Android中Lifecycle的原理详解
作者:一杯清泉
Lifecycle是Android Architecture Components的成员,是一个生命周期感知组件,能够感知Activity、Fragment等组件的生命周期变化,并将变化通知到已注册的观察者。正确的使用有助于更好地组织代码,减少内存泄漏,增强稳定。下面分析他的实现原理,看看到底只怎么感知生命周期的。
一、基本使用
1、引入依赖库
dependencies { def lifecycle_version = "2.5.0-alpha01" def arch_version = "2.1.0" implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version" implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version" implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version" //非java8使用 annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" //java8使用 implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version" }
2、自定义一个类实现LifecycleObserver,在类中监听生命周期回调
public class BaseJsApi implements LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) void create() { onCreate(); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) void destroy() { onDestroy(); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) void resume() { onResume(); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) void pause() { onPause(); } …………………………………… }
3、在Activity或者fragment中注册。
getLifecycle().addObserver(getJsBridge());
通过以上两步就可以感知当前Activity的生命周期了,非常的简便实用。
二、LifecycleObserver接口和LifecycleOwner接口
public interface LifecycleObserver { } public interface LifecycleOwner { @NonNull Lifecycle getLifecycle(); }
LifecycleObserver是一个空接口,LifecycleOwner提供getLifecycle返回Lifecycle,该接口由ComponentActivity实现,提供了getLifecycle的实现。
三、getLifecycle()
getLifecycle方法实际上是调用的ComponentActivity的getLifecycle方法,通过查看,Activity确实也是继承的ComponentActivity,该方法返回mLifecycleRegistry对象,该对象继承自Lifecycle,具体实现了添加观察者,移出观察者的方法,还定义了一个生命周期的枚举。
public class LifecycleRegistry extends 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(); public enum Event { ON_CREATE, ON_START, ON_RESUME, ON_PAUSE, ON_STOP, ON_DESTROY, ON_ANY; ………………………………………… } ……………………………… }
也就可以看出getLifecycle().addObserver实际上就是执行LifecycleRegistry的addObserver的方法添加观察者,做一个小总结如下:
观察者实现LifecycleObserver,实现需要的生命周期。在Activity、Fragment中添通过getLifecycle().addObserver加观察者。实际上调用的是ComponentActivity的getLifecycle方法返回mLifecycleRegistry。调用LifecycleRegistry的addObserver加观察者。三、添加观察者的流程
详情看注释:
@Override public void addObserver(@NonNull LifecycleObserver observer) { //检查是否在主线程 enforceMainThreadIfNeeded("addObserver"); //设置状态值 State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; //将观察者和初始化的值封装到ObserverWithState中 ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); //以观察者为key,statefulObserver为value放到hashMap集合中 ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); //不为null表示之前已经放进去了,否则没放进去,放进去了就添加结束 if (previous != null) { return; } //获取被观察者对象,即Activity,如果被销毁了,则结束 LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); if (lifecycleOwner == null) { // it is null we should be destroyed. Fallback quickly return; } //mAddingObserverCounter 表示正在添加的观察者的数量, //开始添加新的observe之前,对该变量进行++操作,添加完成后,进行--操作。 //mHandlingEvent表示是否正在处理 boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent; //计算状态,进行while循环 State targetState = calculateTargetState(observer); mAddingObserverCounter++; //新添加的LifecycleObserver ,会不断更新自己的生命周期到指定的targetState while ((statefulObserver.mState.compareTo(targetState) < 0 && mObserverMap.contains(observer))) { //添加状态到list,计算当前状态,用于计算状态calculateTargetState pushParentState(statefulObserver.mState); // final Event event = Event.upFrom(statefulObserver.mState); if (event == null) { throw new IllegalStateException("no event up from " + statefulObserver.mState); } //生命周期分发:分发到mLifecycleObserver里面 statefulObserver.dispatchEvent(lifecycleOwner, event); //移除状态到list popParentState(); // mState / subling may have been changed recalculate //重新计算状态 targetState = calculateTargetState(observer); } //没有事件正在处理,或者是添加了observe,就需要重新同步一次状态,下面详细介绍 if (!isReentrance) { // we do sync only on the top level. sync(); } mAddingObserverCounter--; }
