Android-Lifecycle超能解析-生命週期的那些事兒
本文篇幅較長,請大家耐心閱讀。
Lifecycle是什麼?
Lifecycle是一個生命週期感知元件,一般用來響應Activity、Fragment等元件的生命週期變化,並將變化通知到已註冊的觀察者。有助於更好地組織程式碼,讓程式碼邏輯符合生命週期規範,減少記憶體洩漏,增強穩定性。
Lifecycle已經納入新版本的AppCompatActivity和Fragment中了,並且Lifecycle還是Android Jetpack中其他兩個元件LiveData和ViewModel的基礎,意味著這個庫可能將持續伴隨著我們後續的開發,因此有什麼理由不學習一番呢?
按照國際規範,我們先講使用,後解析原始碼。
一、使用
1.新增依賴:
在app或者module目錄下的build.gradle中,新增依賴:
dependencies {
......
implementation "android.arch.lifecycle:runtime:1.1.1"
implementation "android.arch.lifecycle:extensions:1.1.1"
// 如果你使用java8開發,可以新增這個依賴,裡面只有一個類
implementation "android.arch.lifecycle:common-java8:1.1.1"
}
2.實現LifecycleObserver介面
public class Java7Observer implements LifecycleObserver { private static final String TAG = Java7Observer.class.getSimpleName(); @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void onCreate() { Log.d(TAG, "onCreate"); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { Log.d(TAG, "onStart"); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) public void onResume() { Log.d(TAG, "onResume"); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void onPause() { Log.d(TAG, "onPause"); } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { Log.d(TAG, "onStop"); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) public void onDestroy() { Log.d(TAG, "onDestroy"); } }
首先,我們需要實現LifecycleObserver這個介面,然後在我們的方法上加上@OnLifecycleEvent註解,註解值表示該方法對應生命週期的哪個函式,這裡我們把所有生命週期函式都加上了,並簡單地列印了一句Log。
我想有人會問,為什麼這個class要命名Java7Observer ? 沒錯,因為Java8中,官方推薦用另外一種方式:
public class Java8Observer implements DefaultLifecycleObserver {
private static final String TAG = Java8Observer.class.getSimpleName();
@Override
public void onCreate(@NonNull LifecycleOwner owner) { Log.d(TAG, "onCreate"); }
@Override
public void onStart(@NonNull LifecycleOwner owner) { Log.d(TAG, "onStart"); }
@Override
public void onResume(@NonNull LifecycleOwner owner) { Log.d(TAG, "onResume"); }
@Override
public void onPause(@NonNull LifecycleOwner owner) { Log.d(TAG, "onPause"); }
@Override
public void onStop(@NonNull LifecycleOwner owner) { Log.d(TAG, "onStop"); }
@Override
public void onDestroy(@NonNull LifecycleOwner owner) { Log.d(TAG, "onDestroy"); }
}
理由很簡單,Lifecycle原始碼中有一段註釋:
/* .....
* </pre>
* If you use <b>Java 7 Language</b>, Lifecycle events are observed using annotations.
* Once Java 8 Language becomes mainstream on Android, annotations will be deprecated, so between
* {@link DefaultLifecycleObserver} and annotations,
* you must always prefer {@code DefaultLifecycleObserver}.
* <pre>
* ......
*/
翻譯一下就是:“如果你丫的用的是java7,那你就用註解唄,但是我告訴你,一旦java8上位了,你那些註解就過時了,我勸你最好用DefaultLifecycleObserver”
3.將LifecycleObserver新增到Lifecycle的觀察者列表
如果你使用新版本的AppcompatActivity(>26.1.0),可以這麼寫:
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 直接呼叫getLifecycle(),新增Observer
getLifecycle().addObserver(new Java7Observer());
getLifecycle().addObserver(new Java8Observer());
}
}
如果你的AppCompatActivity沒有更新到最新,那麼你需要自實現LifecycleOwner介面,並在Activity生命週期函式中分發事件
public class MainActivity extends Activity implements LifecycleOwner {
private LifecycleRegistry mLifecycleRegistry;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mLifecycleRegistry = new LifecycleRegistry(this);
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
mLifecycleRegistry.addObserver(new TestObserver());
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@Override
public void onStart() {
super.onStart();
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
}
@Override
public void onPause() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
super.onPause();
}
@Override
public void onStop() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
super.onStop();
}
@Override
public void onDestroy() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
super.onDestroy();
}
}
ok,完事兒了,咱們跑起來看一哈,已經成功和生命週期關聯上了。
能舉個熟悉點的例子嗎?
