Android AAC架构解析

• LiveData如何实现数据更新
• LiveData如何实现同activity声明周期绑定
• viewModel如何实现数据共享
• viewModel如何实现数据保存

本文就如上问题结合aac框架源码进行逐步解析 ##一.LiveData实现数据更新 既然是监测数据更新,肯定是使用到观察者模式

• 观察者 GenericLifecycleObserver,其中LifecycleObserver为空接口

public interface GenericLifecycleObserver extends LifecycleObserver {
    void onStateChanged(LifecycleOwner source, Lifecycle.Event event);
}

• 被观察者

public abstract class Lifecycle {
    
    public abstract void addObserver(@NonNull LifecycleObserver observer);

    @MainThread
    public abstract void removeObserver(@NonNull LifecycleObserver observer);

    @MainThread
    @NonNull
    public abstract State getCurrentState();

    @SuppressWarnings("WeakerAccess")
    public enum Event {
        /**
         * Constant for onCreate event of the {@link LifecycleOwner}.
         */
        ON_CREATE,
        /**
         * Constant for onStart event of the {@link LifecycleOwner}.
         */
        ON_START,
        /**
         * Constant for onResume event of the {@link LifecycleOwner}.
         */
        ON_RESUME,
        /**
         * Constant for onPause event of the {@link LifecycleOwner}.
         */
        ON_PAUSE,
        /**
         * Constant for onStop event of the {@link LifecycleOwner}.
         */
        ON_STOP,
        /**
         * Constant for onDestroy event of the {@link LifecycleOwner}.
         */
        ON_DESTROY,
        /**
         * An {@link Event Event} constant that can be used to match all events.
         */
        ON_ANY
    }

    /**
     * Lifecycle states. You can consider the states as the nodes in a graph and
     * {@link Event}s as the edges between these nodes.
     */
    @SuppressWarnings("WeakerAccess")
    public enum State {
        /**
         * Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
         * any more events. For instance, for an {@link android.app.Activity}, this state is reached
         * <b>right before</b> Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
         */
        DESTROYED,

        /**
         * Initialized state for a LifecycleOwner. For an {@link android.app.Activity}, this is
         * the state when it is constructed but has not received
         * {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} yet.
         */
        INITIALIZED,

        /**
         * Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
         * is reached in two cases:
         * <ul>
         *     <li>after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
         *     <li><b>right before</b> {@link android.app.Activity#onStop() onStop} call.
         * </ul>
         */
        CREATED,

        /**
         * Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
         * is reached in two cases:
         * <ul>
         *     <li>after {@link android.app.Activity#onStart() onStart} call;
         *     <li><b>right before</b> {@link android.app.Activity#onPause() onPause} call.
         * </ul>
         */
        STARTED,

        /**
         * Resumed state for a LifecycleOwner. For an {@link android.app.Activity}, this state
         * is reached after {@link android.app.Activity#onResume() onResume} is called.
         */
        RESUMED;

        /**
         * Compares if this State is greater or equal to the given {@code state}.
         *
         * @param state State to compare with
         * @return true if this State is greater or equal to the given {@code state}
         */
        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }
}

我们来看看Lifecycle实现类LifecycleRegistry, 主要看添加观察者以及接收被观察者,对应如下两个方法

@Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

        if (previous != null) {
            return;
        }
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            // it is null we should be destroyed. Fallback quickly
            return;
        }

        boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
        State targetState = calculateTargetState(observer);
        mAddingObserverCounter++;
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            pushParentState(statefulObserver.mState);
            statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
            popParentState();
            // mState / subling may have been changed recalculate
            targetState = calculateTargetState(observer);
        }

        if (!isReentrance) {
            // we do sync only on the top level.
            sync();
        }
        mAddingObserverCounter--;
    }

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        State next = getStateAfter(event);
        moveToState(next);
    }

接着看看ObserverWithState带状态的Observe做了什么,

static class ObserverWithState {
        State mState;
        GenericLifecycleObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.getCallback(observer);
            mState = initialState;
        }

        void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = getStateAfter(event);
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }

