Android ViewModel原理分析

从API调用入手

创建ViewModel实例

//MainActivity
ViewModelProvider(this)[MainViewModel::class.java]

this--->MainActivity

Activity继承关系：

MainActivity->AppCompatActivity->FragmentActivity->ComponentActivity

ComponentActivity实现ViewModelStoreOwner接口重写getViewModelStore()方法

    @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;
    }

ViewModelProvider(this)

    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

    public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;
    }

ViewModelStoreOwner--->ComponentActivity

owner.getViewModelStore()--->ViewModelStore

ViewModelProvider.get()

    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);
    }

    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;
    }

ViewModelStore.get()

    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();
    }

API调用流程总结：
ViewModelProvider(this)传入ViewModelStoreOwner接口实现类ComponentActivity，ComponentActivity重写了抽象方法getViewModelStore()返回ViewModelStore；

ViewModelProvider.get(ViewModel::class.java)通过ViewModelStore获取ViewModel；

ViewModelStore内部维护了一个HashMap，key是ViewModel类名，value是ViewModel实例。

接下来看ViewModel核心功能，Activity重建恢复数据。

回到ComponentActivity.getViewModelStore()

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

nc不为null，通过nc获取viewModelStore

获取nc--->getLastNonConfigurationInstance()

调用父类Activity.getLastNonConfigurationInstance()

    public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;
    }

找一下mLastNonConfigurationInstances赋值处

Activity.attach()

...
mLastNonConfigurationInstances = lastNonConfigurationInstances;
...

至于attach()方法调用处，前面很多篇都说过

ActivityThread.performLaunchActivity(ActivityClientRecord r...)

    activity.attach(appContext, this, getInstrumentation(), r.token,
        r.ident, app, r.intent, r.activityInfo, title, r.parent,
        r.embeddedID, r.lastNonConfigurationInstances, config,
        r.referrer, r.voiceInteractor, window, r.configCallback,
        r.assistToken);

lastNonConfigurationInstances从ActivityClientRecord中获取

lastNonConfigurationInstances何时保存到ActivityClientRecord中的呢？

既然ViewModel在屏幕旋转，Activity销毁重建后可恢复数据，先找到Activity.Destroy()调用处

ActivityThread.performDestroyActivity()

ActivityClientRecord r = mActivities.get(token);

r.lastNonConfigurationInstances = r.activity.retainNonConfigurationInstances();

Activity.retainNonConfigurationInstances()

    NonConfigurationInstances retainNonConfigurationInstances() {
        Object activity = onRetainNonConfigurationInstance();
        HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
        FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();

        mFragments.doLoaderStart();
        mFragments.doLoaderStop(true);
        ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();

        if (activity == null && children == null && fragments == null && loaders == null
                && mVoiceInteractor == null) {
            return null;
        }

        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.activity = activity;
        nci.children = children;
        nci.fragments = fragments;
        nci.loaders = loaders;
        if (mVoiceInteractor != null) {
            mVoiceInteractor.retainInstance();
            nci.voiceInteractor = mVoiceInteractor;
        }
        return nci;
    }

Object activity = onRetainNonConfigurationInstance()

调用子类ComponentActivity.onRetainNonConfigurationInstance()

    public final Object onRetainNonConfigurationInstance() {
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            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;
    }

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

nci.activity = activity

这里activity其实是个Object对象也就是上述方法返回值：

ComponentActivity.NonConfigurationInstances

其内部保存ViewModelStore实例

回看Activity.getLastNonConfigurationInstance()

    public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;
    }

取保存的ComponentActivity.NonConfigurationInstances实例

流程很清晰了：
Activity.Destroy()时保存了ViewModelStore，在Activity启动、实例化时调用Activity.attach()恢复。ViewModelStore内部维护HashMap存储ViewModel实例。