即使在postValue（）Kotlin之后，MutableLiveData ArrayList也为空

Question

I am now stuck and currently wondering why my mutable arraylist returns null even if it is being updated with postvalue(). I tried to display it using Toast and it displayed [] which I think is null. It had no space in between so it looked like a box. I did toString() it as well in order to show the text. How would I be able to solve this problem?

Here is my Main Activity:

val list = ArrayList<String>()
list.add("text1")
list.add("text2")
val viewmodel = ViewModelProviders.of(this).get(viewmodel::class.java)
viewmodel.Testlist.postValue(list)

ViewModel:

class viewmodel: ViewModel() {
    val Testlist: MutableLiveData<ArrayList<String>> = MutableLiveData()
    init {
        Testlist.value = arrayListOf()
    }
}

Fragment:

Top area:

activity?.let {
    val viewmodel = ViewModelProviders.of(this).get(viewmodel::class.java)
    observeInput(viewmodel)
}

Bottom area:

private fun observeInput(viewmodel: viewmodel) {
    viewmodel.Testlist.observe(viewLifecycleOwner, Observer {
        it?.let {
            Toast.makeText(context, it.toString(), Toast.LENGTH_LONG).show()
        }
    })
}

Answer 1

have you used the same instance of your view model? or have you defined another view model in the fragment class? The issue could be that you're accessing a different instance of the view model and not the one were the MutableLiveData was updated

Answer 2

You can use this :

    fun <T : Any?> MutableLiveData<ArrayList<T>>.default(initialValue: ArrayList<T>) = apply { setValue(initialValue) }

and then use this function as below:

    viewmodel.Testlist.default(ArrayList())

For me, I have a BaseActivity that other activities extend from it :

class UAppCompatActivity : AppCompatActivity() {

    protected fun <T : Any?> MutableLiveData<ArrayList<T>>.default(initialValue: ArrayList<T>) = apply { setValue(initialValue) }

    protected fun <T> MutableLiveData<ArrayList<T>>.addItem(item: T) {
        val updatedItems = this.value as ArrayList
        updatedItems.add(item)
        this.value = updatedItems
    }

    protected fun <T> MutableLiveData<ArrayList<T>>.deleteItem(item: T) {
        val updatedItems = this.value as ArrayList
        updatedItems.remove(item)
        this.value = updatedItems
    }

    ...

Answer 3

You post the value to the LiveData object in the activity's viewmodel , which isn't the same instance as the fragment's viewmodel . Let's take look at the way you instantiate the viewmodel in your fragment:

activity?.let {
    // activity can be refered by the implicit parameter `it`
    // `this` refers to the current fragment hence it's the owner of the view model
    val viewmodel = ViewModelProviders.of(this).get(viewmodel::class.java)
    observeInput(viewmodel)
}

To get a viewmodel that is shared between your activity and fragment you have to pass the activity as its owner:

activity?.let { val viewmodel = ViewModelProviders.of(it).get(viewmodel::class.java) }

Answer 4

Probably you can see developer guide example to resolve your problem

https://developer.android.com/topic/libraries/architecture/viewmodel.html#kotlin

// shared viewmodel

 class SharedViewModel : ViewModel() {
    private val usersList: MutableLiveData<List<String>>()

    fun getUsers(): LiveData<List<String>> {
        return usersList
    }

    fun setUsers(users: List<String>) {
        usersList.value = users
    }
 }

// Attach ViewModel In Activity onCreate()

val model = ViewModelProviders.of(this)[SharedViewModel::class.java]

val list = arrayListOf<String>()
list.add("user1")
list.add("user2")

model.setUsers(list)

// Get same ViewModel instance In fragment onCreateView()

model = activity?.run {
        ViewModelProviders.of(this)[SharedViewModel::class.java]
} ?: throw Exception("Invalid Activity")

model.getUsers().observe(this, Observer<List<User>>{ users ->
      // update UI
})