前言
從本篇開始我將跟大家一起來學習Android JetPack系列。JetPack是谷歌推出的一款非常非常重要且有用的庫，它可以極大地提高開發(fā)者的開發(fā)效率，并且減少代碼崩潰率和內(nèi)存泄漏的問題。因此，學習并掌握JetPack系列是我們所有Android開發(fā)者的必修課，本文會跟大家一起學習一下關于Lifecycle的知識。
什么是Lifecycle
Lifecycle是一套專門用來感知Activity或Fragment生命周期變化的組件，當Activity或Fragment的生命周期發(fā)生的變化的時候，將會通知到Lifecycle的訂閱者。這樣我們可以把原本需要卸載Activity或Fragment的代碼抽出來，使Activity或Fragment更加精簡并且易于維護，也可以減少內(nèi)存泄漏。
Lifecycle的工作流程
話不多說，直接開搞。如果我們想要使用JetPack組件,需要在Androidx的環(huán)境下，因為Androidx默認導入了JetPack所有的包，關于Androidx的好處我就不多說了，現(xiàn)在幾乎各大主流App都遷移Androidx了。
在Androidx下的Actvity或Fragment內(nèi)部有個Lifecycle對象,需要觀察Actvity或Fragment的生命周期的觀察者訂閱Lifecycle，當Actvity或Fragment的生命周期發(fā)生變化時Lifecycle就對通知對所有訂閱它的觀察者，大體流程就是這樣，很好理解。
Lifecycle的使用
關于Lifecycle的使用，一般有三種
實現(xiàn)LifecycleObserver接口
無論哪種使用方式，都需要跟Actvity或Fragment建立訂閱關系，這里以Activity為例
（注：當前演示項目處于Androidx環(huán)境下）
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new MyLifeCycleObserver());//建立訂閱關系
}
}
public class MyLifeCycleObserver implements LifecycleObserver {
private static final String TAG = "MyLifeCycleObserver";
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
private void test1() {
Log.d(TAG, "onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
private void test2() {
Log.d(TAG, "onStart");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
private void test3() {
Log.d(TAG, "onResume");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
private void test4() {
Log.d(TAG, "onPause");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private void test5() {
Log.d(TAG, "onStop");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
private void test6() {
Log.d(TAG, "onDestroy");
}
}
關于@OnLifecycleEvent注解我覺得不必多說，基本看一眼就知道啥意義，當Actvity或Fragment的生命周期發(fā)生變化的時候，對應注解標注的方法會得到調(diào)用，我們可以在這些方法中實現(xiàn)我們的業(yè)務代碼。這種使用方式是我們最主流也是最常用的。
傳入Lifecycle對象
public class MyLifeCycleObserver2 implements LifecycleObserver {
private Lifecycle lifecycle;
public MyLifeCycleObserver2(Lifecycle lifecycle) {
this.lifecycle = lifecycle;
}
/**
* onStart()或onStart()以后執(zhí)行
*/
private void doAfterStart() {
if (lifecycle != null) {
if (lifecycle.getCurrentState().isAtLeast(Lifecycle.State.STARTED)) {
//do something
}
}
}
}
一般這種情況是我們當前觀察者需要根據(jù)Activity或Fragment的當前處于哪個生命周期并執(zhí)行相應的操作，如代碼中的例子所示，當Activity或Fragment處于onStart()以后的生命周期我們執(zhí)行某些操作，相對來說用的情況比較少
自定義實現(xiàn)Lifecyele
這種情況就極少了，而且也不推薦大家使用
public class MyActivity extends Activity implements LifecycleOwner {
private LifecycleRegistry lifecycleRegistry;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
lifecycleRegistry = new LifecycleRegistry(this);
lifecycleRegistry.markState(Lifecycle.State.CREATED);
lifecycleRegistry.addObserver(new MyLifeCycleObserver2());
}
@Override
public void onStart() {
super.onStart();
lifecycleRegistry.markState(Lifecycle.State.STARTED);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return lifecycleRegistry;
}
}
相當于給自己找麻煩，大家知道這種情況就好
Lifecycle的源碼分析
知其然更知其所以然，源碼分析是必不可少的。
剛才跟大家提到過Androidx環(huán)境下的Activity內(nèi)部有一個現(xiàn)成的Lifecycle對象，我們直接從MainActivity里面跟到androidx.core.app.ComponentActivity
@RestrictTo(LIBRARY_GROUP_PREFIX)
public class ComponentActivity extends Activity implements
LifecycleOwner,
KeyEventDispatcher.Component {
......................
