android mvvm實例解析_ZenDei技術網路在線

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MVVM架構，將整個應用分為三層，View層，VM層，Model層。其中View層單向引用VM層，VM層單向引用Model層。如上圖。單向引用，而非雙向引用，這是MVVM與MVP最大的區別。View層，只是單向引用VM層，VM層不需要引用View層，但是卻可以更新View層。這是通過VM層的觀察者模式實現的，在這裡使用架構組件LiveData，觀察者註冊LiveData，當LiveData數據發生變更的時候，就會通知註冊的觀察者。 VM層，執行業務邏輯，獲取Model層的數據，Model層的數據由repository來提供。舉例子： ChooseAreaFragment是View層，它持有ViewModel，它可以監聽相關數據，相關數據發生變化的時候，對應的UI就會被更新。比如：dataChanged數據發生變化，就會執行定義的觀察者操作。 viewModel.dataChanged.observe(this, Observer { adapter.notifyDataSetChanged() listView.setSelection(0) closeProgressDialog() })

class ChooseAreaFragment : Fragment() {


    private val viewModel by lazy { ViewModelProviders.of(this, InjectorUtil.getChooseAreaModelFactory()).get(ChooseAreaViewModel::class.java) }
    private var progressDialog: ProgressDialog? = null
    private lateinit var adapter: ArrayAdapter<String>


    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val view = inflater.inflate(R.layout.choose_area, container, false)
        val binding = DataBindingUtil.bind<ChooseAreaBindingImpl>(view)
        binding?.viewModel = viewModel
        return view
    }


    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        adapter = ChooseAreaAdapter(context!!, R.layout.simple_item, viewModel.dataList)
        listView.adapter = adapter
        observe()
    }


    private fun observe() {
        viewModel.currentLevel.observe(this, Observer { level ->
            when (level) {
                LEVEL_PROVINCE -> {
                    titleText.text = "中國"
                    backButton.visibility = View.GONE
                }
                LEVEL_CITY -> {
                    titleText.text = viewModel.selectedProvince?.provinceName
                    backButton.visibility = View.VISIBLE
                }
                LEVEL_COUNTY -> {
                    titleText.text = viewModel.selectedCity?.cityName
                    backButton.visibility = View.VISIBLE
                }
            }
        })


        viewModel.dataChanged.observe(this, Observer {
            adapter.notifyDataSetChanged()
            listView.setSelection(0)
            closeProgressDialog()
        })
        viewModel.isLoading.observe(this, Observer { isLoading ->
            if (isLoading) showProgressDialog()
            else closeProgressDialog()
        })
        viewModel.areaSelected.observe(this, Observer { selected ->
            if (selected && viewModel.selectedCounty != null) {
                if (activity is MainActivity) {
                    val intent = Intent(activity, WeatherActivity::class.java)
                    intent.putExtra("weather_id", viewModel.selectedCounty!!.weatherId)
                    startActivity(intent)
                    activity?.finish()
                } else if (activity is WeatherActivity) {
                    val weatherActivity = activity as WeatherActivity
                    weatherActivity.drawerLayout.closeDrawers()
                    weatherActivity.viewModel.weatherId = viewModel.selectedCounty!!.weatherId
                    weatherActivity.viewModel.refreshWeather()
                }
                viewModel.areaSelected.value = false
            }
        })
        if (viewModel.dataList.isEmpty()) {
            viewModel.getProvinces()
        }
    }


    /**
     * 顯示進度對話框
     */
    private fun showProgressDialog() {
        if (progressDialog == null) {
            progressDialog = ProgressDialog(activity)
            progressDialog?.setMessage("正在載入...")
            progressDialog?.setCanceledOnTouchOutside(false)
        }
        progressDialog?.show()
    }


    /**
     * 關閉進度對話框
     */
    private fun closeProgressDialog() {
        progressDialog?.dismiss()
    }


    companion object {
        const val LEVEL_PROVINCE = 0
        const val LEVEL_CITY = 1
        const val LEVEL_COUNTY = 2
    }