主要是Lifecycling.getCallback(observer)获取GenericLifecycleObserver,在dispatchEvent执行onStateChanged()通知数据更新,而dispatchEvent方法在addObserver中执行

statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));

所以当liveData中执行observe时

  @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
        if (owner.getLifecycle().getCurrentState() == DESTROYED) {
            // ignore
            return;
        }
        LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
            throw new IllegalArgumentException("Cannot add the same observer"
                    + " with different lifecycles");
        }
        if (existing != null) {
            return;
        }
        owner.getLifecycle().addObserver(wrapper);
    }

首先会创建LifecycleBoundObserver,来看看LifecycleBoundObserver代码

class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
        @NonNull final LifecycleOwner mOwner;

        LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<T> observer) {
            super(observer);
            mOwner = owner;
        }

        @Override
        boolean shouldBeActive() {
            return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
        }

        @Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            activeStateChanged(shouldBeActive());
        }

        @Override
        boolean isAttachedTo(LifecycleOwner owner) {
            return mOwner == owner;
        }

        @Override
        void detachObserver() {
            mOwner.getLifecycle().removeObserver(this);
        }
    }

LifecycleBoundObserver实现了GenericLifecycleObserver,并实现onStateChanged方法 接着 owner.getLifecycle().addObserver(wrapper),获取Lifecycle,调用到上面解析的addObserver, 而这里的wrapper为GenericLifecycleObserver,那么Lifecycling.getCallback(observe)直接取到GenericLifecycleObserver,则执行addObserver中dispatchEvent,即调用到onStateChanged, 此时LifecycleBoundObserver中onStateChanged

if (mOwner.getLifecycle().getCurrentState() == DESTROYED) 此判断解决内存泄漏问题

@Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            activeStateChanged(shouldBeActive());
        }

接着执行如下,主要看dispatchingValue

  void activeStateChanged(boolean newActive) {
            if (newActive == mActive) {
                return;
            }
            // immediately set active state, so we'd never dispatch anything to inactive
            // owner
            mActive = newActive;
            boolean wasInactive = LiveData.this.mActiveCount == 0;
            LiveData.this.mActiveCount += mActive ? 1 : -1;
            if (wasInactive && mActive) {
                onActive();
            }
            if (LiveData.this.mActiveCount == 0 && !mActive) {
                onInactive();
            }
            if (mActive) {
                dispatchingValue(this);
            }
        }

 private void dispatchingValue(@Nullable ObserverWrapper initiator) {
        if (mDispatchingValue) {
            mDispatchInvalidated = true;
            return;
        }
        mDispatchingValue = true;
        do {
            mDispatchInvalidated = false;
            if (initiator != null) {
                considerNotify(initiator);
                initiator = null;
            } else {
                for (Iterator<Map.Entry<Observer<T>, ObserverWrapper>> iterator =
                        mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                    considerNotify(iterator.next().getValue());
                    if (mDispatchInvalidated) {
                        break;
                    }
                }
            }
        } while (mDispatchInvalidated);
        mDispatchingValue = false;
    }

这里主要看considerNotify,initiator不为null,直接调用

private void considerNotify(ObserverWrapper observer) {
        if (!observer.mActive) {
            return;
        }
        // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
        //
        // we still first check observer.active to keep it as the entrance for events. So even if
        // the observer moved to an active state, if we've not received that event, we better not
        // notify for a more predictable notification order.
        if (!observer.shouldBeActive()) {
            observer.activeStateChanged(false);
            return;
        }
        if (observer.mLastVersion >= mVersion) {
            return;
        }
        observer.mLastVersion = mVersion;
        //noinspection unchecked
        observer.mObserver.onChanged((T) mData);
    }

最终执行到observer的onChanged,即public void observe(@NonNull LifecycleOwner owner, @NonNull Observer observer)中Observer的onChanged

