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Android Studio – Android Mutablelivedata list item didn't notify changed

I declared variable like this.

var saleData: MutableLiveData<SaleData> = MutableLiveData()

data class

data class SaleData(
    var saleType: Int,
    var saleDetail: Int,
    var salePrice: Int,
    var saleRate: Int,
    var printAmount: Int
)

and then, init the data

init {
 saleData.value = SaleData(saleType = 1, saleDetail = 0, salePrice = 0, saleRate = 0, printAmount = 1)
}

The question is, if one of the components of the data class in SaleData changes, can I be notified of this?
I simply wrote the code as below, but there was no result value.

viewModel

fun changeData() {
    saleData.value?.saleRate = 50
}

fragment – at onCreateView

binding.viewModel = viewModel
binding.lifecycleOwner = this
viewModel.saleData.observe(viewLifecycleOwner, Observer { saleData ->
    Log.d(TAG,"value changed")
})

I can’t get the Log when change the saleRate in saleData

Like this code,

saleData.value = SaleData(saleType = saleType, saleDetail = 0, salePrice = 0, saleRate = 0, printAmount = 1)

I set saleData value, it notify the value changed but I want when change the item of saleData, notify the change

Is there anything else I need to set up?

Tags:

2

Answers


  1. 0

    A call to saleData.value?.saleRate = 50 will not notify observers because the underlying object stored within the MutableLiveData remains unchaged. For LiveData to notify its observers, you need to assign a new object to it.
    Assuming SaleData is a data class, you can make it notify it’s observers by calling,

    saleData.value = saleData.value?.copy(saleRate = 50)

    This will notify all registered observers.

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  2. 0

    As Rafsanjani said, you need to assign an updated reference of the SaleData class to the LiveData. Once you do that, the LiveData observer will notify the changes. For better understanding, please, see the code below.

    SaleDataViewModel

    class SaleDataViewModel: ViewModel() {
   private var _saleData: MutableLiveData<SaleData> = MutableLiveData()
   val saleData: LiveData<SaleData> = _saleData

   init {
      _saleData.value = SaleData(saleType = 1, saleDetail = 0, salePrice = 0, saleRate = 0, printAmount = 1)
   }

   fun updateSale() {
      val saleRate = updatedSaleRate()
      _saleData.value = SaleData( _saleData.value!!.saleType, _saleData.value!!.saleDetail, _saleData.value!!.salePrice, saleRate, _saleData.value!!.printAmount)
   }

   private fun updatedSaleRate(): Int {
      return 50
   }
}

    SaleDataFragment

    class SaleDataFragment: Fragment() {
   ...
   override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
     super.onViewCreated(view, savedInstanceState) {
     viewModel.updateSale()
     viewModel.saleData.observe(viewLifecycleOwner, Observer {
         Log.d(TAG, "Notify change")
     })
  }
}
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