ViewModel源码分析

首先，还是先看一个例子：

public class MyViewModel extends ViewModel {
    private MutableLiveData<List<User>> users;
    public LiveData<List<User>> getUsers() {
        if (users == null) {
            users = new MutableLiveData<List<User>>();
            loadUsers();
        }
        return users;
    }

    private void loadUsers() {
        // Do an asynchronous operation to fetch users.
    }
}
public class MyActivity extends AppCompatActivity {
    public void onCreate(Bundle savedInstanceState) {
        MyViewModel model = new ViewModelProvider(this).get(MyViewModel.class);
    }
}

该例子定义了一个ViewModel的子类MyViewModel，然后通过ViewModelProvider的实例方法get()获取到MyViewModel的实例。

ViewModelProvider

/**

 * Creates {@code ViewModelProvider}. This will create {@code ViewModels}
 * and retain them in a store of the given {@code ViewModelStoreOwner}.
 * <p>
 * This method will use the
 * {@link HasDefaultViewModelProviderFactory#getDefaultViewModelProviderFactory() default factory}
 * if the owner implements {@link HasDefaultViewModelProviderFactory}. Otherwise, a
 * {@link NewInstanceFactory} will be used.
 */
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
    this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
         ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
         : NewInstanceFactory.getInstance());
}
/**
 * Creates {@code ViewModelProvider}, which will create {@code ViewModels} via the given
 * {@code Factory} and retain them in the given {@code store}.
 *
 * @param store   {@code ViewModelStore} where ViewModels will be stored.
 * @param factory factory a {@code Factory} which will be used to instantiate
 *                new {@code ViewModels}
 */
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
    mFactory = factory;
    mViewModelStore = store;
}

从构造方法中可以看出ViewModelProvider需要ViewModelStore和Factory两个类型的成员变量才能构造处理，分别是mViewModelStore和mFactory，ComponentActivity和Fragment分别都实现了ViewModelStoreOwner和HasDefaultViewModelProviderFactory接口，所以都可以从中获取到ViewModelStore和Factory的实例。

@NonNull
@MainThread
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);
}
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
    ViewModel viewModel = mViewModelStore.get(key);


    if (modelClass.isInstance(viewModel)) {
        if (mFactory instanceof OnRequeryFactory) {
            ((OnRequeryFactory) mFactory).onRequery(viewModel);
        }
        return (T) viewModel;
    } else {
        //noinspection StatementWithEmptyBody
        if (viewModel != null) {
            // TODO: log a warning.
        }
    }
    if (mFactory instanceof KeyedFactory) {
        viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
    } else {
        viewModel = mFactory.create(modelClass);
    }
    mViewModelStore.put(key, viewModel);
    return (T) viewModel;
}

get()方法首先尝试通过mViewModelStore的get()方法获取ViewModel的实例，如果没获取到则使用mFactory的create()创建实例，创建出来后则存入到mViewModelStore中。在这里mFactory就是ViewModel的构造工厂，mViewModelStore则是ViewModel的缓存管理者。

ViewModelProvider作为ViewModel的提供者，使用缓存mViewModelStore和工厂mFactory实现，第一次提供ViewModel时会通过工厂创建出来，后续则都是从缓存中拿。

ViewModelStore

public ComponentActivity() {

        ...
        getLifecycle().addObserver(new LifecycleEventObserver() {
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source,
                                   @NonNull Lifecycle.Event event) {
            if (event == Lifecycle.Event.ON_DESTROY) {
                if (!isChangingConfigurations()) {
                    getViewModelStore().clear();
                }
            }
        }
    });
}
@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.");
    }
    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;
}

ViewModelStoreOwner接口getViewModelStore()的实现就是提供一个ViewModelStore实例，而ComponentActivity使用Lifecycle能力在页面销毁时调用ViewModelStore实例的clear方法，清空其中的ViewModel。

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();
    }
}

ViewModelStore中使用HashMap管理ViewModel缓存，它被页面持有，并在页面真正销毁时才清空缓存。

官网的这张图中可以说明ViewModel的生命周期。

SaveStateViewModelFactory

public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {

    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 (mDefaultFactory == null) {
        mDefaultFactory = new SavedStateViewModelFactory(
            getApplication(),
            this,
            getIntent() != null ? getIntent().getExtras() : null);
    }
    return mDefaultFactory;
}

ComponentActivity中getDefaultViewModelProviderFactory()方法通过构造方法创建一个SavedStateViewModelFactory对象，传入了Application、当前ComponentActivity实例和Intent中的数据bundle。

