閱讀LifeCycle 的源碼,需要一定的技術(shù)基礎(chǔ)
1. 反射 市面上大部分框架基本上都使用了反射原來,而且配合反射的使用過程中還會(huì)使用 Map <class,結(jié)果> 來緩存反射后的結(jié)果,為后續(xù)加載提速,已空間換時(shí)間的方法
2. 注解 @Target @Retention 元注解等含義,要不然理解起來也是比較片面的,
3 lifeCycle觀察者模式是有狀態(tài)驅(qū)動(dòng)事件來完成工作的,具體的流程會(huì)在下面分析,但是在分析的過程你需要建立這樣的一個(gè)概念,要不然很容易給自己搞的很頭疼,至于Lifecycle 未什么要設(shè)計(jì)的這么復(fù)雜,原因就是Lifecycle是jetpack所有事件的基礎(chǔ),他在創(chuàng)建之初服務(wù)的對象就不單單是一個(gè)觀察者
如果你覺得以上問題都Ok ,那么下面就可以開始代碼的閱讀了
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycle.addObserver(MyObserve())
}
}
class MyObserve :LifecycleObserver{
@OnLifecycleEvent(Lifecycle.Event.ON_START)
fun onStart(){
}
}
源碼分析的過程就是在 lifecycle.addObserver(MyObserve()) 這里開始的, 這個(gè)方法是將觀察者 MyObserve 與被觀察者 Activity(LifecycleOwner) 建立綁定關(guān)系
建立綁定關(guān)系的代碼在LifecycleRegistry里面
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
enforceMainThreadIfNeeded("addObserver");
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);
final Event event = Event.upFrom(statefulObserver.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + statefulObserver.mState);
}
statefulObserver.dispatchEvent(lifecycleOwner, event);
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
這里我們要關(guān)注 他拿到了LifecycleObserver observer 干了什么,所以就要跟著 observer 來繼續(xù)分析 ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); 這里的代碼
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
將observer 通過 Lifecycling.lifecycleEventObserver 轉(zhuǎn)換成了 LifecycleEventObserver mLifecycleObserver, 繼續(xù)分析
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}
final Class<?> klass = object.getClass();
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
if (constructors.size() == 1) {
GeneratedAdapter generatedAdapter = createGeneratedAdapter(
constructors.get(0), object);
return new SingleGeneratedAdapterObserver(generatedAdapter);
}
GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
for (int i = 0; i < constructors.size(); i++) {
adapters[i] = createGeneratedAdapter(constructors.get(i), object);
}
return new CompositeGeneratedAdaptersObserver(adapters);
}
return new ReflectiveGenericLifecycleObserver(object);
}
這里對這個(gè)object 做了一下分析, 由于我們傳入的是一個(gè)LifecycleObserver ,都不滿足 return 條件,就看下面的解析的過程,看到下面的代碼,如果你研究過注解和反射就知道, 下面代碼的含義就是在運(yùn)行時(shí)通過class 反射獲取所有代碼注解的方法,在通過注解獲取元注解的事件來做最后關(guān)聯(lián),將他們包裝成LifecycleEventObserver 最后返回,到這里解析這條鏈路就結(jié)束了,在準(zhǔn)備知識的過程中,提到過配合反射 大部分框架都會(huì)使用map 來做緩存,我們可以回到 observer 方法中的繼續(xù)看一下下面代碼
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
可以看到了這個(gè)里面有,不僅這里面有,framework 中也包含了非常多的這種用法
observer 的后續(xù)代碼就是就是通過狀態(tài)來判斷是否需要同步狀態(tài),講到這里就要講一下lifecycle 的狀態(tài)與事件的機(jī)制
至于這個(gè)流程是如何來的呢,看下面代碼
@NonNull
public State getTargetState() {
switch (this) {
case ON_CREATE:
case ON_STOP:
return State.CREATED;
case ON_START:
case ON_PAUSE:
return State.STARTED;
case ON_RESUME:
return State.RESUMED;
case ON_DESTROY:
return State.DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException(this + " has no target state");
}
可以看到 ON_CREATE ON_STOP 執(zhí)行后,得到的都是 State.CREATED 這個(gè)狀態(tài) ON_START與 ON_PAUSE 得到的都是 State.STARTED 這個(gè)狀態(tài),如果你想明白了這里,其實(shí)addObserver方法后續(xù)同步狀態(tài)的代碼塊的路基就明白了,
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
enforceMainThreadIfNeeded("addObserver");
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);
final Event event = Event.upFrom(statefulObserver.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + statefulObserver.mState);
}
statefulObserver.dispatchEvent(lifecycleOwner, event);
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
