1.使用
Lifecycle框架用来感知组件执行操作以响应另一个组件（例如Activity和Fragment）的生命周期状态的更改。这些组件可帮助您生成更易于组织且通常更轻量级的代码，这些代码更易于维护。 举个例子： 如果你需要在Activity或者Fragment的声明周期里做一些处理：
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
presenter.onCreate()
}
override fun onStart(){
super.onStart()
presenter.onStart()
}
...
override fun onDestroy(){
super.onDestroy()
presenter.onDestroy()
}
复制代码这种写法需要在Activity或者Fragment的相应生命周期函数里调用presenter的相应方法，这种写法显得比较厚重，不够轻盈也没有解耦presenter和Activity，lifecycle的出现解决了这个问题
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycle.addObserver(Presenter())
}
}
class Presenter():LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate(){
println("presenter oncreate")
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
fun onStart(){
println("presenter onstart")
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResumse(){
println("presenter on resume")
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
fun onPause(){
println("presenter on pause")
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
fun onStop(){
println("presenter on stop")
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun onDestroy(){
println("presenter on destroy")
}
复制代码这样Presenter和Activity的关系就由关联关系转变为依赖关系，降低了耦合度，同时Presenter现在也可以用来感知Activity相应的生命周期，不用再在Activity的声明周期函数里调用Presenter的方法。
2.分析
2.1观察者模式
通过接口和方法的名字大家就可以发现整个Lifecycle其实就是一个观察者模式，下面就是一个观察者模式的UML类图
2.2生命周期感知
Lifecycle是一个生命周期感知的框架，那么它是如何感知Activity的生命周期呢？看SupportActivity的onCretae方法
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
复制代码在这里首先调用了ReportFragment.injectIfNeededIn(this)方法，看这个方法
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();
}
}
复制代码很简单，就是在当前的Activity里添加一个ReportFragment。再看ReportFragment的生命周期函数。
@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);
}
}
}
复制代码你会发现都调用了dispatch方法，而dispatch方法则会判断Activity是否实现了LifecycleOwner接口，如果实现了该接口就调用LifecycleRegister的handleLifecycleEvent，这样生命周期的状态就会借由LifecycleRegistry通知给各个LifecycleObserver从而调用其中对应Lifecycle.Event的方法。这种通过Fragment来感知Activity生命周期的方法其实在Glide的中也是有体现的。这是Activity的声明周期感知，那么Fragment呢？看一下Fragment源码你会发现Fragment也实现了LifecycleOwner接口，也关联了一个LifecycleRegistry对象
void performCreate(Bundle savedInstanceState) {
if (this.mChildFragmentManager != null) {
this.mChildFragmentManager.noteStateNotSaved();
}
this.mState = 1;
this.mCalled = false;
this.onCreate(savedInstanceState);
this.mIsCreated = true;
if (!this.mCalled) {
throw new SuperNotCalledException("Fragment " + this + " did not call through to super.onCreate()");
} else {
this.mLifecycleRegistry.handleLifecycleEvent(Event.ON_CREATE);
}
}
复制代码看它的performCreate里调用了mLifecycleRegistry.handleLifecycleEvent，同理在performStart和performResume方法里也调用了。这里其实有个有趣的现象就是在ReportFragment中dispatch(Lifecycle.Event.ON_CREATE)这个方法是在onActivityCreated里调用的，而fragment的Lifecycle.Event.ON_CREATE的事件的发送是在performCreate里调用的，这就有个问题了，如果你在Activity里的onCreate方法里添加Fragment，同时这个Fragment也添加了LifecycleObserver的监听，那么谁先监听到呢？ 举个例子：
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycle.addObserver(Presenter())
supportFragmentManager.beginTransaction().replace(R.id.container,MainFragment()).commit()
}
}
class Presenter:LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate(){
println("presenter oncreate")
}
class MainFragment:Fragment() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
lifecycle.addObserver(FragmentPresenter())
}
}
class FragmentPresenter:LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate(){
println("presenter fragment oncreate")
}
}
复制代码运行程序你会发现打印结果是presenter oncreate，然后才是presenter fragment oncreate 这就很疑惑了，我们知道fragment的声明周期onCreate是优先于onActivityCreated的，为什么没有先执行FragmentPresenter的onCreate方法，其实原因就在commit是异步的，提交fragment事物后会在何时的时机调用这个事物，所以此时MainFragment还没有被加入，但是ReportFragment调用了manager.executePendingTransactions()这个方法，这个方法会立即执行现有事物，所以这样ReportFragment就已经添加进FragmentManager了，所以出现了生命周期不对应的情况，如果你在MainActivity里MainFragment commit之后也添加manager.executePendingTransactions()，你就会发现打印结果是倒过来的。
2.3LifecycleRegister分析
LifecycleRegister是重点了先看addObserver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--;
}
复制代码这里你会发现生成了一个ObserverWithState，然后放入FastSafeIterableMap里，这个类 是一个自定义列表，用于保存观察者并可在遍历期间处理删除/添加。 观察者1和观察者2的任何时刻：如果addition_order（observer1）<addition_order（observer2），那么 state（observer1）> = state（observer2） 接下来看handleLifecycleEvent方法
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
复制代码首先会根据传入的Event得到相应的State，具体的对象关系在下图
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
+ "new events from it.");
return;
}
while (!isSynced()) {
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
复制代码这里可以看到如果ObserverWithState的state小于当前state，那么就调用forwardPass方法，如果大于当前state，那么就调用backwardPass方法，ObserverWithState默认的State是在addObserver方法里确定的
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
复制代码接着看forwardPass方法
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
复制代码这里调用了ObserverWithState的dispatchEvent方法，看一下这个方法
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
复制代码真相大白，最终调用了 mLifecycleObserver.onStateChanged(owner, event);同时更新了自己的state，这个mLifecycleObserver是怎么得到的，它又具体做了什么，就不具体说了，有兴趣的可以进去看一看，本片文章中的调用方法会生成一个ReflectiveGenericLifecycleObserver对象，这个对象会持有Presenter中声明了@OnLifecycleEvent注解的方法的Method对象，然后最终调用。