ListAdapter不保存旧数据

Question

我有带有MediatorLiveData的ViewModel。我想像这样合并两个LiveData

 private fun mergeData(): LiveData<List<DataIncomeOperations>> {
    val summaryLiveData = MediatorLiveData<List<DataIncomeOperations>>()
    summaryLiveData.addSource(incomeData) {
        summaryLiveData.value = incomeData.value
    }

    summaryLiveData.addSource(costsData) {
        summaryLiveData.value = costsData.value
    }

    return summaryLiveData
}

val summaryLiveData: LiveData<List<DataIncomeOperations>> = mergeData()

像这样将数据设置为RecyclerView

operationsViewModel.summaryLiveData.observe(
            viewLifecycleOwner,
            Observer<List<DataOperations>> {
                adapter.submitList(it)
            })

作为结果，在显示模拟器上显示最后添加的LiveData而不是所有数据 我有点尝试了一切，却找不到解决方法