android mvvm实例解析

MVVM架构，将整个应用分为三层，View层，VM层，Model层。其中View层单向引用VM层，VM层单向引用Model层。如上图。

单向引用，而非双向引用，这是MVVM与MVP最大的区别。View层，只是单向引用VM层，VM层不需要引用View层，但是却可以

更新View层。这是通过VM层的观察者模式实现的，在这里使用架构组件LiveData，观察者注册LiveData，当LiveData数据发生变更

的时候，就会通知注册的观察者。

VM层，执行业务逻辑，获取Model层的数据，Model层的数据由repository来提供。

 

举例子：

ChooseAreaFragment是View层，它持有ViewModel，它可以监听相关数据，相关数据发生变化的时候，对应的UI就会被更新。

比如：dataChanged数据发生变化，就会执行定义的观察者操作。

  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 = "中国"
                    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()
        }
    }

    /**
     * 显示进度对话框
     */
    private fun showProgressDialog() {
        if (progressDialog == null) {
            progressDialog = ProgressDialog(activity)
            progressDialog?.setMessage("正在加载...")
            progressDialog?.setCanceledOnTouchOutside(false)
        }
        progressDialog?.show()
    }

    /**
     * 关闭进度对话框
     */
    private fun closeProgressDialog() {
        progressDialog?.dismiss()
    }

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

}

VM层，ViewModel：

使用LiveData包装被View层监听的数据，在VM层数据发生的变化，会通知到View层，但却无需要View层的引用。

因为LiveData应用了观察者模式，注册的观察者，在数据发生变化的时候，会自动通知观察者。

如下，currentLevel，dataChanged，isLoading等都是使用LiveData包装的，意味着，它们发生变化的时候View层会监听得到，从而进行相应的更新操作。

在VM层，持有Model层的引用，Model层的数据获取，网络请求，都依赖repository实现。

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层：

在这个例子中，Model层对外提供的方法是

getProvinceList，getCityList，getCountyList。

它的数据来源，可能是数据库Dao，或者是网络，各自的实现，再委托到具体的方法去实现。

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的实例解析。应用MVVM的时候，关键是划分功能属于哪一个层次，然后，再确定引用关系。划分功能属于哪个层次，可以依据单一职责原则，让功能代码原子化，再在这一基础上去区分层次。