推展:setValue,postValue之后observer获取到数据更新

 @MainThread
    protected void setValue(T value) {
        assertMainThread("setValue");
        mVersion++;
        mData = value;
        dispatchingValue(null);
    }

private void dispatchingValue(@Nullable ObserverWrapper initiator) {
        if (mDispatchingValue) {
            mDispatchInvalidated = true;
            return;
        }
        mDispatchingValue = true;
        do {
            mDispatchInvalidated = false;
            if (initiator != null) {
                considerNotify(initiator);
                initiator = null;
            } else {
                for (Iterator<Map.Entry<Observer<T>, ObserverWrapper>> iterator =
                        mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                    considerNotify(iterator.next().getValue());
                    if (mDispatchInvalidated) {
                        break;
                    }
                }
            }
        } while (mDispatchInvalidated);
        mDispatchingValue = false;
    }

同上逻辑,只是 dispatchingValue(null);会遍历所有Observer,调用considerNotify

 protected void postValue(T value) {
        boolean postTask;
        synchronized (mDataLock) {
            postTask = mPendingData == NOT_SET;
            mPendingData = value;
        }
        if (!postTask) {
            return;
        }
        ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
    }

private final Runnable mPostValueRunnable = new Runnable() {
        @Override
        public void run() {
            Object newValue;
            synchronized (mDataLock) {
                newValue = mPendingData;
                mPendingData = NOT_SET;
            }
            //noinspection unchecked
            setValue((T) newValue);
        }
    };

而postValue只是post到主线程,执行setValue

##二.LiveData绑定activity声明周期

主要看LifecycleDispatcher这个类

 static void init(Context context) {
        if (sInitialized.getAndSet(true)) {
            return;
        }
        ((Application) context.getApplicationContext())
                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
    }

    @SuppressWarnings("WeakerAccess")
    @VisibleForTesting
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {
        private final FragmentCallback mFragmentCallback;

        DispatcherActivityCallback() {
            mFragmentCallback = new FragmentCallback();
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            if (activity instanceof FragmentActivity) {
                ((FragmentActivity) activity).getSupportFragmentManager()
                        .registerFragmentLifecycleCallbacks(mFragmentCallback, true);
            }
            ReportFragment.injectIfNeededIn(activity);
        }

init初始化之后,在DispatcherActivityCallback中ReportFragment.injectIfNeededIn(activity);将activity注入到了ReportFragment,接着看ReportFragment

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class ReportFragment extends Fragment {
    private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";

    public static void injectIfNeededIn(Activity activity) {
        // ProcessLifecycleOwner should always correctly work and some activities may not extend
        // FragmentActivity from support lib, so we use framework fragments for activities
        android.app.FragmentManager manager = activity.getFragmentManager();
        if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
            manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
            // Hopefully, we are the first to make a transaction.
            manager.executePendingTransactions();
        }
    }

    static ReportFragment get(Activity activity) {
        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                REPORT_FRAGMENT_TAG);
    }

    private ActivityInitializationListener mProcessListener;

    private void dispatchCreate(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onCreate();
        }
    }

    private void dispatchStart(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onStart();
        }
    }

    private void dispatchResume(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onResume();
        }
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }

    @Override
    public void onStart() {
        super.onStart();
        dispatchStart(mProcessListener);
        dispatch(Lifecycle.Event.ON_START);
    }

    @Override
    public void onResume() {
        super.onResume();
        dispatchResume(mProcessListener);
        dispatch(Lifecycle.Event.ON_RESUME);
    }

    @Override
    public void onPause() {
        super.onPause();
        dispatch(Lifecycle.Event.ON_PAUSE);
    }

    @Override
    public void onStop() {
        super.onStop();
        dispatch(Lifecycle.Event.ON_STOP);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        dispatch(Lifecycle.Event.ON_DESTROY);
        // just want to be sure that we won't leak reference to an activity
        mProcessListener = null;
    }

