Jetpack系列(四) — ViewModel-一一网

Jetpack系列(四) — ViewModel

ViewModel简单介绍

初步印象

在MVVM框架中ViewModel 负责逻辑判断，承接View 和 Repository

ViewModel 生命周期存活在整个Activity生命周期，旋转不会重新创建

基本概念

ViewModelProviders 用户维护ViewModelStore 和 ViewModelProvider.Factory关键方法public static ViewModelProvider of(Fragment fragment, Factory factory)

ViewModelStore 通过HashMap存储ViewModels

ViewModel基本使用

实现 ViewModel

创建ViewModel对象，继承ViewModel

// HomeViewModel.kt
class HomeViewModel : ViewModel() {

    private var tapCount = 0

    private val _taps = MutableLiveData<String>("$tapCount taps")

    val taps: LiveData<String>
        get() = _taps

    fun updateTaps() {
        tapCount++
        _taps.postValue("$tapCount taps")
    }
}
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在View中间实例化ViewModel

// HomeFragment.kt
private val viewModel: HomeViewModel by viewModels()
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通过LiveData观察ViewModel当中的数据变化

//HomeViewModel.kt
observe(viewModel.taps, ::tapsUpdate)   // LifecycleOwner 扩展函数

private fun tapsUpdate(s: String?) {
    binding.tvCount.text = s ?: ""
}
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共享Fragment数据

应用场景是在一个Fragment 中操纵数据，在另一个 Fragment用于显示该数据的内容。

创建ViewModel

/**
 * ViewModel
 * 在 Fragment 之间共享数据
 */
class MainViewModel : ViewModel() {

    private var _tapCount = 0
    private var _taps = MutableLiveData<String>("$_tapCount taps")

    val taps: LiveData<String>
        get() = _taps

    fun updateTaps() {
        _tapCount++
        _taps.value = "$_tapCount taps"
    }
}
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在HomeFragment中观察和操作数据

// HomeFragment.kt
class HomeFragment : Fragment() {

    private lateinit var binding: FragmentHomeBinding

    // 方式一： 通过viewModels() 创建ViewModel
    private val viewModel: HomeViewModel by viewModels()
    private val mainViewModel: MainViewModel by activityViewModels()

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        ...
        initListener()
        subscribeUI()
    }

    private fun initListener() {
        binding.btnAdd.setOnClickListener { 
            mainViewModel.updateTaps()
        }
    }

    private fun subscribeUI() {
        // viewModel.taps.observe(viewLifecycleOwner, ::tapsUpdate)
        // 扩展函数
        observe(mainViewModel.taps, ::tapsUpdate) 
    }

    private fun tapsUpdate(s: String?) {
        binding.tvCount.text = s ?: ""
    }
 
}
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在DeepLinkFragment 当中观察数据

class DeepLinkFragment : Fragment() {

    private lateinit var binding: FragmentDeepLinkBinding
    private val mainViewModel: MainViewModel by activityViewModels()

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
		...
        subscribeUI()
    }

    private fun subscribeUI() {
        observe(mainViewModel.taps, ::tapsUpdate)
    }

    private fun tapsUpdate(s: String) {
        binding.tap.text = s
    }
}
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相关知识点

知识点一：实例 ViewModel

上述代码中直接使用viewModels()延时函数，将ViewModel 和 ViewModelStoreOwner自动绑定，然后直接使用ViewModel 实例

// FragmentViewModelLazyKt.java
@MainThread
inline fun <reified VM : ViewModel> Fragment.viewModels(
    noinline ownerProducer: () -> ViewModelStoreOwner = { this },
    noinline factoryProducer: (() -> Factory)? = null
) = createViewModelLazy(VM::class, { ownerProducer().viewModelStore }, factoryProducer)

@MainThread
fun <VM : ViewModel> Fragment.createViewModelLazy(
    viewModelClass: KClass<VM>,
    storeProducer: () -> ViewModelStore,
    factoryProducer: (() -> Factory)? = null
): Lazy<VM> {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }
    return ViewModelLazy(viewModelClass, storeProducer, factoryPromise)
}
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viewModels() 可以传入参数，手动创建XxxxViewModel

//  SearchFragment.kt
private val viewModel: SearchViewModel by viewModels { LiveDataVMFactory }

object LiveDataVMFactory : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        @Suppress("UNCHECKED_CAST")
        return SearchViewModel(RepoRepository(GithubService.create())) as T
    }
}
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viewModels() 实际上做了两个动作，一是创建ViewModel实例，一是绑定ViewModel对象，等效于下述代码

// 注： 代码来自《Android 第一行代码》
// MainActivity.java 
lateinit var viewModel: MainViewModel

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 		...
    	// 绑定ViewModel对象
        viewModel = ViewModelProviders.of(this, MainViewModelFactory(countReserved)).get(MainViewModel::class.java)
        viewModel.counter.observe(this, Observer{ count ->
            infoText.text = count.toString()
        })
        viewModel.user.observe(this, Observer { user ->
            infoText.text = user.firstName
        })
        ...
    }

// MainViewModelFactory.kt
// 创建`ViewModel`实例
class MainViewModelFactory(private val countReserved: Int) : ViewModelProvider.Factory {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return MainViewModel(countReserved) as T
    }

}
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