}

VM層，ViewModel：使用LiveData包裝被View層監聽的數據，在VM層數據發生的變化，會通知到View層，但卻無需要View層的引用。因為LiveData應用了觀察者模式，註冊的觀察者，在數據發生變化的時候，會自動通知觀察者。如下，currentLevel，dataChanged，isLoading等都是使用LiveData包裝的，意味著，它們發生變化的時候View層會監聽得到，從而進行相應的更新操作。在VM層，持有Model層的引用，Model層的數據獲取，網路請求，都依賴repository實現。

class ChooseAreaViewModel(private val repository: PlaceRepository) : ViewModel() {


    var currentLevel = MutableLiveData<Int>()


    var dataChanged = MutableLiveData<Int>()


    var isLoading = MutableLiveData<Boolean>()


    var areaSelected = MutableLiveData<Boolean>()


    var selectedProvince: Province? = null


    var selectedCity: City? = null


    var selectedCounty: County? = null


    lateinit var provinces: MutableList<Province>


    lateinit var cities: MutableList<City>


    lateinit var counties: MutableList<County>


    val dataList = ArrayList<String>()


    fun getProvinces() {
        currentLevel.value = LEVEL_PROVINCE
        launch {
            provinces = repository.getProvinceList()
            dataList.addAll(provinces.map { it.provinceName })
        }
    }


    private fun getCities() = selectedProvince?.let {
        currentLevel.value = LEVEL_CITY
        launch {
            cities = repository.getCityList(it.provinceCode)
            dataList.addAll(cities.map { it.cityName })
        }
    }


    private fun getCounties() = selectedCity?.let {
        currentLevel.value = LEVEL_COUNTY
        launch {
            counties = repository.getCountyList(it.provinceId, it.cityCode)
            dataList.addAll(counties.map { it.countyName })
        }
    }


    fun onListViewItemClick(parent: AdapterView<*>, view: View, position: Int, id: Long) {
        when {
            currentLevel.value == LEVEL_PROVINCE -> {
                selectedProvince = provinces[position]
                getCities()
            }
            currentLevel.value == LEVEL_CITY -> {
                selectedCity = cities[position]
                getCounties()
            }
            currentLevel.value == LEVEL_COUNTY -> {
                selectedCounty = counties[position]
                areaSelected.value = true
            }
        }
    }


    fun onBack() {
        if (currentLevel.value == LEVEL_COUNTY) {
            getCities()
        } else if (currentLevel.value == LEVEL_CITY) {
            getProvinces()
        }
    }


    private fun launch(block: suspend () -> Unit) = viewModelScope.launch {
        try {
            isLoading.value = true
            dataList.clear()
            block()
            dataChanged.value = dataChanged.value?.plus(1)
            isLoading.value = false
        } catch (t: Throwable) {
            t.printStackTrace()
            Toast.makeText(CoolWeatherApplication.context, t.message, Toast.LENGTH_SHORT).show()
            dataChanged.value = dataChanged.value?.plus(1)
            isLoading.value = false
        }
    }


}

Model層：在這個例子中，Model層對外提供的方法是 getProvinceList，getCityList，getCountyList。它的數據來源，可能是資料庫Dao，或者是網路，各自的實現，再委托到具體的方法去實現。

class PlaceRepository private constructor(private val placeDao: PlaceDao, private val network: CoolWeatherNetwork) {


    suspend fun getProvinceList() = withContext(Dispatchers.IO) {
        var list = placeDao.getProvinceList()
        if (list.isEmpty()) {
            list = network.fetchProvinceList()
            placeDao.saveProvinceList(list)
        }
        list
    }


    suspend fun getCityList(provinceId: Int) = withContext(Dispatchers.IO) {
        var list = placeDao.getCityList(provinceId)
        if (list.isEmpty()) {
            list = network.fetchCityList(provinceId)
            list.forEach { it.provinceId = provinceId }
            placeDao.saveCityList(list)
        }
        list
    }


    suspend fun getCountyList(provinceId: Int, cityId: Int) = withContext(Dispatchers.IO) {
        var list = placeDao.getCountyList(cityId)
        if (list.isEmpty()) {
            list = network.fetchCountyList(provinceId, cityId)
            list.forEach { it.cityId = cityId }
            placeDao.saveCountyList(list)
        }
        list
    }


    companion object {


        private var instance: PlaceRepository? = null


        fun getInstance(placeDao: PlaceDao, network: CoolWeatherNetwork): PlaceRepository {
            if (instance == null) {
                synchronized(PlaceRepository::class.java) {
                    if (instance == null) {
                        instance = PlaceRepository(placeDao, network)
                    }
                }
            }
            return instance!!
        }


    }


}

以上就是MVVM的實例解析。應用MVVM的時候，關鍵是劃分功能屬於哪一個層次，然後，再確定引用關係。劃分功能屬於哪個層次，可以依據單一職責原則，讓功能代碼原子化，再在這一基礎上去區分層次。版權聲明：作者：ttylinux 　　　　出處：http://www.cnblogs.com/ttylinux/ 　　　　本文版權歸作者，歡迎轉載，但未經作者同意必須保留此段聲明，且在文章頁面明顯位置給出原文連接，否則保留追究法律責任的權利。