ViewModel 并不是MVVM的VM，而是jetpack的一個(gè)組件庫(kù)，主要目的是保證數(shù)據(jù)安全的

基本的使用

最常見(jiàn)的場(chǎng)景Activity的用戶(hù)數(shù)據(jù)在頁(yè)面重建時(shí)候 數(shù)據(jù)丟失，使用ViewModel來(lái)解決

 var binding: ActivityMainBinding? = null
    var viewModel : MainViewModel ? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        viewModel = ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory(application))
            .get(MainViewModel::class.java)
        binding ?.vm = viewModel
        binding!! .lifecycleOwner = this
    }

使用DataBindingUtil.setContentView 綁定view，然后使用ViewModelProvider獲取到viewmodel ，再把綁定的view和viewmodel建立關(guān)系，同時(shí)把綁定關(guān)聯(lián)生命周期
個(gè)人認(rèn)為使用還是比較繁瑣的，需要4步：1.拿到布局；2.拿到model; 3.布局和model建立關(guān)系；4.布局和生命周期建立關(guān)系

對(duì)應(yīng)的model的數(shù)據(jù)，我們直接直接使用的LiveData 現(xiàn)在繼承ViewModel就能保證數(shù)據(jù)可靠性：

class MainViewModel(application: Application) : AndroidViewModel(application) {
    val testData by lazy { MutableLiveData<String>() }

實(shí)現(xiàn)原理

類(lèi)似LifeCycle的實(shí)現(xiàn)，AppCompatActivity實(shí)現(xiàn)了對(duì)應(yīng)的接口ViewModelStoreOwner：

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,

我們看下調(diào)用的地方

        viewModel = ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory(application))
            .get(MainViewModel::class.java)

ViewModelProvider創(chuàng)建對(duì)象時(shí)傳入owner和Factory，再看下ViewModelStore里面就一個(gè)map 和存 取方法

public class ViewModelStore {

    private final HashMap<String, ViewModel> mMap = new HashMap<>();

    final void put(String key, ViewModel viewModel) {
        ViewModel oldViewModel = mMap.put(key, viewModel);
        if (oldViewModel != null) {
            oldViewModel.onCleared();
        }
    }

    final ViewModel get(String key) {
        return mMap.get(key);
    }

    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}

再看下get方法,反射：

.get(MainViewModel::class.java)

    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }

從ViewModelStore中取出具體對(duì)應(yīng)的viewModel，里面主要是緩存在map里面

再看Activity的生命周期方法，``會(huì)在頁(yè)面重建時(shí) 通過(guò)ASM調(diào)用

    public Object onRetainNonConfigurationInstance() {
        return null;
    }

他的實(shí)現(xiàn)在ComponentActivity中，

public final Object onRetainNonConfigurationInstance() {
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            // No one called getViewModelStore(), so see if there was an existing
            // ViewModelStore from our last NonConfigurationInstance
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                viewModelStore = nc.viewModelStore;
            }
        }

        if (viewModelStore == null && custom == null) {
            return null;
        }

        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

如果mViewModelStore為空 則會(huì)從nc中取，并且存到nci里面，那是什么時(shí)候會(huì)用到這個(gè)呢？
答案是我們調(diào)用時(shí)候。頁(yè)面重建會(huì)執(zhí)行onCreate方法，里面調(diào)用了ViewmodelProvider的方法第一個(gè)參數(shù)getViewModelStore（）,對(duì)應(yīng)的實(shí)現(xiàn)就是component Activity里面的。也就是對(duì)應(yīng)的在頁(yè)面銷(xiāo)毀時(shí)存在nc，在頁(yè)面重建時(shí)候取了nc.viewModelStore

    @Override
    public ViewModelStore getViewModelStore() {
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }

nc是一個(gè)對(duì)象：

    static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

ViewModelProvider 使用 ViewModelStore 來(lái)存儲(chǔ)和管理 ViewModel 實(shí)例。當(dāng) Activity 或 Fragment 被銷(xiāo)毀時(shí)，ViewModelProvider 并不會(huì)立即銷(xiāo)毀 ViewModel 實(shí)例，而是將其保存在 ViewModelStore 中。當(dāng) Activity 或 Fragment 重新創(chuàng)建時(shí)，ViewModelProvider 可以從 ViewModelStore 中恢復(fù) ViewModel 實(shí)例。