class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
       val viewModel =  ViewModelProvider(this , ViewModelProvider.NewInstanceFactory()).get(MyViewModel::class.java)
    }
}

第一步：创建 ViewModelProvider , 拿到 ViewModelStoreOwner（ComponentActivity） 的 ViewModelStore ， 如果有横竖屏切换，则拿上一次的 ViewModelStore ， 否则如果为空 则创建 ViewModelStore

ViewModelProvider.kt

public constructor(owner: ViewModelStoreOwner, factory: Factory) : this(
        owner.viewModelStore,   // 拿到 ViewModelStoreOwner（ComponentActivity） 的  viewModelStore
        factory,
        defaultCreationExtras(owner)
    )

ComponentActivity.kt

void ensureViewModelStore() {
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance(); // 关键
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }

// 实际返回的是  NonConfigurationInstances 
public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;   
    }

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

第二步 ： 通过 ViewModelStore 创建、保存、获取 ViewModel

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) }
    }

其他：
Activity onDestroy 时 ， 判断如果不是 configChange ， 则调用
ViewModelStore.clear() ,
---> ViewModel.clear()
--> ViewModel.onClear()

image.png