SavedStateViewModelFactory构造方法

public SavedStateViewModelFactory(@NonNull Application application,
                                  @NonNull SavedStateRegistryOwner owner,
                                  @Nullable Bundle defaultArgs) {
    mSavedStateRegistry = owner.getSavedStateRegistry();
    mLifecycle = owner.getLifecycle();
    mDefaultArgs = defaultArgs;
    mApplication = application;
    mFactory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}

构造方法接受的参数中，页面实例是SavedStateRegistryOwner接口类型的，通过该接口获取到SavedStateRegistry和Lifecycle。另外成员变量mFactory是AndroidViewModelFactory的单例对象。

SavedStateViewModelFactory的create()

@Override
public <T extends ViewModel> T create(@NonNull String key, @NonNull Class<T> modelClass) {
    boolean isAndroidViewModel = AndroidViewModel.class.isAssignableFrom(modelClass);
    Constructor<T> constructor;
    if (isAndroidViewModel) {
        constructor = findMatchingConstructor(modelClass, ANDROID_VIEWMODEL_SIGNATURE);
    } else {
        constructor = findMatchingConstructor(modelClass, VIEWMODEL_SIGNATURE);
    }
    // doesn't need SavedStateHandle
    if (constructor == null) {
        return mFactory.create(modelClass);
    }

    SavedStateHandleController controller = SavedStateHandleController.create(
        mSavedStateRegistry, mLifecycle, key, mDefaultArgs);
    try {
        T viewmodel;
        if (isAndroidViewModel) {
            viewmodel = constructor.newInstance(mApplication, controller.getHandle());
        } else {
            viewmodel = constructor.newInstance(controller.getHandle());
        }
        viewmodel.setTagIfAbsent(TAG_SAVED_STATE_HANDLE_CONTROLLER, controller);
        return viewmodel;
    } catch (IllegalAccessException e) {
        throw new RuntimeException("Failed to access " + modelClass, e);
    } catch (InstantiationException e) {
        throw new RuntimeException("A " + modelClass + " cannot be instantiated.", e);
    } catch (InvocationTargetException e) {
        throw new RuntimeException("An exception happened in constructor of "
                                   + modelClass, e.getCause());
    }
}

create()方法支持创建三种类型的ViewModel：AndroidViewModel、支持SavedState的ViewModel、普通ViewModel，这里由于篇幅原因，只分析一下普通ViewModel的创建。普通ViewModel通过mFactory的create()方法创建出来。

AndroidViewModelFactory的create()

public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

    ...
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
            //noinspection TryWithIdenticalCatches
            try {
                return modelClass.getConstructor(Application.class).newInstance(mApplication);
            } catch (NoSuchMethodException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InstantiationException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InvocationTargetException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            }
        }
        return super.create(modelClass);
    }
}

AndroidViewModelFactory的create()方法判断如果不是AndroidViewModel类型，就直接通过父类的create()方法创建，而AndroidViewModelFactory的父类是NewInstanceFactory。

NewInstanceFactory的create()

public static class NewInstanceFactory implements Factory {
    ...

    @SuppressWarnings("ClassNewInstance")
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        //noinspection TryWithIdenticalCatches
        try {
            return modelClass.newInstance();
        } catch (InstantiationException e) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, e);
        }
    }
}

NewInstanceFactory的create()则是直接通过反射创建出ViewModel实例。

SaveStateViewModelFactory作为ComponentActivity和Fragment提供的对象，在NewInstanceFactory的基础上增加了对AndroidViewModel和支持SavedStated的ViewModel的创建，但对于普通的ViewModel创建，最后还是降级使用NewInstanceFactory完成。

到此，ViewModel的主要类已经分析完了，接下来可以结合类关系，一定程度上总结出对全局视角的理解。

主要类说明：

• ViewModelProvider：ViewModel提供者

• ViewModelStore：ViewModel缓存管理

• ViewModelProvider.Factory：ViewModel创建工厂

• SavedStateViewModelFactory：ViewModel创建工厂的实现

• NewInstanceFactory：普通ViewModel创建工厂的实现

类关系描述：

ViewModel通过ViewModelProvider的get()方法获取到，ViewModelProvider由缓存ViewModelStore和创建工厂ViewModelProvider.Factory组合而成，ViewModelStore和ViewModelProvider.Factory也是ComponentActivity的一部分，ComponentActivity通过实现ViewModelStoreOwner和HasDefaultViewModelProviderFactory两个接口对外提供ViewModelStore和ViewModelProvider.Factory。其中，ViewModelProvider.Factory在ComponentActivity的具体实现是SavedStateViewModelFactory，SavedStateViewModelFactory一部分由AndroidViewModelFactory组合而成，它提供创建三种ViewModel的能力，其中普通ViewModel的创建是由AndroidViewModelFactory的父类NewInstanceFactory完成。