目录
- 提纲
- 什么是Lifecycle
- 如何使用Lifecycle
- 关系梳理
- Activity是如何实现Lifecycle的
- CompatActivity
- AppCompatActivity
- Fragment是如何实现Lifecycle的
- Lifecycle是如何下发宿主生命周期给观察者的
提纲
1,什么是Lifecycle?
2,如何使用Lifecycle?
3,LifecycleOwner,Lifecycle,LifecycleObserver之间是什么关系?
3,Activity是如何实现Lifecycle的?
4,Fragment是如何实现Lifecycle的?
5,Lifecycle是如何下发宿主生命周期给观察者的?
什么是Lifecycle
Lifecycle是Jetpack组件库中的架构组件,顾名思义就是一个生命周期组件,它可感知宿主的生命周期,并根据生命周期反推出生命周期所属的状态下发给观察者。
如何使用Lifecycle
1,实现其生命周期回调接口,成为生命周期观察者
2,在Activity/Fragment中获取Lifecycle实例并添加观察者
3,实例代码如下,个人比较推荐第一种方式,第二种方式比较繁琐,需要在方法上通过注解来表明想要观察的生命周期事件
class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { //订阅生命周期 lifecycle.addObserver(MyLifecycleEventObserver()) lifecycle.addObserver(MyLifecycleObserver()) } /** * 方式一(个人比较推荐) */ class MyLifecycleEventObserver : LifecycleEventObserver { override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) { when (event) { Lifecycle.Event.ON_CREATE -> println("onCreate") Lifecycle.Event.ON_START -> println("onStart") Lifecycle.Event.ON_RESUME -> println("onResume") Lifecycle.Event.ON_PAUSE -> println("onPause") Lifecycle.Event.ON_STOP -> println("onStop") Lifecycle.Event.ON_DESTROY -> println("onDestroy") } } } /** * 方式二 */ class MyLifecycleObserver : LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) fun onCreate(event: Lifecycle.Event) { println("onCreate") } @OnLifecycleEvent(Lifecycle.Event.ON_START) fun onStart(event: Lifecycle.Event) { println("onStart") } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) fun onResume(event: Lifecycle.Event) { println("onResume") } } }
关系梳理
LifecycleOwner,Lifecycle,LifecycleObserver之间是什么关系?
1,LifecycleOwner:生命周期持有者,我们的Activity/Fragment都实现了这个接口并重写了它的抽象方法getLicycle()返回一个Licycle实例。
2,Lifecycle:LifecycleRegsitry是它的唯一实现类,主要用来负责注册观察者,下发宿主状态给观察者
3,LicycleObserver:是一个接口,主要用来接收宿主的生命周期状态,实现该接口即可成为一个生命周期观察者
4,他们之间的持有关系如下图:
Activity是如何实现Lifecycle的
CompatActivity
如果我们的Activity是继承自CompatActivity,那么CompatActivity需要在Activity上添加一个ReportFragment来实现生命周期下发
(1)在CompatActivity中创建LifecycleRegistry类型的成员变量mLifecycleRegistry
(2)在CompatActivity的onCreate()方法中往Activity中添加一个ReportFragment来下发命周期
public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner, OnBackPressedDispatcherOwner { //创建Lifecycle实例 private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); mSavedStateRegistryController.performRestore(savedInstanceState); //往Activity添加一个ReportFragment来达到下发生命周期的目的 ReportFragment.injectIfNeededIn(this); if (mContentLayoutId != 0) { setContentView(mContentLayoutId); } } }
(3) 将Fragment与Activity进行绑定,添加到Activity中,用于感知Activity生命周期变化
(4)当Activity生命周期发生变化,对应的生命周期回调方法被调用,下发生命周期给观察者
public class ReportFragment extends Fragment { public static void injectIfNeededIn(Activity activity) { //往Activity中存放一个ReportFragment 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(); } } @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; } /** * 下发生命周期事件 */ private void dispatch(Lifecycle.Event event) { //获取Activity的Lifecycle实例,下发生命周期事件 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); } } } }
AppCompatActivity
如果我们的Activity是继承自AppCompatActivity ,不需要往Activity中添加一个ReportFragment来感知生命周期并下发生命周期事件,AppCompatActivity 继承自FragmentActivity,下发生命周期事件都在FragmentActivity的生命周期回调方法中进行
(1)创建LifecycleRegistry类型的变量mFragmentLifecycleRegistry
(2)在其生命周期回调方法中调用mFragmentLifecycleRegistry的handlerLifecycleEvent()方法进行下发生命周期事件
