public static MyFragment newInstance(int title, String message) {
MyFragment fragment = new MyFragment();
Bundle bundle = new Bundle(2);
bundle.putInt(EXTRA_TITLE, title);
bundle.putString(EXTRA_MESSAGE, message);
fragment.setArguments(bundle);
return fragment ;
}
And read these arguments at onCreate
:
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
title = getArguments().getInt(EXTRA_TITLE);
message = getArguments().getString(EXTRA_MESSAGE);
//...
}
This way, if detached and re-attached, the object state can be stored through the arguments, much like bundles
attached to Intent
s.
Solution 4 - Android
If you use parameter for some class. try this
SomeClass mSomeInstance;
public static final MyFragment newInstance(SomeClass someInstance){
MyFragment f = new MyFragment();
f.mSomeInstance = someInstance;
return f;
}
Solution 5 - Android
I think, there is no difference between static constructor and two constructors (empty and parametrized one that stores arguments into a Fragment's arguments bundle), most probably, this rule of thumb is created to reduce probability of forgetting to implement no-arg constructor in Java, which is not implicitly generated when overload present.
In my projects I use Kotlin, and implement fragments with a primary no-arg constructor and secondary constructor for arguments which just stores them into a bundle and sets it as Fragment arguments, everything works fine.
Solution 6 - Android
If fragment uses non-default constructors after configuration changing the fragment will lose all data.