    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }

    void setProcessListener(ActivityInitializationListener processListener) {
        mProcessListener = processListener;
    }

    interface ActivityInitializationListener {
        void onCreate();

        void onStart();

        void onResume();
    }
}

通过injectIfNeededIn方法创建了ReportFragment,通过管理ReportFragment的声明周期,dispatch(Lifecycle.Event.ON_PAUSE);将声明周期枚举进行分发

private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }

最后event赋值给了LifecycleRegistry中mState,并通过getCurrentState获取 则Lifecycle通过getCurrentState()可以取到当前activity的声明周期了

看看为什么不会导致内存泄漏

 @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
        if (owner.getLifecycle().getCurrentState() == DESTROYED) {
            // ignore
            return;
        }
        LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
        ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
        if (existing != null && !existing.isAttachedTo(owner)) {
            throw new IllegalArgumentException("Cannot add the same observer"
                    + " with different lifecycles");
        }
        if (existing != null) {
            return;
        }
        owner.getLifecycle().addObserver(wrapper);
    }

@Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            activeStateChanged(shouldBeActive());
        }

当执行observe和onStateChanged都有判断当前activity是否destroyed,由此可见,activity销毁了,并不会执行数据更新,自然不会有内存泄漏

##三.viewModel如何实现数据共享

主要是同一个HolderFragment,导致有同一个ViewModelStore,同一个ViewModelStore存的key是同一个,所以get获取到的viewModel是同一个,数据自然能共享,我们来一步步解析

class ViewModelDelegate<out T : BaseViewModel>(private val clazz: KClass<T>, private val fromActivity: Boolean) {

    private var viewModel: T? = null

    operator fun getValue(thisRef: BaseActivity, property: KProperty<*>) = buildViewModel(activity = thisRef)

    operator fun getValue(thisRef: BaseFragment, property: KProperty<*>) = if (fromActivity)
        buildViewModel(activity = thisRef.activity as? BaseActivity
                ?: throw IllegalStateException("Activity must be as BaseActivity"))
    else buildViewModel(fragment = thisRef)

    private fun buildViewModel(activity: BaseActivity? = null, fragment: BaseFragment? = null): T {
        if (viewModel != null) return viewModel!!

        activity?.let {
            viewModel = ViewModelProviders.of(it).get(clazz.java)
        } ?: fragment?.let {
            viewModel = ViewModelProviders.of(it).get(clazz.java)
        } ?: throw IllegalStateException("Activity and Fragment null! =(")

        return viewModel!!
    }
}

fun <T : BaseViewModel> BaseActivity.viewModelDelegate(clazz: KClass<T>) = ViewModelDelegate(clazz, true)

// fromActivity默认为true，viewModel生命周期默认跟activity相同 by aaron 2018/7/24
fun <T : BaseViewModel> BaseFragment.viewModelDelegate(clazz: KClass<T>, fromActivity: Boolean = true) = ViewModelDelegate(clazz, fromActivity)

这是通过代理生成的viewModel,我们来看ViewModelProviders.of(it).get(clazz.java)里面做了一些什么

@NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull FragmentActivity activity) {
        return of(activity, null);
    }

 @NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull FragmentActivity activity,
            @Nullable Factory factory) {
        Application application = checkApplication(activity);
        if (factory == null) {
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(ViewModelStores.of(activity), factory);
    }

如上,首先默认传入Factory为null,会通过ViewModelProvider.AndroidViewModelFactory.getInstance(application);去获取单例的factory,接着看ViewModelStores.of(activity)

 @NonNull
    @MainThread
    public static ViewModelStore of(@NonNull FragmentActivity activity) {
        if (activity instanceof ViewModelStoreOwner) {
            return ((ViewModelStoreOwner) activity).getViewModelStore();
        }
        return holderFragmentFor(activity).getViewModelStore();
    }

if (activity instanceof ViewModelStoreOwner)并不会执行,那么走holderFragmentFor(activity).getViewModelStore(); holderFragmentFor(activity)如下,是获取HolderFragment

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    public static HolderFragment holderFragmentFor(FragmentActivity activity) {
        return sHolderFragmentManager.holderFragmentFor(activity);
    }

sHolderFragmentManager是静态内部类HolderFragmentManager,而HolderFragmentManager是饿汉式创建的单例

private static final HolderFragmentManager sHolderFragmentManager = new HolderFragmentManager();

