Android开发：Activity无法观察ViewModel内LiveData的状态变更

• 作为Android开发新手，在宿主Activity中观察关联ViewModel内的Boolean类型LiveData状态时，应用启动阶段能正常收到数据的初始值，后续所有状态变更（比如切换Fragment时改动的数值）都无法触发已设置的观察者。
• 预期目标：通过MyMusicViewOn这个MutableLiveData的状态变化，对应监听ViewModel初始化、ViewModel清理、界面切换的时机。

现有实现代码 #

自定义ViewModel #

class MyMusicViewModel : ViewModel() {
    private var _MyMusicViewOn = MutableLiveData<Boolean>()
    val MyMusicViewOn: LiveData<Boolean> get() = _MyMusicViewOn

    init {
        Timber.i("MyMusicViewModel Init Called!")
        setMyMusicView(true)
    }

    override fun onCleared() {
        super.onCleared()
        Timber.i("MyMusicViewModel Cleared!")
        setMyMusicView(false)
    }

    fun setMyMusicView(setter: Boolean) {
        Timber.i("MyMusicViewModel setter called! %s", setter)
        _MyMusicViewOn.value = setter
    }
}

宿主Activity代码 #

class FullscreenActivity : AppCompatActivity() {
    private val viewModel: MyMusicViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        viewModel.MyMusicViewOn.observe(this, Observer { MyMusicViewOn ->
            Timber.i("Observer called for MyMusicViewOn %s", MyMusicViewOn)
        })
    }
}

关联Fragment代码 #

class MyMusicFragment : Fragment() {
    private lateinit var viewModel: MyMusicViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val binding = DataBindingUtil.inflate<FragmentMyMusicBinding>(
            inflater,
            R.layout.fragment_my_music,
            container,
            false
        )
        // 错误的ViewModel获取逻辑
        viewModel = ViewModelProvider(this).get(MyMusicViewModel::class.java)
        return binding.root
    }

    override fun onResume() {
        super.onResume()
        Timber.i("MyMusicViewFragment resumed!")
        viewModel.setMyMusicView(true)
    }
}

问题根因 #

• 问题出在Fragment获取ViewModel的传参上：代码里写的ViewModelProvider(this)传入的this是Fragment自身，这会让系统从Fragment独有的ViewModelStore存储空间里获取（或新建）一个MyMusicViewModel实例，和宿主Activity里通过by viewModels()从Activity的ViewModelStore里拿到的ViewModel根本不是同一个对象。
• 两个独立的ViewModel实例各持有独立的_MyMusicViewOn LiveData对象，Fragment修改自己持有实例的LiveData值时，Activity观察的是自己手里那个ViewModel的LiveData，两边数据完全不互通，自然无法触发Activity里设置的观察者回调。

修复方案 #

如果要实现Activity和Fragment共享同一个ViewModel实例，Fragment获取ViewModel时需要把绑定源指定为宿主Activity，两种常用写法：

• 使用activityViewModels属性代理，直接绑定和Activity相同的ViewModel实例：

  private val viewModel: MyMusicViewModel by activityViewModels()
  

• 手动创建ViewModelProvider时传入宿主Activity作为绑定源，注意不要传Fragment自己：

  // 传入requireActivity()而非Fragment本身的this
  viewModel = ViewModelProvider(requireActivity()).get(MyMusicViewModel::class.java)
  

内容的提问来源于stack exchange，提问作者Fabrizio Ferrari