LiveData update on object field change

Question

I'm using Android MVVM architecture with LiveData. I have an object like this

public class User {
    private String firstName;
    private String lastName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}

And my view model looks like this

public class InfoViewModel extends AndroidViewModel {
    MutableLiveData<User> user = new MutableLiveData<>();

    public InfoViewModel(@NonNull Application application) {
        super(application);
        User user = new User();
        user.setFirstName("Alireza");
        user.setLastName("Ahmadi");

        this.user.setValue(user);
    }

    public LiveData<User> getUser(){
        return user;
    }

    public void change(){
        user.getValue().setFirstName(user.getValue().getFirstName() + " A ");
    }
}

How can I make sure when some field in user object changes observers get notified? BTW it is important to me to keep this data in the separate object and not use primary values like Strings in my ViewModel.

Solution 1

I don't think there is any best practice as such recommended by android for this. I would suggest you to use the approach which uses cleaner & less boilerplate code.

If you are using android data binding along with LiveData you can go with the following approach:

Your POJO object would look something like this

public class User extends BaseObservable {
    private String firstName;
    private String lastName;

    @Bindable
    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
        notifyPropertyChanged(BR.firstName);
    }

    @Bindable
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
        notifyPropertyChanged(BR.lastName);
    }
}

So you would be already having a class which notifies whenever its property changes. So you can just make use of this property change callback in your MutableLiveData to notify its observer. You can create a custom MutableLiveData for this

public class CustomMutableLiveData<T extends BaseObservable>
        extends MutableLiveData<T> {


    @Override
    public void setValue(T value) {
        super.setValue(value);

        //listen to property changes
        value.addOnPropertyChangedCallback(callback);
    }

    Observable.OnPropertyChangedCallback callback = new Observable.OnPropertyChangedCallback() {
        @Override
        public void onPropertyChanged(Observable sender, int propertyId) {

            //Trigger LiveData observer on change of any property in object
            setValue(getValue());

        }
    };


}

Then all you need to do is use this CustomMutableLiveData instead of MutableLiveData in your View Model

public class InfoViewModel extends AndroidViewModel {

    CustomMutableLiveData<User> user = new CustomMutableLiveData<>();
-----
-----

So by doing this you can notify both view & LiveData observer with little change to existing code. Hope it helps

Solution 2

When using MVVM and LiveData, you can re-bind the object to the layout so it will trigger all changes on the UI.

Given "user" is a MutableLiveData<User> in the ViewModel

ViewModel

class SampleViewModel : ViewModel() {
    val user = MutableLiveData<User>()

    fun onChange() {
        user.value.firstname = "New name"
        user.value = user.value // force postValue to notify Observers
        // can also use user.postValue()
    }
}

Activity/Fragment file:

viewModel = ViewModelProviders
            .of(this)
            .get(SampleViewModel::class.java)

// when viewModel.user changes, this observer get notified and re-bind
// the user model with the layout.
viewModel.user.observe(this, Observer {
    binding.user = it //<- re-binding user
})

Your layout file shouldn't change:

<data>
    <variable
        name="user"
        type="com.project.model.User" />
</data>

...

<TextView
        android:id="@+id/firstname"
        android:text="@{user.firstname}"
        />

Solution 3

If you are using Kotlin and LiveData, I can offer you 2 ways - with and without extension fucntion:

Without extension function

liveData.value = liveData.value?.also { it ->
    // Modify your object here. Data will be auto-updated
    it.name = "Ed Khalturin"
    it.happyNumber = 42
}

Same, but with extension

// Extension. CopyPaste it anywhere in your project
fun <T> MutableLiveData<T>.mutation(actions: (MutableLiveData<T>) -> Unit) {
    actions(this)
    this.value = this.value
}

// Usage
liveData.mutation {
    it.value?.name = "Ed Khalturin"
    it.value?.innerClass?.city= "Moscow" // it works with inner class too
}

Solution 4

From reddit - @cedrickc's answer :

add an extension function to MutableLiveData:

fun <T> MutableLiveData<T>.modifyValue(transform: T.() -> T) {
   this.value = this.value?.run(transform)
}

Solution 5

How can I make sure when some filed in user object changes observers get notified? BTW it is important to me to keep this data in the separate object and not use primary values like Strings in my ViewModel.

You can use androidx.lifecyle.Transformation class to monitor for individual fields.

val user = MutableLiveData<User>();
//to monitor for User.Name
val firstName: LiveData<String>  = Transformations.map(user) {it.firstName}
val lastName: LiveData<String>  = Transformations.map(user) {it.lastName}

you update user as per normal, and listen for firstname/lastname to monitor for changes in those fields.

View more solutions

Comments

[Sup.Ia]

I do not know that why . in my class could not find BR.

[Abhishek V]

@Sup.Ia Re-build the project to generate BR file

[MiguelHincapieC]

is there a way to do this without data binding? I mean, an approach like this one in where you can choose when the liveData's value has changed.

[AouledIssa]

This is working! but does re-binding has any performance drawbacks ?

[Marat]

I was reading this article medium.com/@tylerwalker/… that explains same solution but in Kotlin. However I couldn't get it to work. I think it is outdated because it is using lifecycle version = 1.1.1. How to set the value inside of onPropertyChanged() in Kotlin? value = value does not compile.

[Kwnstantinos Nikoloutsos]

BR stands for? Anyone knows that?

[PhilBlais]

The BR is also giving me issues

[muetzenflo]

BR stands for BindingResources. It is created when compiling your project. For every <variable>-tag in your data-bound layout xml there will be a reference in BR. Similar to normal resources that are also defined in xml (e.g. R.string.something)

[A.K.]

Hi, it works without the CustomMutableLiveData class. Why do i need the class?

[ejdrian313]

It's simpler and more clear approch than marked as best.

[Crearo Rotar]

This just seems wrong to me. Isn't the whole point of using LiveData to not rebind?

[morpheus05]

Who unregisters the listener if we change to a completely different value object? And don't we register a new listener object every time we change one of the User object fields?

[Sean]

Instead of binding user, you can bind viewModel, then you won't need to re-bind.

[Aleksandar Stefanović]

Great idea. I would enhance it further by using actions: MutableLivedata<T>.() -> Unit, to that the lambda could be written without it

[Alireza]

binding viewModel does not work . for me re-binding the object works very well.

[fullmoon]

This is not rebinding, it just refreshes the variable, which will reflect the change on the UI. Don't see a problem here.

[Afilu]

Elegant. The point I missed was that setting the exact same value object triggers the notification to observers. Hopefully it won't be 'optimized' in future releases.

[Carteră Veaceslav]

how it suppose to work? This will be fired only once (during the initialization), not every time the value changes

[Nick M]

how does it behave on additions to an .innerArray ?

[act262]

fun <T> MutableLiveData<T>.setField(transform: T.() -> Unit) { this.value = this.value?.apply(transform) }

[Bilawal muzaffar]

i think @morpheus05 is right.

[Alex Bin Zhao]

It is probably better to create new user instead of using existing one

[Lifes]

This no longer works.