1. 簡(jiǎn)介
很早就聽(tīng)說(shuō)了Google的Lifecycle組件，因?yàn)轫?xiàng)目沒(méi)有使用過(guò)，所以并沒(méi)有過(guò)多的接觸牺荠。不過(guò)最近看到了一篇文章，其中的一條評(píng)論提到了LiveData驴一。恰巧這兩天工作內(nèi)容不多休雌，所以趕緊研究一波！
不過(guò)在看LiveData之前肝断，我覺(jué)得還是先看下Lifecycle吧（Lifecycle更像是LiveData的基礎(chǔ)挑辆，LiveData就不寫(xiě)了，感覺(jué)沒(méi)啥東西）孝情。
2. Lifecycle的簡(jiǎn)單介紹
Lifecycle的介紹，我們還是拿Google的官方文檔作為參考吧洒嗤。
Lifecycle主要解決的是業(yè)務(wù)和Activity/Fragment生命周期相關(guān)的問(wèn)題箫荡。例如：我們?cè)?code>onResume()/onStart()中請(qǐng)求定位，在onPause()/onStop()中停止定位渔隶。那么我們一般的做法：
Google Simple Code
class MyLocationListener {
public MyLocationListener(Context context, Callback callback) {
// ...
}
void start() {
// connect to system location service
}
void stop() {
// disconnect from system location service
}
}
class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;
@Override
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, (location) -> {
// update UI
});
}
@Override
public void onStart() {
super.onStart();
myLocationListener.start();
// manage other components that need to respond
// to the activity lifecycle
}
@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
// manage other components that need to respond
// to the activity lifecycle
}
}
上面的代碼雖然看起來(lái)還可以羔挡，但在真實(shí)的應(yīng)用程序中，可能會(huì)有很多的方法都需要根據(jù)當(dāng)前Activity/Fragment的生命周期來(lái)進(jìn)行不同的操作间唉。因此其生命周期方法中可能會(huì)被放置大量代碼绞灼，例如onStart()和 onStop()中，這使得它們難以維護(hù)呈野。因此Lifecycle應(yīng)運(yùn)而生低矮！
3. Lifecycle的使用
Lifecycle已經(jīng)發(fā)布了release版，所以其中的一些默認(rèn)支持已經(jīng)包含到了support-v7:26.1.0以及更高被冒。
添加依賴：
implementation "android.arch.lifecycle:extensions:1.1.1"
annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
使用方式1：實(shí)現(xiàn)LifecycleObserver接口军掂，使用@OnLifecycleEvent注解，通過(guò)編譯時(shí)注解生成代碼：
public class MyLifecycleObserver implements LifecycleObserver {
private static final String TAG = MyLifecycleObserver.class.getSimpleName();
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void start(LifecycleOwner lifecycleOwner) {
Lifecycle.State currentState = lifecycleOwner.getLifecycle().getCurrentState();
Log.d(TAG, "start: " + currentState);
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void stop(LifecycleOwner lifecycleOwner) {
Log.d(TAG, "stop: " + lifecycleOwner.getLifecycle().getCurrentState());
}
}
使用方式2：實(shí)現(xiàn)
GenericLifecycleObserver接口昨悼，實(shí)現(xiàn)其onStateChanged方法：
public class MyLifecycleObserver implements GenericLifecycleObserver {
private static final String TAG = MyLifecycleObserver.class.getSimpleName();
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
Log.d(TAG, event.name());
}
}
創(chuàng)建完成后蝗锥，我們需要將其添加：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new MyLifecycleObserver());
}
}
結(jié)果圖：
4. 源碼分析
- Lifecycle的獲取
根據(jù)源碼追蹤我們可以看到Lifecycle是在SupportActivity中：
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
這里還有個(gè)挺重要的代碼：
SupportActivity.java
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
ReportFragment.java
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();
}
}
這里在Activity中添加了一個(gè)Fragment，至于作用率触，留到后面在講终议。
- addObserver
addObserver方法的話，還是簡(jiǎn)單來(lái)看吧：
LifecycleRegistry.java
@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);
......
