如何使用翻新GET获取LiveData列表以填充recyclerView

Question

如何使用翻新GET获取LiveData列表以填充recyclerView。我的代码未在屏幕上显示数据。如果我不更改现有列表，而是第一次从新数据开始，是否调用onChange（）？

MainActivity，我在其中调用ViewModel来检查数据并在设置到适配器中时加载：

var mContactViewModel = ViewModelProviders.of(this).get(ContactViewModel::class.java)
mContactViewModel.getContacts(this)?.observe(this, object : Observer<List<Contact>> {
    override fun onChanged(contactsList: List<Contact>?) {    
        loading_contacts_list.visibility = View.GONE
        if(contactsList!!.isNotEmpty()){
            rv_users.adapter = ContactsAdapter(contactsList, onContactClick, context)
        } else {
            //TODO throw exception
            println("list of contacts returned empty")
        }
    }
})

ContactViewModel类，其中包装了ContactRepository的getContact函数：

class ContactViewModel(application: Application) : AndroidViewModel(application) { 
    private val mRepository: ContactRepository

    init {
        mRepository = ContactRepository()
    }

    fun getContacts(context: Context) : LiveData<List<Contact>>? {
        return mRepository.getContacts(context)
    }
}

ContactRepository类，其中Retrofit发出GET请求，将LiveData>放在此类的列表中并返回：

class ContactRepository {
    var allContacts: LiveData<List<Contact>>? = null
    var call: Call<LiveData<List<Contact>>>

    init {
        var requestService: RequestService = RetrofitClient().getClient(MY_URL)!!.create(RequestService::class.java)
        call = requestService.getContacts()
    }

    fun getContacts(context: Context) : LiveData<List<Contact>>? {
        call.enqueue(object : Callback<LiveData<List<Contact>>> {
            override fun onFailure(call: Call<LiveData<List<Contact>>>, t: Throwable) {
                Log.e("Tag onfailure", "retrofit on failure", t)
            }

            override fun onResponse(call: Call<LiveData<List<Contact>>>, response: Response<LiveData<List<Contact>>>) {
                if(response.isSuccessful)
                    allContacts = response.body()
            }
        })
        return allContacts
    }
}