class UserViewModel : ViewModel() {
    var age: Int = 0
}
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val userViewModel =
            ViewModelProvider(this, ViewModelProvider.NewInstanceFactory())[UserViewModel::class.java]
        val mView = ActivityMainBinding.inflate(layoutInflater)
        setContentView(mView.root)
        mView.tv.text = userViewModel.age.toString()
        var sum = 0
        mView.tv.setOnClickListener {
            sum = sum.inc()
            userViewModel.age = sum
        }
    }
}

public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}
//代碼很簡潔 一個抽象方法 返回值ViewModelStore 顧名思義這個類的功能也就呼之欲出，存儲viewmodel，那我們就看實現(xiàn)類中怎么處理的
//ComponentActivity中實現(xiàn) 
  @NonNull
    @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.");
        }
        ensureViewModelStore();
        return mViewModelStore;
    }
//我們直接看ensureViewModelStore()方法
void ensureViewModelStore() {
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }
 //ensureViewModelStore方法看來是為了獲取ViewModelStore，那我們在具體看下ViewModelStore內(nèi)部做了什么?
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());
    }
    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}
//看到這里就明了了，果然和我們猜想的一樣，ViewModelStore是用來緩存ViewModel的

public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        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;
    }

  val userViewModel =
            ViewModelProvider(this, ViewModelProvider.NewInstanceFactory())
                .get(UserViewModel::class.java)

 public constructor(owner: ViewModelStoreOwner, factory: Factory) : this(
        owner.viewModelStore,
        factory,
        defaultCreationExtras(owner)
    )

  public open class NewInstanceFactory : Factory {
        @Suppress("DocumentExceptions")
        override fun <T : ViewModel> create(modelClass: Class<T>): T {
            return try {
                modelClass.newInstance()
            } catch (e: InstantiationException) {
                throw RuntimeException("Cannot create an instance of $modelClass", e)
            } catch (e: IllegalAccessException) {
                throw RuntimeException("Cannot create an instance of $modelClass", e)
            }
        }

 @MainThread
    public open operator fun <T : ViewModel> get(key: String, modelClass: Class<T>): T {
        val viewModel = store[key]
        if (modelClass.isInstance(viewModel)) {
            (factory as? OnRequeryFactory)?.onRequery(viewModel)
            return viewModel as T
        } else {
            @Suppress("ControlFlowWithEmptyBody")
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        val extras = MutableCreationExtras(defaultCreationExtras)
        extras[VIEW_MODEL_KEY] = key
        // AGP has some desugaring issues associated with compileOnly dependencies so we need to
        // fall back to the other create method to keep from crashing.
        return try {
            factory.create(modelClass, extras)
        } catch (e: AbstractMethodError) {
            factory.create(modelClass)
        }.also { store.put(key, it) }
    }

 getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });