Android Lifecycle
猪飞啦~ 人气:0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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