MVP想必大家都用過,或者聽說過,View層通過Presenter層和Model層進行通訊,因此Presenter層如果做了什麼超出View生命週期的事,又沒有及時釋放記憶體,就會造成記憶體洩漏。
所以,原來的Presenter,為了響應生命週期,我們可能會這麼寫:
先定義所有的生命週期函式
public class MainPresenter {
public void onCreate() { }
public void onStart() { }
public void onResume() { }
public void onPause() { }
public void onStop() { }
public void onDestroy() { }
}
然後再與Activity生命週期一一繫結,如果有多個類似的需要響應生命週期的類,Activity的生命週期函式就會變得非常臃腫
public class MainActivity extends AppCompatActivity {
private MainPresenter mPresenter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mPresenter = new MainPresenter();
mPresenter.onCreate();
}
@Override
protected void onResume() {
super.onResume();
mPresenter.onCreate();
// VideoPlayer.onResume();
// ...
}
@Override
protected void onStart() {
super.onStart();
mPresenter.onStart();
// VideoPlayer.onStart();
// ...
}
@Override
protected void onPause() {
super.onPause();
mPresenter.onPause();
// VideoPlayer.onPause();
// ...
}
@Override
protected void onStop() {
super.onStop();
mPresenter.onStop();
// VideoPlayer.onStop();
// ...
}
@Override
protected void onDestroy() {
super.onDestroy();
mPresenter.onDestroy();
// VideoPlayer.onDestroy();
// ...
}
}
因此,Lifecycle就是為了解決這個痛點,將生命週期的響應分發到各個觀察者中去,我們只需要在Activity中呼叫一行程式碼:
getLifecycle().addObserver(mPresenter);
程式碼量蹭蹭就下來了。諸如音視訊播放、圖片載入,這些吃記憶體大戶,都應該響應生命週期,及時釋放記憶體,否則很可能就會造成OOM。
二、知其然,還得知其所以然
在專案中引入一個庫,卻不懂是怎麼實現的,是一件非常危險的行為,因此我們不僅要學會怎麼用,還得知道其中的原理。
先看一張官方的圖:
可以看到兩個很顯眼的單詞:State和Event,這也是貫穿整個Lifecycle的兩個概念:狀態和事件。
Lifecycle將Activity的生命週期函式對應成State,生命週期改變,會造成State改變,而State變化將觸發Event事件,從而被LifecycleObser接收。
-
State:狀態,是Lifecycle中對應Activity生命週期的一種狀態標識,從圖中可以看到,它有INITIALIZED、DESTROYED、CREATED、STARTED、RESUMED這5中狀態。
- INITIALIZED:對應Activity的onCreate之前的生命週期
- DESTROYED:對應Activity的onDestroy
- CREATED:對應Activity的onCreate到onStop之間的生命週期
- STARTED:對應Activity的onStart到onPause之間的生命週期
- RESUMED:對應Activity的onResume
- Event:事件,當State發生變化時,Lifecycle會向已註冊的LifecycleObserver傳送事件,例如:當State從INITIALIZED變化到CREATED時,就會發出ON_CREATE事件。
因此,弄懂Lifecycle,其實也就是需要弄懂兩件事:
- State是如何與Activity/Fragment的生命週期繫結的?
- Event事件是如何分發到LifecycleObserver的?
ok,帶著這兩個問題,我們開始看原始碼:
1.從AppCompatActivity的getLifecycle()開始
直接ctrl+左鍵跟進原始碼,由於AppCompatActivity繼承自SupportActivity,我們發現,AppCompatActivity.getLifecycle(),最終定位在了SupportActivity.getLifecycle()方法中
public class SupportActivity extends Activity implements LifecycleOwner {
......
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
......
}
不難看到:
- 最終getLifecycle()返回的是一個LifecycleRegistry 物件,所以所有對LifecycleObserver的操作都是由LifecycleRegistry 完成的
- 在onCreate()方法中,我們還看到了一個ReportFragment.injectIfNeededIn(this),先做個標記,後面再說
2.LifecycleRegistry
字面意思:生命週期登記處
這個類是Lifecycle中最重要的一個類,它是Lifecycle的子類,起著新增觀察者,響應生命週期事件,分發生命週期事件的作用
public class LifecycleRegistry extends Lifecycle {
// LifecycleObserver Map,每一個Observer都有一個State
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>();
// 當前的狀態
private State mState;
// 生命週期擁有者,上述的SupportActivity繼承了LifecycleOwner
private final WeakReference<LifecycleOwner> mLifecycleOwner;
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
/**
* 新增LifecycleObserver觀察者,並將之前的狀態分發給這個Observer,例如我們在onResume之後註冊這個Observer,
* 該Observer依然能收到ON_CREATE事件
*/
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
......
