android - 如何在 Android Studio 中比较两个 LiveData 是否相等？

Question

我有两个 LiveData，aMVoice1 和 aMVoice2。

我希望检查它们是否相等。

我知道我需要使用 observe 来获取 LiveData 的值。

所以我认为 isEqual = (mDetailViewModel.aMVoice1.value==mDetailViewMode2.aMVoice1.value ) 是错误的。

但我认为 fun observeVoice() 存在一些问题，我该如何解决？

class FragmentDetail : Fragment() {

    private lateinit var binding: LayoutDetailBinding
 
    private val mDetailViewModel by lazy {
       ...
    }

    var isEqual=false

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        ...
        binding.lifecycleOwner = this.viewLifecycleOwner
        binding.aDetailViewModel=mDetailViewModel

        isEqual = (mDetailViewModel.aMVoice1.value==mDetailViewMode2.aMVoice1.value ) // I don't think it's correct.

        observeVoice()
        return binding.root
    }

    fun observeVoice() {
        mDetailViewModel.aMVoice1.observe(viewLifecycleOwner){value1->
           isEqual = (value1==mDetailViewModel.aMVoice2.value)  // mDetailViewModel.aMVoice2.value maybe null
        }
    }

}


class DetailViewModel(private val mDBVoiceRepository: DBVoiceRepository, private val voiceId1:Int,private val voiceId2:Int) : ViewModel() {
   val aMVoice1=mDBVoiceRepository.getVoiceById(voiceId1)
   val aMVoice2=mDBVoiceRepository.getVoiceById(voiceId2)
}

class DBVoiceRepository private constructor(private val mDBVoiceDao: DBVoiceDao){
    fun getVoiceById(id:Int)=mDBVoiceDao.getVoiceById(id)
}


@Dao
interface DBVoiceDao{
   @Query("SELECT * FROM voice_table where id=:id")
   fun getVoiceById(id:Int):LiveData<MVoice>
}

data class MVoice(
    @PrimaryKey (autoGenerate = true) @ColumnInfo(name = "id") var id: Int = 0,
    var name:          String = "",
    var path:          String = ""
)

添加的内容

下面的代码可以吗？

fun observeVoice() {
    mDetailViewModel.aMVoice1.observe(viewLifecycleOwner){value1->
       mDetailViewModel.aMVoice2.observe（viewLifecycleOwner）{value2->
             isEqual = (value1==value2)
       }
    }
 }

Answer 1

据官方documents ，实现此类情况的解决方案的最佳方法是使用 MediatorLiveData 作为 LiveData 合并。使用它，您可以在将新值发布到任一 LiveData 时检查值的相等性:

class DetailViewModel(...) : ViewModel() {

    val areMVoicesEqual = MediatorLiveData<Boolean>().apply {
        addSource(aMVoice1) { postValue(it == aMVoice2.value) }
        addSource(aMVoice2) { postValue(it == aMVoice1.value) }
    }
}

然后:

fun observeVoice() {
    mDetailViewModel.areMVoicesEqual.observe(viewLifecycleOwner){ equality ->
       // do whatever you want with `equality`
    }
 }

请注意，您提到的Added Content fragment 不正确。事实上，在这种情况下，每次在 aMVoice1 上观察到一个值时，都会有一个新的 Observer 开始在 aMVoice2 上观察，这是不对的.