- LiveData 如何实现数据更新
- LiveData 如何实现同 activity 申明周期绑定
- viewModel 如何实现数据共享
- viewModel 如何实现数据保留
本文就如上问题联合 aac 框架源码进行逐渐解析 ## 一.LiveData 实现数据更新 既然是监测数据更新, 必定是应用到观察者模式
- 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;
}
}
}
咱们来看看 Lifecycle 实现类 LifecycleRegistry,
次要看增加观察者以及接管被观察者, 对应如下两个办法
@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;
}
}
次要是 Lifecycling.getCallback(observer) 获取 GenericLifecycleObserver, 在 dispatchEvent 执行 onStateChanged() 告诉数据更新, 而 dispatchEvent 办法在 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);
}
首先会创立 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 办法
接着 owner.getLifecycle().addObserver(wrapper), 获取 Lifecycle, 调用到下面解析的 addObserver, 而这里的 wrapper 为 GenericLifecycleObserver, 那么 Lifecycling.getCallback(observe) 间接取到 GenericLifecycleObserver, 则执行 addObserver 中 dispatchEvent, 即调用到 onStateChanged, 此时 LifecycleBoundObserver 中 onStateChanged
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) 此判断解决内存透露问题
@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, 次要看 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;
}
这里次要看 considerNotify,initiator 不为 null, 间接调用
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 申明周期
次要看 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 办法创立了 ReportFragment, 通过治理 ReportFragment 的申明周期,dispatch(Lifecycle.Event.ON\_PAUSE); 将申明周期枚举进行散发
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);
}
}
}
最初 event 赋值给了 LifecycleRegistry 中 mState, 并通过 getCurrentState 获取
则 Lifecycle 通过 getCurrentState() 能够取到以后 activity 的申明周期了
看看为什么不会导致内存透露
@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 都有判断以后 activity 是否 destroyed, 由此可见,activity 销毁了, 并不会执行数据更新, 天然不会有内存透露
三.viewModel 如何实现数据共享
次要是同一个 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, 数据天然能共享, 咱们来一步步解析
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 雷同 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, 咱们来看 ViewModelProviders.of(it).get(clazz.java) 外面做了一些什么
@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 是动态外部类 HolderFragmentManager, 而 HolderFragmentManager 是饿汉式创立的单例
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 不为空间接返回, 如果为空通过 mNotCommittedActivityHolders.get(activity), 如果 HolderFragment 为空, 通过 createHolderFragment() 创立 ,tag 为 HOLDER\_TAG, 并办法 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;
}
再回到下面 holderFragmentFor(activity).getViewModelStore(), 看 getViewModelStore() 办法
private ViewModelStore mViewModelStore = new ViewModelStore();
@NonNull
@Override
public ViewModelStore getViewModelStore() {return mViewModelStore;}
在 HolderFragment 中创立的 ViewModelStore, 则为同一个 ViewModelStore
最初再看 ViewModelProviders.of(it).get(clazz.java) 中 get 办法
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;
}
次要看 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;
}
首先是咱们从 mViewModelStore 取出咱们想要的 ViewModel.
有的话就返回 而 mViewModelStore 是通过 ViewModelStores.of(activity) 取到的以后 activity 中的同一个 mViewModelStore 通过 key, 则获取了咱们想要的 viewModel
没有的话就利用工厂模式反射生产咱们所要的 ViewModel 对象，同时把咱们的 ViewModel 对象放入 mViewModelStore。同时返回咱们的 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 数据天然能共享
四.viewModel 实现数据保留
次要是横屏竖屏下,activity 会重启, 数据会从新获取, 应用 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），这也保障了 HolderFragment 中的 ViewModelStore（寄存咱们 ViewModel 的中央）不被销毁，而后咱们取出咱们所要的 ViewModel, 进行数据读取
相干教程
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