// 在 Activity 中使用
class MainActivity : AppCompatActivity() {
   // 使用 Activity 的作用域
   private val viewModel : MainViewModel by viewModels()
}
// 在 Fragment 中使用
class MainFragment : Fragment() {
   // 使用 Activity 的作用域，與 MainActivity 使用同一個對象
   val activityViewModel : MainViewModel by activityViewModels()
   // 使用 Fragment 的作用域
   val viewModel : MainViewModel by viewModels()
}

// ViewModel 創(chuàng)建工廠
private final Factory mFactory;
// ViewModel 存儲容器
private final ViewModelStore mViewModelStore;
// 默認使用 NewInstanceFactory 反射創(chuàng)建 ViewModel
public ViewModelProvider(ViewModelStoreOwner owner) {
    this(owner.getViewModelStore(), ... NewInstanceFactory.getInstance());
}
// 自定義 ViewModel 創(chuàng)建工廠
public ViewModelProvider(ViewModelStoreOwner owner, Factory factory) {
    this(owner.getViewModelStore(), factory);
}
// 記錄宿主的 ViewModelStore 和 ViewModel 工廠
public ViewModelProvider(ViewModelStore store, Factory factory) {
    mFactory = factory;
    mViewModelStore = store;
}
@NonNull
@MainThread
public <T extends ViewModel> T get(Class<T> modelClass) {
    String canonicalName = modelClass.getCanonicalName();
    if (canonicalName == null) {
        throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
    }
    // 使用類名作為緩存的 KEY
    return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
// Fragment
@NonNull
@MainThread
public <T extends ViewModel> T get(String key, Class<T> modelClass) {
    // 1. 先從 ViewModelStore 中取緩存
    ViewModel viewModel = mViewModelStore.get(key);
    if (modelClass.isInstance(viewModel)) {
        return (T) viewModel;
    }
    // 2. 使用 ViewModel 工廠創(chuàng)建實例
    viewModel = mFactory.create(modelClass);
    ...
    // 3. 存儲到 ViewModelStore
    mViewModelStore.put(key, viewModel);
    return (T) viewModel;
}
// 默認的 ViewModel 工廠
public static class NewInstanceFactory implements Factory {
    private static NewInstanceFactory sInstance;
    @NonNull
    static NewInstanceFactory getInstance() {
        if (sInstance == null) {
            sInstance = new NewInstanceFactory();
        }
        return sInstance;
    }
    @NonNull
    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        // 反射創(chuàng)建 ViewModel 對象
        return modelClass.newInstance();
    }
}

// ViewModel 本質(zhì)上就是一個映射表而已
public class ViewModelStore {
   // <String - ViewModel> 哈希表
   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();
   }
}

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
    ContextAware,
    LifecycleOwner,
    ViewModelStoreOwner ... {
    // ViewModel 的存儲容器
    private ViewModelStore mViewModelStore;
    // ViewModel 的創(chuàng)建工廠
    private ViewModelProvider.Factory mDefaultFactory;
    @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        if (mViewModelStore == null) {
            // 已簡化過程
            mViewModelStore = new ViewModelStore();
        }
        return mViewModelStore;
    }
}