在創(chuàng)建之初的的狀態(tài)是INITIALIZED ,沒有之前的同步狀態(tài)結(jié)束,如果存在綁定出現(xiàn)狀態(tài)不一致,就會(huì)啟動(dòng)的他下年的While 判斷,至于為什么用while,如果狀態(tài)差大于1 ,就會(huì)同步多次, 這個(gè)情況大概會(huì)出現(xiàn)在on Start 等狀態(tài)中綁定,會(huì)就存在狀態(tài)不一致的情況
到了這里綁定的流程就完成了, 下面分析時(shí)間分發(fā)的流程
事件分發(fā)的流程
事件分發(fā)是從 ComponentActivity 這個(gè)activity 中的onCreate 方法里面開始,這里比較有意思的是他的調(diào)用過程也借鑒了Glide 的方式,給這個(gè)Activity 添加了一個(gè)沒有頁面的activity 來感知activity 的生命周期狀態(tài),
ComponentActivity . onCreate
protected void onCreate(@Nullable Bundle savedInstanceState) {
// Restore the Saved State first so that it is available to
// OnContextAvailableListener instances
mSavedStateRegistryController.performRestore(savedInstanceState);
mContextAwareHelper.dispatchOnContextAvailable(this);
super.onCreate(savedInstanceState);
mActivityResultRegistry.onRestoreInstanceState(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
ReportFragment injectIfNeededIn
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
LifecycleCallbacks.registerIn(activity);
}
// Prior to API 29 and to maintain compatibility with older versions of
// ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
// need to support activities that don't extend from FragmentActivity from support lib),
// use a framework fragment to get the correct timing of Lifecycle events
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();
}
}
這么做的優(yōu)勢是什么?
1.代碼分離
- 如果老版本activity 想要實(shí)現(xiàn)這個(gè)功能,可以主動(dòng)添加這個(gè)fragment,后續(xù)功能自動(dòng)封裝實(shí)現(xiàn)
知道了消息是有誰來發(fā)出,我們就來跟蹤一個(gè) onStart 事件看看接下來的流程
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}
這里是發(fā)送事件
private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
// Only dispatch events from ReportFragment on API levels prior
// to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
// added in ReportFragment.injectIfNeededIn
dispatch(getActivity(), event);
}
}
判斷了不同的系統(tǒng)版本的事件分發(fā)方式
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
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);
}
}
}
根據(jù)不同的角色來進(jìn)行不同事件分發(fā),我們查看的ComponentActivity 實(shí)現(xiàn)了LifecycleOwner,看下面的代碼
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
enforceMainThreadIfNeeded("handleLifecycleEvent");
moveToState(event.getTargetState());
}
檢查了一下就開始狀態(tài)的切換了,
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;
}
這里判斷了一下是否是重復(fù)事件,也判斷了一下是否正在處理事件,或者沒有觀察值,就不執(zhí)行下面代碼了,
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);
}
Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
這里就是事件和狀態(tài)容易讓人搞亂的地方,他是根據(jù)當(dāng)前狀態(tài)和上一個(gè)狀態(tài)來判斷當(dāng)前是前進(jìn)操作還是后退操作,
而從while 這個(gè)操作來看的話,這個(gè)狀態(tài)是一步一步來操作的, 如果 觀察者現(xiàn)在狀態(tài)是2,而被觀察現(xiàn)在的狀態(tài)是5,那么觀察者的狀態(tài)變化是從 2-->3-->4-->5,這種設(shè)計(jì)模式是為了更好的適配其他jetpack,對外提供狀態(tài)
接下來就是事件分發(fā)
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
就又回到了我們綁定觀察者分析時(shí)候的方法,最后event 流向了 mLifecycleObserver.onStateChanged(owner, event); 方法
class CompositeGeneratedAdaptersObserver implements LifecycleEventObserver {
private final GeneratedAdapter[] mGeneratedAdapters;
CompositeGeneratedAdaptersObserver(GeneratedAdapter[] generatedAdapters) {
mGeneratedAdapters = generatedAdapters;
}
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
MethodCallsLogger logger = new MethodCallsLogger();
for (GeneratedAdapter mGenerated: mGeneratedAdapters) {
mGenerated.callMethods(source, event, false, logger);
}
for (GeneratedAdapter mGenerated: mGeneratedAdapters) {
mGenerated.callMethods(source, event, true, logger);
}
}
}
到 CompositeGeneratedAdaptersObserver 后 找到反射后的方法 cellMethods 方法就完成了
