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Best practice for instantiating a new Android Fragment

Posted by: admin March 10, 2020 Leave a comment

Questions:

I have seen two general practices to instantiate a new Fragment in an application:

Fragment newFragment = new MyFragment();

and

Fragment newFragment = MyFragment.newInstance();

The second option makes use of a static method newInstance() and generally contains the following method.

public static Fragment newInstance() 
{
    MyFragment myFragment = new MyFragment();
    return myFragment;
}

At first, I thought the main benefit was the fact that I could overload the newInstance() method to give flexibility when creating new instances of a Fragment – but I could also do this by creating an overloaded constructor for the Fragment.

Did I miss something?

What are the benefits of one approach over the other? Or is it just good practice?

How to&Answers:

If Android decides to recreate your Fragment later, it’s going to call the no-argument constructor of your fragment. So overloading the constructor is not a solution.

With that being said, the way to pass stuff to your Fragment so that they are available after a Fragment is recreated by Android is to pass a bundle to the setArguments method.

So, for example, if we wanted to pass an integer to the fragment we would use something like:

public static MyFragment newInstance(int someInt) {
    MyFragment myFragment = new MyFragment();

    Bundle args = new Bundle();
    args.putInt("someInt", someInt);
    myFragment.setArguments(args);

    return myFragment;
}

And later in the Fragment onCreate() you can access that integer by using:

getArguments().getInt("someInt", 0);

This Bundle will be available even if the Fragment is somehow recreated by Android.

Also note: setArguments can only be called before the Fragment is attached to the Activity.

This approach is also documented in the android developer reference: https://developer.android.com/reference/android/app/Fragment.html

Answer:

The only benefit in using the newInstance() that I see are the following:

  1. You will have a single place where all the arguments used by the fragment could be bundled up and you don’t have to write the code below everytime you instantiate a fragment.

    Bundle args = new Bundle();
    args.putInt("someInt", someInt);
    args.putString("someString", someString);
    // Put any other arguments
    myFragment.setArguments(args);
    
  2. Its a good way to tell other classes what arguments it expects to work faithfully(though you should be able to handle cases if no arguments are bundled in the fragment instance).

So, my take is that using a static newInstance() to instantiate a fragment is a good practice.

Answer:

There is also another way:

Fragment.instantiate(context, MyFragment.class.getName(), myBundle)

Answer:

While @yydl gives a compelling reason on why the newInstance method is better:

If Android decides to recreate your Fragment later, it’s going to call
the no-argument constructor of your fragment. So overloading the
constructor is not a solution.

it’s still quite possible to use a constructor. To see why this is, first we need to see why the above workaround is used by Android.

Before a fragment can be used, an instance is needed. Android calls YourFragment() (the no arguments constructor) to construct an instance of the fragment. Here any overloaded constructor that you write will be ignored, as Android can’t know which one to use.

In the lifetime of an Activity the fragment gets created as above and destroyed multiple times by Android. This means that if you put data in the fragment object itself, it will be lost once the fragment is destroyed.

To workaround, android asks that you store data using a Bundle (calling setArguments()), which can then be accessed from YourFragment. Argument bundles are protected by Android, and hence are guaranteed to be persistent.

One way to set this bundle is by using a static newInstance method:

public static YourFragment newInstance (int data) {
    YourFragment yf = new YourFragment()
    /* See this code gets executed immediately on your object construction */
    Bundle args = new Bundle();
    args.putInt("data", data);
    yf.setArguments(args);
    return yf;
}

However, a constructor:

public YourFragment(int data) {
    Bundle args = new Bundle();
    args.putInt("data", data);
    setArguments(args);
}

can do exactly the same thing as the newInstance method.

Naturally, this would fail, and is one of the reasons Android wants you to use the newInstance method:

public YourFragment(int data) {
    this.data = data; // Don't do this
}

As further explaination, here’s Android’s Fragment Class:

/**
 * Supply the construction arguments for this fragment.  This can only
 * be called before the fragment has been attached to its activity; that
 * is, you should call it immediately after constructing the fragment.  The
 * arguments supplied here will be retained across fragment destroy and
 * creation.
 */
public void setArguments(Bundle args) {
    if (mIndex >= 0) {
        throw new IllegalStateException("Fragment already active");
    }
    mArguments = args;
}

Note that Android asks that the arguments be set only at construction, and guarantees that these will be retained.

EDIT: As pointed out in the comments by @JHH, if you are providing a custom constructor that requires some arguments, then Java won’t provide your fragment with a no arg default constructor. So this would require you to define a no arg constructor, which is code that you could avoid with the newInstance factory method.

EDIT: Android doesn’t allow using an overloaded constructor for fragments anymore. You must use the newInstance method.

Answer:

I disagree with yydi answer saying:

If Android decides to recreate your Fragment later, it’s going to call
the no-argument constructor of your fragment. So overloading the
constructor is not a solution.

I think it is a solution and a good one, this is exactly the reason it been developed by Java core language.

Its true that Android system can destroy and recreate your Fragment. So you can do this:

public MyFragment() {
//  An empty constructor for Android System to use, otherwise exception may occur.
}

public MyFragment(int someInt) {
    Bundle args = new Bundle();
    args.putInt("someInt", someInt);
    setArguments(args);
}

It will allow you to pull someInt from getArguments() latter on, even if the Fragment been recreated by the system. This is more elegant solution than static constructor.

