- LiveData如何实现数据更新 https://www.jianshu.com/p/334d674cf65b
- 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<T> 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
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