public class FragmentActivity extends ComponentActivity implements ActivityCompat.OnRequestPermissionsResultCallback, ActivityCompat.RequestPermissionsRequestCodeValidator { //创建Lifecycle实例 final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this); @Override protected void onCreate(@Nullable Bundle savedInstanceState) { //下发生命周期事件 mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); } @Override protected void onDestroy() { super.onDestroy(); mFragments.dispatchDestroy(); //下发生命周期事件 mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY); } @Override protected void onPause() { super.onPause(); mResumed = false; mFragments.dispatchPause(); //下发生命周期事件 mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE); } protected void onResumeFragments() { //下发生命周期事件 mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME); mFragments.dispatchResume(); } }
Fragment是如何实现Lifecycle的
其实Fragment实现Lifecycle,下发其生命周期的操作跟AppCompatActivity 是一样的套路,在其生命周期回调方法中进行生命周期事件下发
(1)创建Fragment时调用initLifecycle()方法给LifecycleRegistry类型的mLifecycleRegistry变量赋值
(2)在其生命周期回调方法中调用mLifecycleRegistry的handlerLifecycleEvent()方法下发生命周期事件给观察者
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner, ViewModelStoreOwner, SavedStateRegistryOwner { LifecycleRegistry mLifecycleRegistry; public Fragment() { initLifecycle(); } private void initLifecycle() { //创建Lifecycle实例 mLifecycleRegistry = new LifecycleRegistry(this); } void performCreate(Bundle savedInstanceState) { onCreate(savedInstanceState); //下发生命周期事件给观察者 mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); } void performStart() { onStart(); //下发生命周期事件给观察者 mChildFragmentManager.dispatchStart(); } void performResume() { onResume(); //下发生命周期事件给观察者 mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME); } }
Lifecycle是如何下发宿主生命周期给观察者的
(1)在我们调用Lifecycle的addObserver()方法时就已经开始下发生命周期事件了,接下来让我们先从addObserver()这个入口看看其实现逻辑,大致逻辑如下
public class LifecycleRegistry extends Lifecycle { @Override public void addObserver(@NonNull LifecycleObserver observer) { //首次添加观察者,如果宿主状态不是DESTROYED,那么观察者初始状态都是INITIALIZED State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; //把传进去的生命周期观察者以及初始状态包装成ObserverWithState对象 ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); //把包装好的ObserverWithState对象存放到观察者集合中,如果此前已经添加过则会返回此前添加的值,否则返回null ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); //判断此前是否添加过,如果添加过则直接结束方法 if (previous != null) { return; } //获取宿主实例 LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); if (lifecycleOwner == null) { // it is null we should be destroyed. Fallback quickly return; } //计算观察者的目标状态 State targetState = calculateTargetState(observer); //循环比对观察者的状态和宿主的状态,如果没有对齐则下发对应的生命周期事件 //拿观察者的状态和宿主的状态做比较 如果小于0代表状态还没有对齐,需要继续下发生命周期状态给观察者 //假设是在Activity的onResume()方法中注册的观察者,那么就需要给观察者下发onCreate,onStart,onResume这些事件 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); } } }
(2)分析完addObserver()做了哪些事,那么我们再来分析下handlerLifecycleEvent()方法做了什么事
public class LifecycleRegistry extends Lifecycle { public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { //根据生命周期事件推算出其状态 State next = getStateAfter(event); //移动到新状态 moveToState(next); } private void moveToState(State next) { //如果当前状态和新状态相等 结束方法 if (mState == next) { return; } //记录新状态 mState = next; //如果当前正在下发生命周期事件 或 当前正在添加观察者 结束方法 if (mHandlingEvent || mAddingObserverCounter != 0) { mNewEventOccurred = true; return; } //同步新状态给观察者 mHandlingEvent = true; sync(); mHandlingEvent = false; } private void sync() { //获取宿主实例 LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); //停止循环的条件是已经同步状态完成 或 没有观察者 while (!isSynced()) { mNewEventOccurred = false; //获取观察者集合中最先添加的那个元素 拿当前状态和观察者状态作比较 判断当前是不是向后移动状态 STARTED -> ON_STOP