Kotlin ObserveForver是否了解生命周期

是否可以永远观察生命周期？

No, that's why it's called observeForever.

I have implemented observeForever() from the ViewModel, but I don't think that is a good idea

不，不是，你应该用Transformations.switchMap {.

since I don't know if observeForever() will be cleared on onCleared() if my view is destroyed, so it won't keep the observer alive if the view dies.

Well if you're not clearing it in onCleared() using removeObserver(observer), then it won't clear itself, because it observes forever.

这就是我真的怀疑我使用的方法是否正确的地方，

不，通过被动的方法，你可以做得更好.

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    button.setOnClickListener {
        val deviceId = editText.text.toString().trim()
        viewModel.onSelectedDeviceChanged(deviceId)
    }

    viewModel.selectedDevice.observe(this, Observer { device ->
        textView.text = "Tipo: ${device.devType}"
    })
}

和

class MainViewModel(
    private val savedStateHandle: SavedStateHandle,
): ViewModel() {
    private val repo = Repo() // TODO: move to Constructor Argument with ViewModelProvider.Factory

    private val selectedDeviceId: MutableLiveData<String> = savedStateHandle.getLiveData<String>("selectedDeviceId")

    fun onSelectedDeviceChanged(deviceId: String) {
        selectedDeviceId.value = deviceId
    }

    val selectedDevice = Transformations.switchMap(selectedDeviceId) { deviceId ->
        repo.getDeviceData(deviceId)
    }
}

和

class Repo {
    private val db = FirebaseDatabase.getInstance().reference // TODO: move to constructor arg? Probably

    fun getDeviceData(deviceId:String) : LiveData<Device> {
        return object: MutableLiveData<Device>() {
            private val mutableLiveData = this

            private var query: Query? = null
            private val listener: ValueEventListener = object: ValueEventListener {
                override fun onDataChange(dataSnapshot: DataSnapshot) {
                    val device = dataSnapshot.getValue(Device::class.java)
                    mutableLiveData.value = device
                }

                override fun onCancelled(dataError: DatabaseError) {
                    Log.e("Error","handle error callback")
                }
            }

            override fun onActive() {
                query?.removeEventListener(listener)
                val query = db.child(deviceId).child("config/device")
                this.query = query
                query.addValueEventListener(listener)
            }
    
            override fun onInactive() {
                query?.removeEventListener(listener)
                query = null
            }
        }
    }
}

This way, you can observe for changes made in Firebase (and therefore be notified of future changes made to your values) using LiveData, rather than only execute a single fetch and then not be aware of changes made elsewhere to the same data.