android MVVM(2)用数据绑定关联VM 与 V ; viewModel 扩展包

1.官方文档

　　https://developer.android.com/topic/libraries/data-binding/architecture

2.简介

　　数据绑定库 可与MVVM 架构组件无缝协作。指定一下 binding的 lifecycleOwner 就可以了。如下：

class PageFrgmt (var number : Int) : Fragment() {

    lateinit
    var binding     :   PageBinding
    val viewModel   :   DataViewModel by viewModels()

    fun initBinding(){
        binding.lifecycleOwner = this
        binding.number = number
        binding.data = viewModel.data
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = PageBinding.inflate(inflater,container,false)
        initBinding()
        return binding.root
    }
}

3.使用数据绑定与LiveData区别

使用数据绑定时，有两种方式实现数据更新通知 ：

a. 使用  ObservableInt,ObservableFloat ... ObservableField<XX> 等

• 缺点是这系列类不知道观察者(在mvvm架构中，通常是Activity,Fragment)的生命期变化，
• 优点是不用在代码里写观察代码，如

         viewModel.loadUser().observe(viewLifecycleOwner){user->
              nameValue.setText(user.name)
              ageValue.setText(user.age.toString())
          }

• 在android 3.1以上，可以直接把ObservableXXX 系列类，换成LiveData系列。

b. 使用 BaseObservable 基类，这时无法替换成LiveData

4.示例

4.1 定义Data与Model

Data:

class Data : BaseObservable {

    constructor():super(){
        Log.e("MVVM_Data","constructor(), value = $value")
    }
    init{
        Log.e("MVVM_Data","init, value = $value")
        ++value
        name = "data$value"
    }
    @get:Bindable
    var name    =   "unknown_"
        set(value) {
            field = value
            notifyPropertyChanged(BR.name)
        }

    companion object : BaseObservable(){
        @get:Bindable
        @JvmStatic var value   =   0
            set(value) {
                field = value
                notifyPropertyChanged(BR.value)
            }
    }
}

Model

class DataModel(var args : Bundle) {

    val liveData: MutableLiveData<Data> by lazy { initLiveData() }
    val data    :   Data  by lazy { initData() }

    fun initLiveData() : MutableLiveData<Data>{
        val data = Data().apply {
            name = "liveData"
        }
        return MutableLiveData(data)
    }

    fun initData() : Data{
        return Data().apply {
            name = "Data"
        }
    }

    fun loadData() : Data{
        return data
    }

    fun loadLiveData() : MutableLiveData<Data>{
        return liveData
    }
}

4.2 定义ViewModel

class DataViewModel(var args : Bundle): ViewModel(),Observable {

    val model = DataModel(args)

    fun loadData() = model.loadData()
    fun loadLiveData() = model.loadLiveData()

    override fun onCleared() {
        super.onCleared()
    }

    override fun removeOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {
    }

    override fun addOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {
    }
}

由于ViewModel有参数，默认的 ViewModelFactory不支持，需要自定义一个 ViewModelFactory

class DataViewModelFactory(var args : Bundle) : ViewModelProvider.Factory {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(DataViewModel::class.java)) {
            return DataViewModel(args) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

4.3 在View中用数据绑定关联View与ViewModel

class PageFrgmt (var number : Int) : Fragment() {

    companion object {  @JvmStatic var  VMSOwner  =   false }

    lateinit
    var binding     :   PageBinding

    val viewModel1  :   DataViewModel by lazy { initViewModel() }
    val viewModel2  :   DataViewModel by activityViewModels(this::factory)
    val viewModel3  :   DataViewModel by viewModels(this::ownerProducer,this::factory)


    fun factory() : DataViewModelFactory{
        return DataViewModelFactory(Bundle())
    }
    fun ownerProducer() : ViewModelStoreOwner {
        return this
    }

    fun initViewModel() : DataViewModel{
        return if (VMSOwner){
//            ViewModelProvider(requireActivity()).get(DataViewModel::class.java)
            ViewModelProvider(requireActivity(), DataViewModelFactory(Bundle())).get(DataViewModel::class.java)
        }else{
//            ViewModelProvider(this).get(DataViewModel::class.java)
            ViewModelProvider(this,DataViewModelFactory(Bundle())).get(DataViewModel::class.java)
        }
    }

    fun initBinding(){
        binding.lifecycleOwner = this
//        val viewModel2  :   DataViewModel by activityViewModels(this::factory)
//        val viewModel3  :   DataViewModel by viewModels(this::ownerProducer,this::factory)
        binding.data = if (VMSOwner) viewModel2.loadData() else viewModel3.loadData()
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        Log.e("MVVM_TAG","PageFrgmt page $number onCreateView ,VMSOwner = $VMSOwner")
        binding = PageBinding.inflate(inflater,container,false)
        initBinding()
        return binding.root
    }
    //...
}

• 其中第8、9、10行：用3种方法定义了ViewModel成员
• 第31行：指定了binding的生命期
• 第34行：关联了view与ViewModel

绑定布局

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <data>
        <variable name="data" type="com.example.mvvm2.model.Data" />
    </data>
    <androidx.constraintlayout.widget.ConstraintLayout
        android:background="#ffffff"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <TextView
            ...
            android:text="@{@string/data_format(data.name,data.value),default=@string/data_format}"
           />


    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

4.4 完整示例

　　https://github.com/f9q/mvvm2 

5.ViewModel与application绑定

在指定ViewModel关联的生命期对象时，系统提供了activity和fragment，可以使用Application与ViewModel绑定。

5.1在application中定义 modelStoreOwner

class XXXApp : MultiDexApplication(), ViewModelStoreOwner {
    val modelStoreOwner     =   ViewModelStore()
    override fun getViewModelStore(): ViewModelStore {
        return modelStoreOwner
    }
    //...
}    

5.2 关联ViewModel与Application

class DeviceFrgmt : Fragment() {
    val viewModel      : DeviceViewModel   by lazy { initDeviceModel() }
    fun initDeviceModel() : DeviceViewModel{
        val app = activity?.application as XXXApp
        val vm = ViewModelProvider(app, DeviceViewModelFactory(app)).get(DeviceViewModel::class.java).apply {
            loadCallback    = this@DeviceFrgmt::loadDeviceCallback
            ...
        }
        return vm
    }
    fun initBinding(){
        binding.lifecycleOwner = this
        //...    
    }
}

6. 扩展包

6.1 viewModels、activityViewModels

　　4.3 节代码中的 activityViewModels 与 viewModels 可以简单构造viewModel ，它们所在包：

　　implementation 'androidx.fragment:fragment-ktx:1.2.5'

6.2 ViewModelProvider　

　4.3 节代码中的  ViewModelProvider 所在包：

　　implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

class NurseApp : MultiDexApplication(), ViewModelStoreOwner {
    val modelStoreOwner     =   ViewModelStore()
    override fun getViewModelStore(): ViewModelStore {
        return modelStoreOwner
}