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众所周知,Android凡是须要展现给用户看的,都包含着生命周期这个概念,例如Activity、Fragment、View等都与生命周期息息相关,在生命周期函数里,它们各自完成建立、渲染、销毁等工做。android
可是一旦咱们往某个生命周期函数中,加入咱们本身的逻辑,如果处理不当,就有可能破坏其原有的生命周期,形成内存泄漏甚至应用崩溃等问题。程序员
Android官方彷佛也注意到了这一点,所以在Google IO 2018大会上,推出了Android Jetpack开发工具包,其中就包含了一个很是重要的生命周期组件,即本篇的主角-Lifecycle微信
本文篇幅较长,请你们耐心阅读。app
Lifecycle是一个生命周期感知组件,通常用来响应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。有助于更好地组织代码,让代码逻辑符合生命周期规范,减小内存泄漏,加强稳定性。ide
Lifecycle已经归入新版本的AppCompatActivity和Fragment中了,而且Lifecycle仍是Android Jetpack中其余两个组件LiveData和ViewModel的基础,意味着这个库可能将持续伴随着咱们后续的开发,所以有什么理由不学习一番呢?函数
按照国际规范,咱们先讲使用,后解析源码。工具
在app或者module目录下的build.gradle中,添加依赖:post
dependencies { ...... implementation "android.arch.lifecycle:runtime:1.1.1" implementation "android.arch.lifecycle:extensions:1.1.1" // 若是你使用java8开发,能够添加这个依赖,里面只有一个类 implementation "android.arch.lifecycle:common-java8:1.1.1" }
public class Java7Observer implements LifecycleObserver { private static final String TAG = Java7Observer.class.getSimpleName(); @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void onCreate() { Log.d(TAG, "onCreate"); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { Log.d(TAG, "onStart"); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) public void onResume() { Log.d(TAG, "onResume"); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void onPause() { Log.d(TAG, "onPause"); } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { Log.d(TAG, "onStop"); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) public void onDestroy() { Log.d(TAG, "onDestroy"); } }
首先,咱们须要实现LifecycleObserver这个接口,而后在咱们的方法上加上@OnLifecycleEvent注解,注解值表示该方法对应生命周期的哪一个函数,这里咱们把全部生命周期函数都加上了,并简单地打印了一句Log。学习
我想有人会问,为何这个class要命名Java7Observer ? 没错,由于Java8中,官方推荐用另一种方式:
public class Java8Observer implements DefaultLifecycleObserver { private static final String TAG = Java8Observer.class.getSimpleName(); @Override public void onCreate(@NonNull LifecycleOwner owner) { Log.d(TAG, "onCreate"); } @Override public void onStart(@NonNull LifecycleOwner owner) { Log.d(TAG, "onStart"); } @Override public void onResume(@NonNull LifecycleOwner owner) { Log.d(TAG, "onResume"); } @Override public void onPause(@NonNull LifecycleOwner owner) { Log.d(TAG, "onPause"); } @Override public void onStop(@NonNull LifecycleOwner owner) { Log.d(TAG, "onStop"); } @Override public void onDestroy(@NonNull LifecycleOwner owner) { Log.d(TAG, "onDestroy"); } }
理由很简单,Lifecycle源码中有一段注释:
/* ..... * </pre> * If you use <b>Java 7 Language</b>, Lifecycle events are observed using annotations. * Once Java 8 Language becomes mainstream on Android, annotations will be deprecated, so between * {@link DefaultLifecycleObserver} and annotations, * you must always prefer {@code DefaultLifecycleObserver}. * <pre> * ...... */
翻译一下就是:“若是你丫的用的是java7,那你就用注解呗,可是我告诉你,一旦java8上位了,你那些注解就过期了,我劝你最好用DefaultLifecycleObserver”
若是你使用新版本的AppcompatActivity(>26.1.0),能够这么写:
