android Lifecycle原始碼分析--原始碼閱讀100天（1）
1.使用
Lifecycle框架用來感知元件執行操作以響應另一個元件（例如Activity和Fragment）的生命週期狀態的更改。這些元件可幫助您生成更易於組織且通常更輕量級的程式碼，這些程式碼更易於維護。
舉個例子：
如果你需要在Activity或者Fragment的宣告週期裡做一些處理：
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
presenter.onCreate()
}
override fun onStart(){
super.onStart()
presenter.onStart()
}
...
override fun onDestroy(){
super.onDestroy()
presenter.onDestroy()
}
這種寫法需要在Activity或者Fragment的相應生命週期函式裡呼叫presenter的相應方法，這種寫法顯得比較厚重，不夠輕盈也沒有解耦presenter和Activity，lifecycle的出現解決了這個問題
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")
}
這樣Presenter和Activity的關係就由關聯關係轉變為依賴關係，降低了耦合度，同時Presenter現在也可以用來感知Activity相應的生命週期，不用再在Activity的宣告周期函式裡呼叫Presenter的方法。
2.分析
2.1觀察者模式
通過介面和方法的名字大家就可以發現整個Lifecycle其實就是一個觀察者模式，下面就是一個觀察者模式的UML類圖
在這裡插入圖片描述
那麼duiyingLifecycle又是什麼樣的呢？下面是我自己畫的一個Lifecycle的UML類圖
在這裡插入圖片描述
我們自定義的Activity都繼承自SupportActivity，可以發現SupportActivity實現了LifecycleOwner介面，同時持有實現了Lifecycle的LifecycleRegistry物件，這個物件就可以將其理解為觀察者模式中的Subject，LifecycleRegistr聚合多個LifecycleObserver，生命週期改變時通知LifecycleObserver進行相應的方法呼叫。
2.2生命週期感知
Lifecycle是一個生命週期感知的框架，那麼它是如何感知Activity的生命週期呢？看SupportActivity的onCretae方法
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
在這裡首先呼叫了ReportFragment.injectIfNeededIn(this)方法，看這個方法
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();
}
}
很簡單，就是在當前的Activity裡新增一個ReportFragment。再看ReportFragment的生命週期函式。
@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);
}
}
}
你會發現都呼叫了dispatch方法，而dispatch方法則會判斷Activity是否實現了LifecycleOwner介面，如果實現了該介面就呼叫LifecycleRegister的handleLifecycleEvent，這樣生命週期的狀態就會藉由LifecycleRegistry通知給各個LifecycleObserver從而呼叫其中對應Lifecycle.Event的方法。這種通過Fragment來感知Activity生命週期的方法其實在Glide的中也是有體現的。這是Activity的宣告週期感知，那麼Fragment呢？看一下Fragment原始碼你會發現Fragment也實現了LifecycleOwner介面，也關聯了一個LifecycleRegistry物件
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);
}
}
看它的performCreate裡呼叫了mLifecycleRegistry.handleLifecycleEvent，同理在performStart和performResume方法裡也呼叫了。這裡其實有個有趣的現象就是在ReportFragment中dispatch(Lifecycle.Event.ON_CREATE)這個方法是在onActivityCreated裡呼叫的，而fragment的Lifecycle.Event.ON_CREATE的事件的傳送是在performCreate裡呼叫的，這就有個問題了，如果你在Activity裡的onCreate方法裡新增Fragment，同時這個Fragment也添加了LifecycleObserver的監聽，那麼誰先監聽到呢？
舉個例子：
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")
}
}
執行程式你會發現列印結果是presenter oncreate，然後才是presenter fragment oncreate
這就很疑惑了，我們知道fragment的宣告週期onCreate是優先於onActivityCreated的，為什麼沒有先執行FragmentPresenter的onCreate方法，其實原因就在commit是非同步的，提交fragment事物後會在何時的時機呼叫這個事物，所以此時MainFragment還沒有被加入，但是ReportFragment呼叫了manager.executePendingTransactions()這個方法，這個方法會立即執行現有事物，所以這樣ReportFragment就已經新增進FragmentManager了，所以出現了生命週期不對應的情況，如果你在MainActivity裡MainFragment commit之後也新增manager.executePendingTransactions()，你就會發現列印結果是倒過來的。
2.3LifecycleRegister分析
LifecycleRegister是重點了先看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--;
}
這裡你會發現生成了一個ObserverWithState，然後放入FastSafeIterableMap裡，這個類
是一個自定義列表，用於儲存觀察者並可在遍歷期間處理刪除/新增。
觀察者1和觀察者2的任何時刻：如果addition_order（observer1）<addition_order（observer2），那麼 state（observer1）> = state（observer2）
接下來看handleLifecycleEvent方法
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
首先會根據傳入的Event得到相應的State，具體的物件關係在下圖
在這裡插入圖片描述
在moveToState方法裡，會呼叫sync方法
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;
}
這裡可以看到如果ObserverWithState的state小於當前state，那麼就呼叫forwardPass方法，如果大於當前state，那麼就呼叫backwardPass方法，ObserverWithState預設的State是在addObserver方法裡確定的
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
接著看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();
}
}
}
這裡呼叫了ObserverWithState的dispatchEvent方法，看一下這個方法
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
真相大白，最終呼叫了 mLifecycleObserver.onStateChanged(owner, event);同時更新了自己的state，這個mLifecycleObserver是怎麼得到的，它又具體做了什麼，就不具體說了，有興趣的可以進去看一看，本片文章中的呼叫方法會生成一個ReflectiveGenericLifecycleObserver物件，這個物件會持有Presenter中聲明瞭@OnLifecycleEvent註解的方法的Method物件，然後最終呼叫。
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