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Android中Lifecycle的作用是什么？相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
Lifecycle的简单介绍
Lifecycle的介绍，我们还是拿Google的官方文档作为参考吧。
Lifecycle主要解决的是业务和Activity/Fragment生命周期相关的问题。例如：我们在onResume()/onStart()中请求定位，在onPause()/onStop()中停止定位。那么我们一般的做法：
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
}
}上面的代码虽然看起来还可以，但在真实的应用程序中，可能会有很多的方法都需要根据当前Activity/Fragment的生命周期来进行不同的操作。因此其生命周期方法中可能会被放置大量代码，例如onStart()和 onStop()中，这使得它们难以维护。因此Lifecycle应运而生！
3. Lifecycle的使用
Lifecycle已经发布了release版，所以其中的一些默认支持已经包含到了support-v7:26.1.0以及更高。
添加依赖：
implementation "android.arch.lifecycle:extensions:1.1.1" annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
使用方式1：实现LifecycleObserver接口，使用@OnLifecycleEvent注解，通过编译时注解生成代码：
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());
}
} 
APT生成的代码
使用方式2：实现GenericLifecycleObserver接口，实现其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());
}
}创建完成后，我们需要将其添加：
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new MyLifecycleObserver());
}
}结果图：
方法1
方法2
4. 源码分析
1.Lifecycle的获取
根据源码追踪我们可以看到Lifecycle是在SupportActivity中：
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}这里还有个挺重要的代码：
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();
}
}这里在Activity中添加了一个Fragment，至于作用，留到后面在讲。
2.addObserver
addObserver方法的话，还是简单来看吧：
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);
......
// 对状态的修正，这里会对已经过去的状态进行分发。也就是说如果添加observer时状态已经改变，那么也会被通知到！
}
// 有状态的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;
}
}我们将实现的接口通过装饰者（我认为是这样）模式转成ObserverWithState对象，并将该对象添加到mObserverMap中。
在ObserverWithState的构造方法中，有对我们传入的LifecycleObserver进行包装：
Lifecycling.java
@NonNull
static GenericLifecycleObserver getCallback(Object object) {
// 这里应该算第三种实现方式，然而FullLifecycleObserver不是public，所以不能使用
if (object instanceof FullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
}
// 我们的第一种方式
if (object instanceof GenericLifecycleObserver) {
return (GenericLifecycleObserver) object;
}
// 通过注解生成的
final Class klass = object.getClass();
// 将生成的MyLifecycleObserver_LifecycleAdapter放到Map中
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List> 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);
} 根据我们传入的对象进行解析，最终返回结果为：GenericLifecycleObserver或者GenericLifecycleObserver的实现类。附获取生成MyLifecycleObserver_LifecycleAdapter代码：
// 通过名称获取
public static String getAdapterName(String className) {
return className.replace(".", "_") + "_LifecycleAdapter";
}最终通过Class.forName方式获得Class。
3.事件分发
Lifecycle的事件分发在ObserverWithState类中：
// 事件分发
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}看下哪里调用了dispatchEvent方法：
调用
我们看这个方法，其中一个在addObserver中调用，另外的两个都会通过sync()方法调用：
// 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;
}
// 最新的状态和当前状态不一致，则需要进行状态修改
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 newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
// 是否状态同步的
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
// 最新的和最后添加的Observer状态一致，并且当前的状态和最新状态一致，则已经同步了
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
} 这里的同步方法用于同步当前Observer的状态，如果最新的和最老的Observer的状态不一致或者当前的状态和最新的状态不一致时，那么需要进行状态同步。同步包括了向前同步和向后同步。
调用sync()方法一共就两处，一处在addObserver方法，另一处是moveToState方法，而调用moveToState方法也有两处：
@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方法有很多处调用：
handleLifecycleEvent调用
看图的话，就可以知道Fragment在不同的生命周期调用了handleLifecycleEvent方法，随便看一个吧：
ON_CREATE
还记得上面说的ReportFragment吗？这里也出现了，我们看下ReportFragment到底做什么的：
// 调用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，ReportFragment只是作为一个中间层，通过它来分发各种事件！
由于篇幅原因（已经啰嗦了很多了），这里就不写LifecycleDispatcher和ProcessLifecycleOwner了，这两个通过自定义的内容提供者ProcessLifecycleOwnerInitializer进行初始化，并且通过registerActivityLifecycleCallbacks和registerFragmentLifecycleCallbacks注册统一的观察回调。
看完上述内容，你们掌握Android中Lifecycle的作用是什么的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注创新互联行业资讯频道，感谢各位的阅读！
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