LifeCycle用于幫助開發(fā)者管理Activity和Fragment 的生命周期送挑。
介紹Lifecycle之前绑莺，首先需要了解觀察者模式。
觀察者模式首先需要被觀察者和被觀察者惕耕，被觀察者里面可以添加觀察者和刪除觀察者纺裁，以及有事件的時候通知觀察的方法。而觀察者則事收到事件的回調(diào)之后司澎，即做出相應(yīng)的處理欺缘。
而Lifecycle則是典型的觀察者模式。
LifecycleOwner可以理解為被觀察者挤安。而它所有的處理全部在LifecycleRegistry中谚殊。
那那些又可以當(dāng)作被觀察者了？其實只要我們在一個類中取實現(xiàn)LifecycleOwner蛤铜，然后在對應(yīng)的生命周期去設(shè)置不同的Lifecycle.State嫩絮，這樣即可。
public class MyActivity extends Activity implements LifecycleOwner {
private LifecycleRegistry mLifecycleRegistry;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mLifecycleRegistry = new LifecycleRegistry(this);
mLifecycleRegistry.setCurrentState(Lifecycle.State.CREATED);
}
@Override
protected void onDestroy() {
super.onDestroy();
mLifecycleRegistry.setCurrentState(Lifecycle.State.DESTROYED);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
那同樣的道理就是我們只要實現(xiàn)了LifecycleObserver就可以作為觀察者围肥。
public class LifecycleObserverImpl implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate(){
Log.e("zzf","---------onCreate--------");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart(){
Log.e("zzf","---------onStart--------");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume(){
Log.e("zzf","---------onResume--------");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause(){
Log.e("zzf","---------onPause--------");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop(){
Log.e("zzf","---------onStop--------");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestory(){
Log.e("zzf","---------onDestory--------");
}
}
那么有個問題來了剿干，我們在觀察者上面對應(yīng)的Lifecycle.Event，而在被觀察者的是設(shè)置Lifecycle.State穆刻，那這個Event和State是怎樣關(guān)聯(lián)起來的置尔？
那下面以ComponentActivity為例來講講整個流程。
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner {
}
```LifecycleOwner
我們可以看到蛹批，ComponentActivity是實現(xiàn)了LifecycleOwner接口撰洗。
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
然后在oncreate中添加一個沒有頁面的ReportFragment，我們知道Fragment的生命周期和activity的生命周期是一致的腐芍，這讓我們想到Glide的生命周期管理好像也是采用這種方法差导。
所以接下來我們看在ReportFragment的生命周期中做了啥。
@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;
}
在生命周期中調(diào)用dispatch方法.
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
繼續(xù)調(diào)用LifecycleRegistry的handleLifecycleEvent()猪勇。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}
進(jìn)入狀態(tài)同步方法sync()设褐。
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
+ "garbage collected. It is too late to change lifecycle state.");
}
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;
}
mState.compareTo(mObserverMap.eldest().getValue().mState) < 0 從緩存的觀察者中拿出最舊的狀態(tài)，與當(dāng)前狀態(tài)進(jìn)行比較泣刹，如果小于 0助析， 說明觀察者的狀態(tài)提前于當(dāng)前狀態(tài)，那么就執(zhí)行一個backwardPass(lifecycleOwner) 方法椅您，讓觀察者的狀態(tài)回退到當(dāng)前狀態(tài)上
mState.compareTo(newest.getValue().mState) > 0 這個判斷正好和上面相反外冀，說明當(dāng)前的觀察者狀態(tài)落后于當(dāng)前狀態(tài)，那么就讓觀察者的狀態(tài)追上當(dāng)前狀態(tài)掀泳，執(zhí)行 forwardPass(lifecycleOwner)
通過分析 forwardPass 和 backwardPass 方法雪隧，我們看到其內(nèi)部又調(diào)用了兩個方法, downEvent 和 upEvent西轩，我們稱呼為升級事件和降級事件
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);
}
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);
}
在這個就回答了前面的哪個疑問，Event和state是怎樣轉(zhuǎn)換的脑沿。
在backwardPass或forwardPass方法中最終調(diào)用的是dispatchEvent方法藕畔，進(jìn)而調(diào)用mLifecycleObserver.onStateChanged(owner, event)。
前面我們說了一大堆庄拇，都是對 Lifecycle 如何處理注服，并監(jiān)聽我們的生命周期的方法，也就是 Lifecycle是如何處理監(jiān)聽的措近。那么我們前面一直都沒有說過溶弟，觀察者那里來的？？？蒜田？凭峡？？苇本？袜茧？？
好瓣窄，我們繼續(xù)觀察 LifecycleRegister 中的 addObserver(LifecycleObserver) 這個也是我們自定義觀察者后笛厦，需要調(diào)用的方法
@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--;
}
創(chuàng)建了一個 ObserverWithState ，這是一個有狀態(tài)的觀察者俺夕，從名字上我們就可以看出來裳凸。兩個參數(shù)分別是我們傳入的觀察者和我們剛剛創(chuàng)建好的狀態(tài)。這是個裝飾器模式劝贸，目的是給原對象姨谷，增加一個新屬性，但又不改變原數(shù)據(jù)結(jié)構(gòu)的方案映九。
緊接著又把這個觀察者和帶有狀態(tài)的觀察者梦湘，放進(jìn)了事先已經(jīng)創(chuàng)建好的 Map 中，也就是 mObserverMap 然后返回了個東西件甥， 閱讀后發(fā)現(xiàn)捌议，又和 Lifecycle 的注釋對上了。
之后引有，計算出當(dāng)前 Activity 或 Fragment 的狀態(tài)瓣颅，并判斷，如果當(dāng)前觀察者的狀態(tài)落后于 Activity 或 Fragment 的狀態(tài)譬正，則立即執(zhí)行dispatchEvent() 方法宫补，執(zhí)行回調(diào)檬姥。