For my opinion static constructors are useless and should not be used. Also they will limit you if in the future you would like to extend this Fragment and add more functionality to the constructor. With static constructor you can’t do this.

Update:

Android added inspection that flag all non-default constructors with an error.
I recommend to disable it, for the reasons mentioned above.

Answer:

Some kotlin code:

companion object {
    fun newInstance(first: String, second: String) : SampleFragment {
        return SampleFragment().apply {
            arguments = Bundle().apply {
                putString("firstString", first)
                putString("secondString", second)
            }
        }
    }
}

And you can get arguments with this:

val first: String by lazy { arguments?.getString("firstString") ?: "default"}
val second: String by lazy { arguments?.getString("secondString") ?: "default"}

Answer:

Best practice to instance fragments with arguments in android is to have static factory method in your fragment.

public static MyFragment newInstance(String name, int age) {
    Bundle bundle = new Bundle();
    bundle.putString("name", name);
    bundle.putInt("age", age);

    MyFragment fragment = new MyFragment();
    fragment.setArguments(bundle);

    return fragment;
}

You should avoid setting your fields with the instance of a fragment. Because whenever android system recreate your fragment, if it feels that the system needs more memory, than it will recreate your fragment by using constructor with no arguments.

You can find more info about best practice to instantiate fragments with arguments here.

Answer:

Since the questions about best practice, I would add, that very often good idea to use hybrid approach for creating fragment when working with some REST web services

We can’t pass complex objects, for example some User model, for case of displaying user fragment

But what we can do, is to check in onCreate that user!=null and if not – then bring him from data layer, otherwise – use existing.

This way we gain both ability to recreate by userId in case of fragment recreation by Android and snappiness for user actions, as well as ability to create fragments by holding to object itself or only it’s id

Something likes this:

public class UserFragment extends Fragment {
    public final static String USER_ID="user_id";
    private User user;
    private long userId;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        userId = getArguments().getLong(USER_ID);
        if(user==null){
            //
            // Recreating here user from user id(i.e requesting from your data model,
            // which could be services, direct request to rest, or data layer sitting
            // on application model
            //
             user = bringUser();
        }
    }

    public static UserFragment newInstance(User user, long user_id){
        UserFragment userFragment = new UserFragment();
        Bundle args = new Bundle();
        args.putLong(USER_ID,user_id);
        if(user!=null){
            userFragment.user=user;
        }
        userFragment.setArguments(args);
        return userFragment;

    }

    public static UserFragment newInstance(long user_id){
        return newInstance(null,user_id);
    }

    public static UserFragment newInstance(User user){
        return newInstance(user,user.id);
    }
}

Answer:

use this code 100% fix your problem

enter this code in firstFragment

public static yourNameParentFragment newInstance() {

    Bundle args = new Bundle();
    args.putBoolean("yourKey",yourValue);
    YourFragment fragment = new YourFragment();
    fragment.setArguments(args);
    return fragment;
}

this sample send boolean data

and in SecendFragment

yourNameParentFragment name =yourNameParentFragment.newInstance();
   Bundle bundle;
   bundle=sellDiamondFragments2.getArguments();
  boolean a= bundle.getBoolean("yourKey");

must value in first fragment is static

happy code

Answer:

Best way to instantiate the fragment is use default Fragment.instantiate method or create factory method to instantiate the the fragment

Caution: always create one empty constructor in fragment other while restoring fragment memory will throw run-time exception.

Answer:

I’m lately here. But somethings I just known that might help you a bit.

If you are using Java, there is nothing much to change. But for kotlin developers, here is some following snippet I think that can make you a basement to run on:

  • Parent fragment:
inline fun <reified T : SampleFragment> newInstance(text: String): T {
    return T::class.java.newInstance().apply {
        arguments = Bundle().also { it.putString("key_text_arg", text) }
    }
}
  • Normal call
val f: SampleFragment = SampleFragment.newInstance("ABC")
// or val f = SampleFragment.newInstance<SampleFragment>("ABC")
  • You can extend the parent init operation in child fragment class by:
fun newInstance(): ChildSampleFragment {
    val child = UserProfileFragment.newInstance<ChildSampleFragment>("XYZ")
    // Do anything with the current initialized args bundle here
    // with child.arguments = ....
    return child
}

Happy coding.

Answer:

setArguments() is useless. It only brings a mess.

public class MyFragment extends Fragment {

    public String mTitle;
    public String mInitialTitle;

    public static MyFragment newInstance(String param1) {
        MyFragment f = new MyFragment();
        f.mInitialTitle = param1;
        f.mTitle = param1;
        return f;
    }

    @Override
    public void onSaveInstanceState(Bundle state) {
        state.putString("mInitialTitle", mInitialTitle);
        state.putString("mTitle", mTitle);
        super.onSaveInstanceState(state);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle state) {
        if (state != null) {
            mInitialTitle = state.getString("mInitialTitle");
            mTitle = state.getString("mTitle");
        } 
        ...
    }
}

Answer:

I believe I have a much simpeler solution for this.

public class MyFragment extends Fragment{

   private String mTitle;
   private List<MyObject> mObjects;

   public static MyFragment newInstance(String title, List<MyObject> objects)
   MyFragment myFrag = new MyFragment();
   myFrag.mTitle = title;
   myFrag.mObjects = objects;
   return myFrag;
   }