ViewModel 并不是MVVM的VM，而是jetpack的一个组件库，主要目的是保证数据安全的

基本的使用

最常见的场景Activity的用户数据在页面重建时候 数据丢失，使用ViewModel来解决

 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 绑定view，然后使用ViewModelProvider获取到viewmodel ，再把绑定的view和viewmodel建立关系，同时把绑定关联生命周期
个人认为使用还是比较繁琐的，需要4步：1.拿到布局；2.拿到model; 3.布局和model建立关系；4.布局和生命周期建立关系

对应的model的数据，我们直接直接使用的LiveData 现在继承ViewModel就能保证数据可靠性：

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

实现原理

类似LifeCycle的实现，AppCompatActivity实现了对应的接口ViewModelStoreOwner：

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

我们看下调用的地方

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

ViewModelProvider创建对象时传入owner和Factory，再看下ViewModelStore里面就一个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);
    }

从ViewModelStore中取出具体对应的viewModel，里面主要是缓存在map里面

再看Activity的生命周期方法，``会在页面重建时 通过ASM调用

    public Object onRetainNonConfigurationInstance() {
        return null;
    }

他的实现在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为空 则会从nc中取，并且存到nci里面，那是什么时候会用到这个呢？
答案是我们调用时候。页面重建会执行onCreate方法，里面调用了ViewmodelProvider的方法第一个参数getViewModelStore（）,对应的实现就是component Activity里面的。也就是对应的在页面销毁时存在nc，在页面重建时候取了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是一个对象：

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

ViewModelProvider 使用 ViewModelStore 来存储和管理 ViewModel 实例。当 Activity 或 Fragment 被销毁时，ViewModelProvider 并不会立即销毁 ViewModel 实例，而是将其保存在 ViewModelStore 中。当 Activity 或 Fragment 重新创建时，ViewModelProvider 可以从 ViewModelStore 中恢复 ViewModel 实例。