Lifecycle簡(jiǎn)介
Lifecycle是一個(gè)生命周期感知組件，一般用來(lái)響應(yīng)Activity、Fragment等組件的生命周期變化，并將變化通知到已注冊(cè)的觀(guān)察者。有助于更好地組織代碼械蹋，讓代碼邏輯符合生命周期規(guī)范，減少內(nèi)存泄漏羞芍，增強(qiáng)穩(wěn)定性哗戈。
使用方法
1、在app或者相關(guān)module下的build.gradle文件下添加如下依賴(lài)：
- androidX項(xiàng)目：直接使用
- 非androidX項(xiàng)目：
//運(yùn)行時(shí)
implementation "android.arch.lifecycle:runtime:1.1.1"
// 編譯期
annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
2荷科、生命周期觀(guān)察者組件實(shí)現(xiàn)LifecycleObserver接口
public class LocationUtil implements LifecycleObserver {
private static final String TAG = "LocationUtil";
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void createLocation(){
Log.d(TAG,"lifecycle:onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void startLocation(){
Log.d(TAG,"lifecycle:onStart");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void resumeLocation(){
Log.d(TAG,"lifecycle:onResume");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void pauseLocation(){
Log.d(TAG,"lifecycle:onPause");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void stopLocation(){
Log.d(TAG,"lifecycle:onStop");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void destoryLocation(){
Log.d(TAG,"lifecycle:onDestory");
}
}
3唯咬、將組件添加到Lifecycle的觀(guān)察者列表
-
假如你使用新版本的AppcompatActivity（>26.1.0）[我使用的是27.1.1],因?yàn)锳ppcompatActivity繼承的ComponentActivity實(shí)現(xiàn)了LifecycleOwner接口，所以寫(xiě)法如下：
image.png
public class LifeCycleActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_life_cycle);
getLifecycle().addObserver(new LocationUtil());
}
}
如此步做，LocationUtil便可以關(guān)聯(lián)到Activity的生命周期
- AppcompatActivity<26.1.0,這種情況下也可以自己實(shí)現(xiàn)LifecycleOwner接口：
1副渴、實(shí)現(xiàn)LifecycleOwner 接口的方法：getLifecycle()
2、注冊(cè)Lifecycle new LifecycleRegistry(this)
3全度、在Activity生命周期函數(shù)中分發(fā)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();
}
}
源碼分析
Lifecycle相關(guān)類(lèi)圖：
回顧LocationUtil代碼：
public class LocationUtil implements LifecycleObserver {
private static final String TAG = "LocationUtil";
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void createLocation(){
Log.d(TAG,"lifecycle:onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void startLocation(){
Log.d(TAG,"lifecycle:onStart");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void resumeLocation(){
Log.d(TAG,"lifecycle:onResume");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void pauseLocation(){
Log.d(TAG,"lifecycle:onPause");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void stopLocation(){
Log.d(TAG,"lifecycle:onStop");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void destoryLocation(){
Log.d(TAG,"lifecycle:onDestory");
}
}
可以很清楚地知道Lifecycle是通過(guò)@OnLifecycleEvent（參數(shù)）注解來(lái)關(guān)聯(lián)生命周期的煮剧。
我們從引用代碼處出發(fā)：
public class LifeCycleActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_life_cycle);
getLifecycle().addObserver(new LocationUtil());
}
}
發(fā)現(xiàn)僅僅一行g(shù)etLifecycle().addObserver(new LocationUtil());代碼。從這行代碼我們可以知道Lifecycle使用了觀(guān)察者模式将鸵，通過(guò)通過(guò)關(guān)聯(lián)生命周期通知訂閱的觀(guān)察者勉盅。那么這行代碼是如何使Lifecycle關(guān)聯(lián)上Activity的生命周期的呢？
帶著這個(gè)疑問(wèn)顶掉，我們查看getLifecycle()的源碼：
ComponentActivity(AppCompatActivity的父類(lèi))
-----------------------------
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
獲取一個(gè)mLifecycleRegistry實(shí)例
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
mLifecycleRegistry是LifecycleRegistry類(lèi)的一個(gè)實(shí)例草娜，
查看ComponentActivity的生命周期onCreate方法：
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
繼續(xù)查看 ReportFragment.injectIfNeededIn(this)：
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();
}
}
發(fā)現(xiàn)injectIfNeededIn（）方法其實(shí)就是給當(dāng)前Activity添加一個(gè)Fragment,如此ReportFragment便能感知到當(dāng)前Activity的生命周期。
再看ReportFragment的生命周期函數(shù)：
@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);
}
}
}
發(fā)現(xiàn)全都調(diào)用了dispatch（）方法痒筒，而dispatch（）方法則會(huì)判斷activity是否實(shí)現(xiàn)了LifecycleOwner接口宰闰，如果實(shí)現(xiàn)了該接口接調(diào)用LifecycleRegister的handleLifecycleEvent()茬贵，這樣生命周期的狀態(tài)就會(huì)借由LifecycleRegister通知給各個(gè)LifecycleObserver從而調(diào)用其中對(duì)應(yīng)Lifecycle.Event的方法。
這是Activity的聲明周期感知移袍，那么Fragment呢解藻？看一下Fragment源碼你會(huì)發(fā)現(xiàn)Fragment也實(shí)現(xiàn)了LifecycleOwner接口，也關(guān)聯(lián)了一個(gè)LifecycleRegistry對(duì)象葡盗。
我們接著看handleLifecycleEvent（）方法：
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
getStateAfter(event):
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;
}
throw new IllegalArgumentException("Unexpected event value " + 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;
}
sync():
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;
}
這里可以看到
如果ObserverWithState的state小于當(dāng)前state螟左，那么就調(diào)用forwardPass方法，
如果大于當(dāng)前state觅够，那么就調(diào)用backwardPass方法胶背。
繼續(xù)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();
}
}
}
上面的observer其實(shí)是一個(gè)ObserverWithState對(duì)象,它是一個(gè)靜態(tài)內(nèi)部類(lèi)
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 = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
通過(guò)lifecycleEventObserver（）方法獲取mLifecycleObserver實(shí)例，調(diào)用onStateChanged（owner喘先，event）通知實(shí)現(xiàn)了 LifecycleObserver的類(lèi)钳吟，生命周期發(fā)生了變化。至此Lifecycle的原理實(shí)現(xiàn)就分析完了苹祟。
Q:為什么要使用ReportFragment砸抛？
我認(rèn)為應(yīng)該是為了降低侵入性，所以封裝一個(gè)具有同樣生命周期的Fragment來(lái)給Lifecycle分發(fā)生命周期事件树枫。
這樣只需要在onCreate方法里面調(diào)用ReportFragment.injectIfNeededIn(this);即可
