作者: Insane (Insane_胖子), 時間: 2018.6 .10
在過去和今年的谷歌IO大會上，Google推出了許多新的元件架構，個人也在慢慢接觸學習這些新的元件架構。今天給大家介紹的一個能處理 Activity 和 Fragment 生命週期的元件 —— Lifecycle. 官網（科學上網）：https://developer.android.com/topic/libraries/architecture/lifecycle
簡單案例
class MainActivity : AppCompatActivity() {
private var lifeListenter:LifeListenter?=null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifeListenter= LifeListenter()
lifecycle.addObserver(lifeListenter!!)
}
}
複製程式碼open class LifeListenter : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate(owner: LifecycleOwner) {
Log.e("TAG-----CREATE", "CREATE")
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
fun onStart(owner: LifecycleOwner) {
Log.e("TAG-----START", "START")
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume(owner: LifecycleOwner) {
Log.e("TAG-----RESUME", "RESUME")
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun onDestroy(owner: LifecycleOwner) {
Log.e("TAG-----DESTROY", "DESTROY")
}
}
複製程式碼列印的Log:
06-03 22:20:00.242 20096-20096/com.lifecycle.lifecycle E/TAG-----CREATE: CREATE
06-03 22:20:00.242 20096-20096/com.lifecycle.lifecycle E/TAG-----START: START
06-03 22:20:00.252 20096-20096/com.lifecycle.lifecycle E/TAG-----RESUME: RESUME
//關掉APP
06-03 22:21:29.822 20723-20723/com.lifecycle.lifecycle E/TAG-----DESTROY: DESTROY
複製程式碼可以看出來，我們可以清楚的知道 actvity 的生命週期，並且只寫了 一行程式碼 lifecycle.addObserver(lifeListenter!!)。
那麼有人要就會問，這個元件能幹嘛？
學會了這個之後，就不需要再去寫那些類似呼叫並且會導致 onCreate() 和 onDestroy() 臃腫的方法。 例如：那些需要在 onCreate 方法中初始化，在 onStop 進行一些處理，onDestoty 進行釋放的操作。（MVP 中的 Presenter,MediaPlayerd等）。
使用：
首先需要知道三個關鍵的東西：
1.LifecycleOwner
生命週期的事件分發者，在 Activity/Fragment 他們的生命週期發生變化的時候 發出相應的 Event 給LifecycleRegistry。
2.LifecycleObserver
生命週期的觀察者，通過註解將處理函式與希望監聽的Event繫結,當相應的Event發生時,LifecycleRegistry會通知相應的函式進行處理。
3.LifecycleRegistry
控制中心。它負責控制state的轉換、接受分發event事件
通過上面的案例就可以知道，在我們需要對 生命週期進行監聽的類中
1.繼承自 LifecycleObserver
2.在類的方法上 加上註解 @OnLifecycleEvent(Lifecycle.Event.XXXX)
3.在類的方法加上 LifecycleOwner 作為引數
4.在 Activity/Fragment 中 例項化類後 加上 getLifecycle().addObserver(xxx)/Kotlin是（ lifecycle.addObserver(xxxx)）;
複製程式碼其中類的註解方法有如下：
public enum Event {
/**
* Constant for onCreate event of the {@link LifecycleOwner}.
*/
ON_CREATE,
/**
* Constant for onStart event of the {@link LifecycleOwner}.
*/
ON_START,
/**
* Constant for onResume event of the {@link LifecycleOwner}.
*/
ON_RESUME,
/**
* Constant for onPause event of the {@link LifecycleOwner}.
*/
ON_PAUSE,
/**
* Constant for onStop event of the {@link LifecycleOwner}.
*/
ON_STOP,
/**
* Constant for onDestroy event of the {@link LifecycleOwner}.
*/
ON_DESTROY,
/**
* An {@link Event Event} constant that can be used to match all events.
*/
ON_ANY
}
複製程式碼原理分析：
既然他這麼方便就能處理 Activity/Fragment 的生命週期，我們肯定要去知道為什麼他可以這樣的。
//入口：
lifecycle.addObserver(lifeListenter!!)
點選 addObserver 發現他是 Lifecycle 一個抽象方法。
點選 lifecycle 。
複製程式碼//跟進
@Override
public Lifecycle getLifecycle() {
return super.getLifecycle();
}
//繼續跟進....