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
.................
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
這里我們只看跟Lifecycle相關的代碼，ComponentActivity實現(xiàn)了LifecycleOwner重寫getLifecycle()方法，這里返回的是LifecycleRegistry對象。LifecycleRegistry繼承自Lifecycle，我們先看一下Lifecycle
public abstract class Lifecycle {
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
}
關鍵的就這三個方法，訂閱、取消觀察者以及獲取當前狀態(tài)
我們在看看Lifecycle的實現(xiàn)子類LifecycleRegistry
public class LifecycleRegistry extends Lifecycle {
//使用map來存儲所有的訂閱者以及訂閱者的狀態(tài)
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =new FastSafeIterableMap<>();
//當Activity或Fragment的生命周期發(fā)生變化時會調(diào)用該方法
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);//再跟到這個方法里面，這里會調(diào)用 sync()
}
}
/**
* 同步所有訂閱者的狀態(tài)
*/
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;
}
這里再跟到backwardPass()或forwardPass()方法里面，這兩個方法都是通知觀察者相應生命周期變化，看一個就行
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);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);//給訂閱發(fā)發(fā)送生命周期事件，跟進去
popParentState();
}
}
}
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);//調(diào)用觀察者的onStateChanged(),整個流程結束
mState = newState;
}
}
以上源碼就是Lifecycle的核心內(nèi)容，再看一張圖會更加清晰
關于Lifecycle內(nèi)部的源碼我們分析得差不多了，我們在看看Activity或者Fragment是如何通知Lifecycle生命周期變化的
先看Activity的，還是回到androidx.core.app.ComponentActivity中去
public class ComponentActivity extends Activity implements
LifecycleOwner,
KeyEventDispatcher.Component {
@SuppressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);//這里注入了一個ReportFragment，跟進去
}
}
public class ReportFragment extends Fragment {
@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;
}
}
看到這里就非常清晰了，原來是ReportFragment幫助ComponentActivity完成的事件通知，點開dispatch(Lifecycle.Event event)看一下
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);
}
}
}
到這里就跟我們剛才的Lifecycle分析連在一起了，清晰明了
接著我們看看Fragment，這里點開androidx.fragment.app.Fragment,注意是Androidx下來的哦
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
ViewModelStoreOwner {//這里同樣實現(xiàn)了LifecycleOwner接口
LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
void performCreate(Bundle savedInstanceState) {
..................
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
void performStart() {
......................
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
}
......................
}
這里源碼就不全部放出來，在Fragment就更加簡單，直接就是在生命周期被調(diào)用的時候調(diào)用mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event event)
源碼分析到這里，你可能還有一個疑問，就是我們訂閱者實現(xiàn)LifecycleObserver接口時并沒有重寫onStateChanged()，那么在最后調(diào)用mLifecycleObserver.onStateChanged(owner, event);我們的觀察者中的方法是被通知到的呢？答案就在LifecycleRegistry的addObserver()方法中
public void addObserver(@NonNull LifecycleObserver observer) {
.......
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);//跟進去看一下
..........
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);//再跟進去
mState = initialState;
}
}
}
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();
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);
}
我們傳入的Observer對象會在Lifecycling的getCallback()方法中一頓加工操作一個GenericLifecycleObserver對象，而我們點開GenericLifecycleObserver會發(fā)現(xiàn)它定義了 void onStateChanged(LifecycleOwner source, Lifecycle.Event event)方法，也就是雖然我們自定義的LifeObserver沒有實現(xiàn)onStateChanged()，但是Lifecycling最終幫我們實現(xiàn)了，至此，整個Lifecycle的工作流程串聯(lián)完畢。
總結
本文給大家介紹了Lifecycle使用以及源碼分析，相對來說Lifecycle是Jetpack推出的組件中比較簡單，大家只要認真讀一遍就能把知識完全掌握，在下一篇文章中我將給大家?guī)黻P于ViewModel的介紹。如果您覺得本文還不錯，記得給個贊，謝謝~