接着往下看

HolderFragment holderFragmentFor(FragmentActivity activity) {
            FragmentManager fm = activity.getSupportFragmentManager();
            HolderFragment holder = findHolderFragment(fm);
            if (holder != null) {
                return holder;
            }
            holder = mNotCommittedActivityHolders.get(activity);
            if (holder != null) {
                return holder;
            }

            if (!mActivityCallbacksIsAdded) {
                mActivityCallbacksIsAdded = true;
                activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
            }
            holder = createHolderFragment(fm);
            mNotCommittedActivityHolders.put(activity, holder);
            return holder;
        }

首先是findHolderFragment(fm),在activity的supportFragmentManager中查找,如下,通过同一个tag获取到HolderFragment, 如果HolderFragment不为空直接返回,如果为空通过mNotCommittedActivityHolders.get(activity), 如果HolderFragment为空,通过createHolderFragment()创建 ,tag为HOLDER_TAG,并方法mNotCommittedActivityHolders的map中 由此可见findHolderFragment,mNotCommittedActivityHolders.get(activity),在多个activity中获取的HolderFragment可能是同一个

  private static HolderFragment createHolderFragment(FragmentManager fragmentManager) {
            HolderFragment holder = new HolderFragment();
            fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
            return holder;
        }

private static HolderFragment findHolderFragment(FragmentManager manager) {
            if (manager.isDestroyed()) {
                throw new IllegalStateException("Can't access ViewModels from onDestroy");
            }

            Fragment fragmentByTag = manager.findFragmentByTag(HOLDER_TAG);
            if (fragmentByTag != null && !(fragmentByTag instanceof HolderFragment)) {
                throw new IllegalStateException("Unexpected "
                        + "fragment instance was returned by HOLDER_TAG");
            }
            return (HolderFragment) fragmentByTag;
        }

再回到上面holderFragmentFor(activity).getViewModelStore(),看getViewModelStore()方法

private ViewModelStore mViewModelStore = new ViewModelStore();

@NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        return mViewModelStore;
    }

在HolderFragment中创建的ViewModelStore,则为同一个ViewModelStore

最后再看ViewModelProviders.of(it).get(clazz.java)中get方法

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);
    }

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

        if (modelClass.isInstance(viewModel)) {
            //noinspection unchecked
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }

        viewModel = mFactory.create(modelClass);
        mViewModelStore.put(key, viewModel);
        //noinspection unchecked
        return (T) viewModel;
    }

主要看ViewModel viewModel = mViewModelStore.get(key);viewmodel的获取是通过mViewModelStore取到

 @NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull FragmentActivity activity,
            @Nullable Factory factory) {
        Application application = checkApplication(activity);
        if (factory == null) {
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(ViewModelStores.of(activity), factory);
    }

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

首先是我们从mViewModelStore取出我们想要的ViewModel. 有的话就返回 而mViewModelStore是通过ViewModelStores.of(activity)取到的当前activity中的同一个mViewModelStore通过key,则获取了我们想要的viewModel

没有的话就利用工厂模式反射生产我们所要的ViewModel对象，同时把我们的ViewModel对象放入mViewModelStore。同时返回我们的ViewModel.

由此可见,同一个activity中获取的对应的viewmodel是同一个,那么fragment中viewmodel数据自然能共享

##四.viewModel实现数据保存

主要是横屏竖屏下,activity会重启,数据会重新获取,使用viewModel时,viewModel是存在HoldFragment中的ViewModelStore中

public HolderFragment() {
        setRetainInstance(true);
    }

可见HolderFragment调用setsetRetainInstance(true),使得HolderFragment在FragmentMannager调用FindFragmentBytag,找到的是同一个HolderFragment对象（无论Activity是否重绘），这也保证了HolderFragment中的ViewModelStore（存放我们ViewModel的地方）不被销毁，然后我们取出我们所要的ViewModel,进行数据读取