无法在 Activity 和 Fragment 之间共享 Viewmodel：

Question

Android Studio 3.6

这是我的视图模型：

import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.ViewModelProviders
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.*
class BluetoothPageViewModel(application: Application) : AndroidViewModel(application) {
    private val isSearchingTableModeLiveData = MutableLiveData<Boolean>()
    private val isInitModeLiveData = MutableLiveData<Boolean>()
    private val errorMessageLiveData = MutableLiveData<String>()
private val toastMessageLiveData = MutableLiveData<String>()

 fun isInitModeLiveData(): LiveData<Boolean> {
        return isInitModeLiveData
    }

    fun isSearchingTableModeLiveData(): LiveData<Boolean> {
        return isSearchingTableModeLiveData
    }

    fun getErrorMessageLiveData(): LiveData<String> {
        return errorMessageLiveData
    }


    fun getToastMessageLiveData(): LiveData<String> {
        return toastMessageLiveData
    }

此处片段订阅此 viewmodel 并成功调用 Observer.onChanged()

class BluetoothPageFragment : Fragment() {
private lateinit var bluetoothPageViewModel: BluetoothPageViewModel

 override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        dataBinding =
            DataBindingUtil.inflate(inflater, R.layout.bluetooth_page_fragment, container, false)
        val view = dataBinding.getRoot()
        dataBinding.setHandler(this)
        init()
        return view
    }

    private fun init() {
        val context = this.context
        val viewViewModelProvider = ViewModelProviders.of(this)
        bluetoothPageViewModel = viewViewModelProvider.get(BluetoothPageViewModel::class.java)

        bluetoothPageViewModel.isInitModeLiveData().observe(this, // SUCCESS CALL
            Observer<Boolean> { isInitMode ->

            })
}

这里是我的 activity 订阅这个 viewmodel 并且 NOT call Observer.onChanged()

import androidx.lifecycle.ViewModelProviders
class QRBluetoothSwipeActivity : AppCompatActivity() {
private lateinit var bluetoothPageViewModel: BluetoothPageViewModel

 private fun init() {
        val viewViewModelProvider = ViewModelProviders.of(this)
        bluetoothPageViewModel = viewViewModelProvider.get(BluetoothPageViewModel::class.java)

        val customFragmentStateAdapter = CustomFragmentStateAdapter(this)
        customFragmentStateAdapter.addFragment(QrPageFragment())
        bluetoothPageFragment = BluetoothPageFragment()
        customFragmentStateAdapter.addFragment(bluetoothPageFragment)
        dataBinding.viewPager2.adapter = customFragmentStateAdapter

        initLogic()
    }

    private fun initLogic() {
        dataBinding.viewPager2.registerOnPageChangeCallback(object :
            ViewPager2.OnPageChangeCallback() {
            override fun onPageSelected(position: Int) {
                positionObservable.set(position)
            }
        })


         bluetoothPageViewModel.getToastMessageLiveData()  // this not call
            .observe(this,
                Observer<String> { message ->
                    Toast.makeText(this, message, Toast.LENGTH_LONG).show()
                })
}

为什么不打电话给 getToastMessageLiveData()？

Answer 1

您在 activity 和片段中都调用了 ViewModelProviders.of(this)，但这是不同的上下文。因此，在您的情况下，您实例化了 2 个不同的 BluetoothPageViewModel 实例，因此未调用 onChanged 回调。

为了在 activity 和 Fragment 之间共享一个实例，您应该从同一上下文中获取 viewModelProvider。在你的 activity 中： ViewModelProviders.of(this) 在你的片段中： ViewModelProviders.of(activity) 或

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

    bluetoothPageViewModel.isInitModeLiveData().observe(this, // SUCCESS CALL
            Observer<Boolean> { isInitMode ->

            })
}

Answer 2

在这两种情况下，您都在使用

ViewModelProviders.of(this)

这意味着您希望此视图模型具有不同的范围。一个来自 Activity Scope，一个来自 Fragment 范围。如果你想分享它。如果你想共享视图模型，你必须使用单一范围。我建议使用更大元素的范围，在本例中为 activity。在片段中你应该调用

ViewModelProviders.of(activity)

这应该可以解决您的问题。

无法在 Activity 和 Fragment 之间共享 Viewmodel：

Can't share Viewmodel between Activity and Fragment:

android

android-livedata

android-viewmodel