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())
}
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")
}
}
}
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;
}
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;
}
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();
}
}
}
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);
}
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();
}
}
}
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
到此这篇关于Android Jetpack库剖析之Lifecycle组件篇的文章就介绍到这了,更多相关Android Lifecycle内容请搜索编程网以前的文章或继续浏览下面的相关文章希望大家以后多多支持编程网！
