当我尝试从中获取数据时，ViewModel 返回清除对象

Question

I'm trying to use ViewModel , for saving data to share between Fragments . That is it:

class AnimalViewModel : ViewModel() {
    private var data = MutableLiveData<AnimalWithHints>()

    fun setData(animalData: AnimalWithHints) {
        data.value = animalData
        Timber.e("Saved?")
    }

    fun getData(): LiveData<AnimalWithHints> {
        if (data.value == null) {
            Timber.e("Null?")
            data.value = AnimalWithHints()
        }
        return data
    }
}

So I've filled non-null object (checked it) and I write before transaction:

ViewModelProviders.of(this).get(AnimalViewModel::class.java).setData(animalWithHints)

I see "Saved?" when it happens, so I think save working good. I logged some field of AnimalWithHints , and I saw it was correct.

Then I try to get my object:

val animalWithHints = ViewModelProviders.of(this).get(AnimalViewModel::class.java).getData().value

And I see "Null?" . And it's true, data.value is rly new empty AnimalWithHints object.

Strange thing is I have only two lines of code, using ViewModel . I only save and get object, there is no place when I can clear it or modify somehow.

I need you help to understand what's wrong.

Answer 1

When you want to share data between your Fragments that are in same Activity , you'll need to create instance of ViewModel with context to Activity .

Below syntax provides you instance of ViewModel that is at Fragment level only (You can share it at Fragment itself with some child fragments inside of that Frament) :

ViewModelProviders.of(this@Fragment)[SomeViewModel::class.java] // This will give you view model at fragment level

---

While passing Activity object to your ViewModelProvider inside your Fragment gives you ViewModel at Activity level. Hence You're having multiple Fragments inside same Activity meaning context of Activity is same gives your same ViewModel object at both Fragments .

So for sharing ViewModel between fragments, use below syntax :

ViewModelProviders.of(activity)[SomeViewModel::class.java] // This will provide you shared view model across multiple fragments.