// 對(duì)狀態(tài)的修正，這里會(huì)對(duì)已經(jīng)過(guò)去的狀態(tài)進(jìn)行分發(fā)穴张。也就是說(shuō)如果添加observer時(shí)狀態(tài)已經(jīng)改變细燎，那么也會(huì)被通知到！
}
// 有狀態(tài)的Observer
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
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;
}
}
我們將實(shí)現(xiàn)的接口通過(guò)裝飾者（我認(rèn)為是這樣）模式轉(zhuǎn)成ObserverWithState對(duì)象陆馁，并將該對(duì)象添加到mObserverMap中找颓。
在ObserverWithState的構(gòu)造方法中，有對(duì)我們傳入的LifecycleObserver進(jìn)行包裝：
Lifecycling.java
@NonNull
static GenericLifecycleObserver getCallback(Object object) {
// 這里應(yīng)該算第三種實(shí)現(xiàn)方式叮贩，然而FullLifecycleObserver不是public击狮，所以不能使用
if (object instanceof FullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
}
// 我們的第一種方式
if (object instanceof GenericLifecycleObserver) {
return (GenericLifecycleObserver) object;
}
// 通過(guò)注解生成的
final Class<?> klass = object.getClass();
// 將生成的MyLifecycleObserver_LifecycleAdapter放到Map中
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);
}
根據(jù)我們傳入的對(duì)象進(jìn)行解析，最終返回結(jié)果為：GenericLifecycleObserver或者GenericLifecycleObserver的實(shí)現(xiàn)類益老。附獲取生成MyLifecycleObserver_LifecycleAdapter代碼：
// 通過(guò)名稱獲取
public static String getAdapterName(String className) {
return className.replace(".", "_") + "_LifecycleAdapter";
}
最終通過(guò)Class.forName方式獲得Class彪蓬。
- 事件分發(fā)
Lifecycle的事件分發(fā)在ObserverWithState類中：
// 事件分發(fā)
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
看下哪里調(diào)用了dispatchEvent方法：
我們看這個(gè)方法，其中一個(gè)在
addObserver中調(diào)用捺萌，另外的兩個(gè)都會(huì)通過(guò)sync()方法調(diào)用：
// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
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;
}
// 最新的狀態(tài)和當(dāng)前狀態(tài)不一致档冬，則需要進(jìn)行狀態(tài)修改
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;
}
// 是否狀態(tài)同步的
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
// 最新的和最后添加的Observer狀態(tài)一致，并且當(dāng)前的狀態(tài)和最新?tīng)顟B(tài)一致桃纯，則已經(jīng)同步了
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
這里的同步方法用于同步當(dāng)前Observer的狀態(tài)酷誓，如果最新的和最老的Observer的狀態(tài)不一致或者當(dāng)前的狀態(tài)和最新的狀態(tài)不一致時(shí)，那么需要進(jìn)行狀態(tài)同步态坦。同步包括了向前同步和向后同步盐数。
調(diào)用sync()方法一共就兩處，一處在addObserver方法伞梯，另一處是moveToState方法玫氢，而調(diào)用moveToState方法也有兩處：
@SuppressWarnings("WeakerAccess")
@MainThread
public void markState(@NonNull State state) {
moveToState(state);
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
handleLifecycleEvent方法有很多處調(diào)用：
看圖的話，就可以知道
Fragment在不同的生命周期調(diào)用了handleLifecycleEvent方法谜诫，隨便看一個(gè)吧：
還記得上面說(shuō)的
ReportFragment嗎漾峡？這里也出現(xiàn)了，我們看下ReportFragment到底做什么的：
// 調(diào)用Activity的
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);
}
}
}
// 生命周期
@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;
}
可以看到喻旷，ReportFragment只是作為一個(gè)中間層生逸，通過(guò)它來(lái)分發(fā)各種事件！
由于篇幅原因（已經(jīng)啰嗦了很多了）且预，這里就不寫(xiě)LifecycleDispatcher和ProcessLifecycleOwner了牺陶，這兩個(gè)通過(guò)自定義的內(nèi)容提供者ProcessLifecycleOwnerInitializer進(jìn)行初始化，并且通過(guò)registerActivityLifecycleCallbacks和registerFragmentLifecycleCallbacks注冊(cè)統(tǒng)一的觀察回調(diào)辣之。有興趣的話掰伸，自己看看吧。
5 總結(jié)
Lifecycle簡(jiǎn)單來(lái)說(shuō)就是用于處理和生命周期相關(guān)的業(yè)務(wù)怀估，其原理以及實(shí)現(xiàn)還是很簡(jiǎn)單的狮鸭。當(dāng)然了合搅，項(xiàng)目中并沒(méi)有使用到Lifecycle，所以實(shí)際應(yīng)用效果怎樣只能靠猜想了??歧蕉。
