LiveData update on object field change

问题

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 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.

回答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

回答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 = viewModel.user //<- re-binding
})

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}"
        />

回答3:

For your observer get notified you should use setValue if you do this user.getValue().setFirstName(user.getValue().getFirstName() + " A "); your observer will not be notified !

View Model

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

Activity / Fragment

mModel = ViewModelProviders.of(this).get(InfoViewModel.class);
mModel.getUser().observe(this, s -> {
    // User has been modified
});

Somewhere in your activity / fragment

This will trigger the observer :

mModel.getUser().setValue(user);

If you want to update only one field from an object instead of update the whole object you should have multiples MutableLiveData<String>

// View Model
private MutableLiveData<String>         firstName;
private MutableLiveData<String>         lastName;

//Somewhere in your code
mModel.getFirstName().setValue(user.getValue().getFirstName() + " A ");
mModel.getFirstName().observe(this, s -> {
    // Firstname has been modified
});

来源：https://stackoverflow.com/questions/48020377/livedata-update-on-object-field-change