// 例如:Observer初始狀態是INITIALIZED,當前狀態是RESUMED,需要將INITIALIZED到RESUMED之間的
// 所有事件分發給Observer
//
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
targetState = calculateTargetState(observer);
}
......
}
/**
* 處理生命週期事件
*/
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
/**
* 改變狀態
*/
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
......
sync();
......
}
/**
* 同步Observer狀態,並分發事件
*/
private void sync() {
LifecycleOwner lfecycleOwner = 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;
// State中,狀態值是從DESTROYED-INITIALIZED-CREATED-STARTED-RESUMED增大
// 如果當前狀態值 < Observer狀態值,需要通知Observer減小狀態值,直到等於當前狀態值
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
// 如果當前狀態值 > Observer狀態值,需要通知Observer增大狀態值,直到等於當前狀態值
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
/**
* 向前傳遞事件,對應圖中的INITIALIZED -> RESUMED
* 增加Observer的狀態值,直到狀態值等於當前狀態值
*/
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);
// 分發狀態改變事件
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
/**
* 向後傳遞事件,對應圖中的RESUMED -> DESTROYED
* 減小Observer的狀態值,直到狀態值等於當前狀態值
*/
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()))) {
Event event = downEvent(observer.mState);
// 分發狀態改變事件
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
}
我們再看看新版AppCompatActivity是如何將事件分發給LifecycleRegistry的,還記得SupportActivity中有這麼一段邏輯嗎:
public class SupportActivity extends Activity implements LifecycleOwner {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
}
我們看看ReportFragment是什麼
3.ReportFragment
public class ReportFragment extends Fragment {
public static void injectIfNeededIn(Activity activity) {
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}
// ...... 其他生命週期也是同樣呼叫了dispatch(Lifecycle.Event.xxx)分發事件
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
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);
}
}
}
}
可以看到,SupportActivity添加了一個沒有頁面的ReportFragment,在ReportFragment的生命週期函式中,呼叫了LifecycleRegistry.handleLifecycleEvent()方法來分發生命週期事件。用一張圖總結一下就是:
AppCompatActivity中添加了一個ReportFragment,其生命週期變化時,呼叫LifecycleRegistry.handleLifecycleEvent()方法通知LifecycleRegistry改變狀態,LifecycleRegistry內部呼叫moveToState()改變狀態,並呼叫每個LifecycleObserver.onStateChange()方法通知生命週期變化。
為什麼不直接在SupportActivity的生命週期函式中給Lifecycle分發生命週期事件,而是要加一個Fragment呢?
因為不是所有的頁面都繼承AppCompatActivity,為了相容非AppCompatActivity,所以封裝一個同樣具有生命週期的Fragment來給Lifecycle分發生命週期事件。
- 那我們不繼承新版本AppCompatActivity時,Lifecycle是如何通過ReportFragment來分發生命週期事件的呢?