在 SupportActivity 中
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
由： LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
可以知道 mLifecycleRegistry 是 LifecycleRegistry 的物件
複製程式碼跟進 LifecycleRegistry ：
//在這個類中他繼承自 Lifecycle ，所以 Activirty 的addObserver 實際是 LifecycleRegistry 的
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//把帶著狀態的 observer 封裝成 ObserverWithState
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--;
}
複製程式碼跟進看一下 ObserverWithState
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//getCallback() 通過不同的 observer 放回不同的 mLifecycleObserver
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
複製程式碼跟進 getCallbak()
@NonNull
static GenericLifecycleObserver getCallback(Object object) {
if (object instanceof FullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
}
if (object instanceof GenericLifecycleObserver) {
return (GenericLifecycleObserver) object;
}
final Class<?> klass = object.getClass();
//跟進（1）這個方法，看下面的跟進
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
//這裡是在獲取到構造方法後，根據數量建立不同的 GeneratedAdapteObserver
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);
}
（1）.一直跟進到這裡，可以發現上面我們跟進的方法就是通過反射，獲取構造方法
@Nullable
private static Constructor<? extends GeneratedAdapter> generatedConstructor(Class<?> klass) {
try {
Package aPackage = klass.getPackage();
String name = klass.getCanonicalName();
final String fullPackage = aPackage != null ? aPackage.getName() : "";
final String adapterName = getAdapterName(fullPackage.isEmpty() ? name :
name.substring(fullPackage.length() + 1));
@SuppressWarnings("unchecked") final Class<? extends GeneratedAdapter> aClass =
(Class<? extends GeneratedAdapter>) Class.forName(
fullPackage.isEmpty() ? adapterName : fullPackage + "." + adapterName);
Constructor<? extends GeneratedAdapter> constructor =
aClass.getDeclaredConstructor(klass);
if (!constructor.isAccessible()) {
constructor.setAccessible(true);
}
return constructor;
} catch (ClassNotFoundException e) {
return null;
} catch (NoSuchMethodException e) {
// this should not happen
throw new RuntimeException(e);
}
}
------------------------------------------------------
我們繼續跟進其中一個 GeneratedAdapte 以：SingleGeneratedAdapterObserver 為例
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class SingleGeneratedAdapterObserver implements GenericLifecycleObserver {
private final GeneratedAdapter mGeneratedAdapter;
SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) {
mGeneratedAdapter = generatedAdapter;
}
//這裡有個 onStateChanged 方法，用於狀態改變
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
mGeneratedAdapter.callMethods(source, event, false, null);
mGeneratedAdapter.callMethods(source, event, true, null);
}
}
//讓我們回到 ObserverWithState 中，可以知道 這裡的 onStateChanged 就是我們上面生成的 SingleGeneratedAdapterObserver 裡面的方法。
void dispatchEvent(LifecycleOwner owner, Event event) {
//得到 State
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
複製程式碼總結一下： acitivty 中呼叫 LifecycleRegistry 的 ddObserver (實際是 LifecycleRegistry 的) LifecycleObserver 被封裝成了 ObserverWithState 並存入我們集合中， 在 ObserverWithState 中，呼叫了 Lifecycling.getCallback(observer) getCallback(observer) 呼叫了createGeneratedAdapter() createGeneratedAdapter通過構造方法的數量,反射生成了不同的 GenericLifecycleObserver 物件 在ObserverWithState的dispatchEvent()方法呼叫了GenericLifecycleObserver 物件的onStateChanged方法，
回到 SuppertActivity 中
@Override
@SuppressWarnings("RestrictedApi")
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
//可以知道這裡建立了 ReportFragment
複製程式碼//在 ReportFragment 中 可以發現他和我們的 Activity 關聯,並且在生命週期方法中都掉去分發一個 event,跟進一下
private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
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();
}
}
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
private ActivityInitializationListener mProcessListener;
private void dispatchCreate(ActivityInitializationListener listener) {
if (listener != null) {
listener.onCreate();
}
}
private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
listener.onStart();
}
}
private void dispatchResume(ActivityInitializationListener listener) {
if (listener != null) {
listener.onResume();
}
}
@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;
}
複製程式碼 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);
}
}
}
//而 handleLifecycleEvent 方法是在 LifecycleRegistry 去呼叫並且會去執行
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);//獲取下一個state
moveToState(next);
}
複製程式碼分析之前 看一下官網的圖：
什麼意思呢 大概意思就是可以通過不同的 events 知道不同的 States 。例如：當你的 events 是 ON_RESUME 的時候就代表他當前的 State 是 STARTE 所以 next State 就是 RESUMED, 分析 getStateAfter：
//對照著表其實很簡單， events 是 ON_CREATE 和 ON_STOP 的下一個狀態指向都是 CREATED，所以返回 CREATED，其他的類似。
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;
}
複製程式碼所以 getStateAfter 獲取的是下一個 State 也就是下一個生命週期。 那麼 moveToState(next); 呢？
rivate void moveToState(State next) {
//判斷 當前 state 是不是 等於我們上面獲取的
if (mState == next) {
return;
}
//不是 重新賦值
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
//然後同步
sync();
mHandlingEvent = false;
}
複製程式碼 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;
}
// 當前的 State 和最早的 State 或最新的 State 進行比較。
// mObserverMap 這個集合存放的是 ObserverWithState ，而 ObserverWithState 有我們的狀態
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;
}
複製程式碼//第一個while循壞遍歷我們儲存觀察者的集合,
//第二個是要處理各個狀態經過的event
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);
//upEvent 返回所要經歷的event
//例如：當前是 STARTED , 那麼他的經過的 events 就是 ON_RESUME
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
複製程式碼 private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
// downEvent 返回下一個 events
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
複製程式碼這兩個都會執行 observer.dispatchEvent （），而這個方法就會去改變 State ,從而我們就知道了生命週期的變化。附上一個簡單的流程圖：