//计算状态规则 private State calculateTargetState(LifecycleObserver observer) { //在map中获取到当前observer的前一个observer Map.Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer); //前一个observer的state State siblingState = previous != null ? previous.getValue().mState : null; //如果是嵌套添加observer,获取父级observer的状态 State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1) : null; //获取最小状态,DESTROYED < INITIALIZED < CREATED < STARTED < RESUMED return min(min(mState, siblingState), parentState); }
四、绑定生命周期
上面介绍了addObserver的流程,在ComponentActivity的onCreate方法中:
ReportFragment.injectIfNeededIn(this);
ReportFragment是一个Fragment,它负责分派生命周期的事件,injectIfNeededIn()就是在当前的Activity里添加一个ReportFragment,和glide的绑定生命周期比较类似。
public static void injectIfNeededIn(Activity activity) { //当API>=29注册registerIn绑定生命周期 if (Build.VERSION.SDK_INT >= 29) { // On API 29+, we can register for the correct Lifecycle callbacks directly LifecycleCallbacks.registerIn(activity); } 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(); } }
当API>=29是通过Application.ActivityLifecycleCallbacks的方式注册生命周期, 以onStart为例,其他onResume、onPause等类似:
@Override public void onStart() { super.onStart(); dispatchStart(mProcessListener); dispatch(Lifecycle.Event.ON_START); }
重点看看dispatch:
private void dispatch(@NonNull Lifecycle.Event event) { //当API<29分发dispatch绑定生命周期 if (Build.VERSION.SDK_INT < 29) { // Only dispatch events from ReportFragment on API levels prior // to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks // added in ReportFragment.injectIfNeededIn dispatch(getActivity(), event); } }
不管是pplication.ActivityLifecycleCallbacks还是当前fragment的生命周期方式都会执行如下dispatch:
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) { 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); } } }
调用LifecycleRegistry的handleLifecycleEvent将生命周期分发出去,LifecycleRegistry上面已经介绍过了addObserve逻辑了,代码比较长,只显示关键代码:
handleLifecycleEvent -> moveToState -> 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."); } //没有同步过,则同步 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) { //如果ObserverWithState的state小于当前state,那么就调用forwardPass方法, backwardPass(lifecycleOwner); } Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest(); if (!mNewEventOccurred && newest != null && mState.compareTo(newest.getValue().mState) > 0) { //如果大于当前state,那么就调用backwardPass方法。 forwardPass(lifecycleOwner); } } mNewEventOccurred = false; }
backwardPass和forwardPass的代码都会调用:
observer.dispatchEvent(lifecycleOwner, event)
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 = event.getTargetState(); mState = min(mState, newState); mLifecycleObserver.onStateChanged(owner, event); mState = newState; } }
调用了Lifecycling.getCallback方法来获得mLifecycleObserver的实例,通过这个实例调用onStateChanged:
static LifecycleEventObserver lifecycleEventObserver(Object object) { boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver; boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver; if (isLifecycleEventObserver && isFullLifecycleObserver) { return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, (LifecycleEventObserver) object); } if (isFullLifecycleObserver) { return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null); } if (isLifecycleEventObserver) { return (LifecycleEventObserver) object; } //通过反射,找到Lifecycle 自动生成的类 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); 最终使用生成的Adapter,创建SingleGeneratedAdapterObserver 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); }
class SingleGeneratedAdapterObserver implements LifecycleEventObserver { private final GeneratedAdapter mGeneratedAdapter; SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) { mGeneratedAdapter = generatedAdapter; } @Override public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) { mGeneratedAdapter.callMethods(source, event, false, null); mGeneratedAdapter.callMethods(source, event, true, null); } }
到这里会根据依赖中添加的annotationProcessor "androidx.lifecycle:lifecycle-compiler:2.2.0"根据不同生命周期注解生成不同的类,整个过程还是比较复杂的。整体过程如下图:
总结
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