這裡我們直接使用AndroidStudio強大的搜尋功能,alt+F7搜尋ReportFragment的呼叫者,我們發現:除了SupportActivity以外,還有兩個地方使用到了ReportFragment:LifecycleDispatcher和ProcessLifecycleOwner
4.LifecycleDispatcher
字面意思:生命週期分發者
class LifecycleDispatcher {
static void init(Context context) {
((Application) context.getApplicationContext())
.registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
}
/**
* 給所有子Fragment設定State
*/
private static void markState(FragmentManager manager, State state) {
Collection<Fragment> fragments = manager.getFragments();
if (fragments == null) {
return;
}
for (Fragment fragment : fragments) {
if (fragment == null) {
continue;
}
markStateIn(fragment, state);
if (fragment.isAdded()) {
markState(fragment.getChildFragmentManager(), state);
}
}
}
private static void markStateIn(Object object, State state) {
if (object instanceof LifecycleRegistryOwner) {
LifecycleRegistry registry = ((LifecycleRegistryOwner) object).getLifecycle();
registry.markState(state);
}
}
// 通過註冊Application.registerActivityLifecycleCallbacks來獲取Activity的生命週期回撥
static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
if (activity instanceof FragmentActivity) {
((FragmentActivity) activity).getSupportFragmentManager()
.registerFragmentLifecycleCallbacks(mFragmentCallback, true);
}
// 給每個Activity新增ReportFragment
ReportFragment.injectIfNeededIn(activity);
}
@Override
public void onActivityStopped(Activity activity) {
if (activity instanceof FragmentActivity) {
markState((FragmentActivity) activity, CREATED);
}
}
@Override
public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
if (activity instanceof FragmentActivity) {
markState((FragmentActivity) activity, CREATED);
}
}
}
}
可以看到LifecycleDispatcher是通過註冊Application.registerActivityLifecycleCallbacks來監聽Activity的生命週期回撥的。
- 在onActivityCreated()中新增ReportFragment,將Activity的生命週期交給ReportFragment去分發給LifecycleRegistry
- 在onActivityStopped()以及onActivitySaveInstanceState()中,將Activity及其所有子Fragment的State置為CREATED
5.ProcessLifecycleOwner
字面意思:執行緒生命週期擁有者
public class ProcessLifecycleOwner implements LifecycleOwner {
public static LifecycleOwner get() { return sInstance; }
static void init(Context context) {
sInstance.attach(context);
}
void activityResumed() {
mResumedCounter++;
if (mResumedCounter == 1) {
if (mPauseSent) {
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
mPauseSent = false;
} else {
mHandler.removeCallbacks(mDelayedPauseRunnable);
}
}
}
void activityPaused() {
mResumedCounter--;
if (mResumedCounter == 0) {
mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);
}
}
private ActivityInitializationListener mInitializationListener = new ActivityInitializationListener() {
@Override
public void onCreate() {}
@Override
public void onStart() {
activityStarted();
}
@Override
public void onResume() {
activityResumed();
}
};
void attach(Context context) {
mHandler = new Handler();
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
Application app = (Application) context.getApplicationContext();
app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
ReportFragment.get(activity).setProcessListener(mInitializationListener);
}
@Override
public void onActivityPaused(Activity activity) {
activityPaused();
}
@Override
public void onActivityStopped(Activity activity) {
activityStopped();
}
});
}
}
根據官方註釋,我們可以瞭解到:
- ProcessLifecycleOwner是用來監聽Application生命週期的,因此它只會分發一次ON_CREATE事件,並且不會分發ON_DESTROY事件。
- ProcessLifecycleOwner在Activity的onResume和onStop方法中都採用了Handle.postDelayed()方法,是為了處理Activity重建時比如橫豎螢幕切換時,不會發送事件。
- ProcessLifecycleOwner一般用來判斷應用是在前臺還是後臺。但由於使用了Handle.postDelayed(),因此這個判斷不是即時的,有預設700ms的延遲。
- ProcessLifecycleOwner與LifecycleDispatcher一樣,都是通過註冊Application.registerActivityLifecycleCallbacks來監聽Activity的生命週期回撥,來給每個Activity新增ReportFragment的。
那麼問題又來了,ProcessLifecycleOwner和LifecycleDispatcher兩個類是在哪裡初始化呢?
從原始碼中我們看到,他們的入口都是init(Context),繼續搜尋原始碼,我們發現:
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
@Override
public boolean onCreate() {
LifecycleDispatcher.init(getContext());
ProcessLifecycleOwner.init(getContext());
return true;
}
// ......
}
AndroidManifest.xml
<manifest xmlns:android="http://schemas.android.com/apk/res/android">
......
<provider
android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer"
android:authorities="me.baron.achitecturelearning.lifecycle-trojan"
android:exported="false"
android:multiprocess="true" />
</manifest>
瞭然,Lifecycle自動在我們的AndroidManifest.xml中添加了一個ContentProvider,用於初始化ProcessLifecycleOwner和LifecycleDispatcher,這麼做的好處是,不需要我們在Application中顯示呼叫,不需要我們寫一行程式碼。
總結
回到我們最初的兩個問題:
- State是如何與Activity/Fragment的生命週期繫結的?
在Activity中新增一個ReportFragment(如果你的Activity繼承AppCompatActivity,會在父類的onCreate()中新增ReportFragment,否則由LifecycleDispatcher新增),在ReportFragment生命週期函式中呼叫LifecycleRegistry.handleLifecycleEvent()方法改變State。
- Event事件是如何分發到LifecycleObserver的?
LifecycleRegistry在收到handleLifecycleEvent()後,內部呼叫moveToState()方法,改變State值,每一次State值改變,都會呼叫LifecycleObserver.onStateChanged()方法將Event分發到LifecycleObserver
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