if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) { backwardPass(lifecycleOwner); } //获取观察者集合中最新添加的那个元素 拿当前状态和观察者状态作比较 判断当前是不是向前移动状态 STARTED -> ON_RESUME Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest(); if (!mNewEventOccurred && newest != null && mState.compareTo(newest.getValue().mState) > 0) { forwardPass(lifecycleOwner); } } mNewEventOccurred = false; } /** * 向前移动状态 */ private void forwardPass(LifecycleOwner lifecycleOwner) { Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator = mObserverMap.iteratorWithAdditions(); //遍历所有观察者 while (ascendingIterator.hasNext() && !mNewEventOccurred) { Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next(); ObserverWithState observer = entry.getValue(); //拿观察者的状态和宿主的状态做比较 如果小于0代表状态还没有对齐 //假设当前宿主在RESUMED状态 观察者在CREATED状态 则需要下发:ON_START,ON_RESUME生命周期事件 需要循环两次 while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) { pushParentState(observer.mState); //根据观察者状态反推向前移动事件 下发生命周期事件 observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState)); popParentState(); } } } /** * 通过观察者状态反推向前移动事件 */ private static Event upEvent(State state) { switch (state) { case INITIALIZED: case DESTROYED: return ON_CREATE; case CREATED: return ON_START; case STARTED: return ON_RESUME; case RESUMED: throw new IllegalArgumentException(); } throw new IllegalArgumentException("Unexpected state value " + state); } /** * 向后移动状态 */ private void backwardPass(LifecycleOwner lifecycleOwner) { Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator = mObserverMap.descendingIterator(); //遍历所有观察者 while (descendingIterator.hasNext() && !mNewEventOccurred) { Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next(); ObserverWithState observer = entry.getValue(); //拿观察者的状态和宿主的状态做比较 如果大于0代表状态还没有对齐 //假设当前观察者在RESUMED状态 宿主在DESTROYED状态 那么需要下发:ON_PAUSE,ON_STOP,ON_DESTROY这些生命周期事件 循环三次 while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) { //根据观察者状态反推出向后移动事件 Event event = downEvent(observer.mState); pushParentState(getStateAfter(event)); //下发该生命周期事件给观察者 observer.dispatchEvent(lifecycleOwner, event); popParentState(); } } } /** * 通过观察者状态反推向后移动的事件 */ private static Event downEvent(State state) { switch (state) { case INITIALIZED: throw new IllegalArgumentException(); case CREATED: return ON_DESTROY; case STARTED: return ON_STOP; case RESUMED: return ON_PAUSE; case DESTROYED: throw new IllegalArgumentException(); } throw new IllegalArgumentException("Unexpected state value " + state); } }
(3)接下来我们看看ObserverWitchState的dispatchEvent()方法是如何下发生命周期事件给观察者的,我们知道在调用Lifecycle的addObserver()方法时就把我们传进去的LifecycleObserver封装成了一个ObserverWitchState对象,并存放到生命周期观察者集合中
static class ObserverWithState { //记录当前状态 State mState; //生命周期观察者 LifecycleEventObserver mLifecycleObserver; ObserverWithState(LifecycleObserver observer, State initialState) { mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer); mState = initialState; } //下发生命周期事件给生命周期观察者 void dispatchEvent(LifecycleOwner owner, Event event) { //根据生命周期事件推算出生命周期状态 State newState = getStateAfter(event); mState = min(mState, newState); //调用生命周期观察者的onStateChanged()方法通知生命周期观察者生命周期发生变化 mLifecycleObserver.onStateChanged(owner, event); mState = newState; } } static State getStateAfter(Event event) { switch (event) { case ON_CREATE: case ON_STOP: return CREATED; case ON_START: case ON_PAUSE: return STARTED; case ON_RESUME: return RESUMED; case ON_DESTROY: return DESTROYED; case ON_ANY: break; } throw new IllegalArgumentException("Unexpected event value " + event); }
(4)到此结束,希望对读者有所帮助
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