public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // 直接调用getLifecycle(),添加Observer getLifecycle().addObserver(new Java7Observer()); getLifecycle().addObserver(new Java8Observer()); } }
若是你的AppCompatActivity没有更新到最新,那么你须要自实现LifecycleOwner接口,并在Activity生命周期函数中分发事件
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(); } }
ok,完事儿了,我们跑起来看一哈,已经成功和生命周期关联上了。
MVP想必你们都用过,或者据说过,View层经过Presenter层和Model层进行通讯,所以Presenter层若是作了什么超出View生命周期的事,又没有及时释放内存,就会形成内存泄漏。
因此,原来的Presenter,为了响应生命周期,咱们可能会这么写:
先定义全部的生命周期函数
public class MainPresenter { public void onCreate() { } public void onStart() { } public void onResume() { } public void onPause() { } public void onStop() { } public void onDestroy() { } }
而后再与Activity生命周期一一绑定,若是有多个相似的须要响应生命周期的类,Activity的生命周期函数就会变得很是臃肿
public class MainActivity extends AppCompatActivity { private MainPresenter mPresenter; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mPresenter = new MainPresenter(); mPresenter.onCreate(); } @Override protected void onResume() { super.onResume(); mPresenter.onCreate(); // VideoPlayer.onResume(); // ... } @Override protected void onStart() { super.onStart(); mPresenter.onStart(); // VideoPlayer.onStart(); // ... } @Override protected void onPause() { super.onPause(); mPresenter.onPause(); // VideoPlayer.onPause(); // ... } @Override protected void onStop() { super.onStop(); mPresenter.onStop(); // VideoPlayer.onStop(); // ... } @Override protected void onDestroy() { super.onDestroy(); mPresenter.onDestroy(); // VideoPlayer.onDestroy(); // ... } }
所以,Lifecycle就是为了解决这个痛点,将生命周期的响应分发到各个观察者中去,咱们只须要在Activity中调用一行代码:
getLifecycle().addObserver(mPresenter);
代码量蹭蹭就下来了。诸如音视频播放、图片加载,这些吃内存大户,都应该响应生命周期,及时释放内存,不然极可能就会形成OOM。
在项目中引入一个库,却不懂是怎么实现的,是一件很是危险的行为,所以咱们不只要学会怎么用,还得知道其中的原理。
先看一张官方的图:
lifecycle-states
能够看到两个很显眼的单词:State和Event,这也是贯穿整个Lifecycle的两个概念:状态和事件。
Lifecycle将Activity的生命周期函数对应成State,生命周期改变,会形成State改变,而State变化将触发Event事件,从而被LifecycleObser接收。
State:状态,是Lifecycle中对应Activity生命周期的一种状态标识,从图中能够看到,它有INITIALIZED、DESTROYED、CREATED、STARTED、RESUMED这5中状态。
Event:事件,当State发生变化时,Lifecycle会向已注册的LifecycleObserver发送事件,例如:当State从INITIALIZED变化到CREATED时,就会发出ON_CREATE事件。
所以,弄懂Lifecycle,其实也就是须要弄懂两件事:
ok,带着这两个问题,咱们开始看源码:
直接ctrl+左键跟进源码,因为AppCompatActivity继承自SupportActivity,咱们发现,AppCompatActivity.getLifecycle(),最终定位在了SupportActivity.getLifecycle()方法中
public class SupportActivity extends Activity implements LifecycleOwner { ...... private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this); } @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; } ...... }
不难看到:
字面意思:生命周期登记处
这个类是Lifecycle中最重要的一个类,它是Lifecycle的子类,起着添加观察者,响应生命周期事件,分发生命周期事件的做用
public class LifecycleRegistry extends Lifecycle { // LifecycleObserver Map,每个Observer都有一个State private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>(); // 当前的状态 private State mState; // 生命周期拥有者,上述的SupportActivity继承了LifecycleOwner private final WeakReference<LifecycleOwner> mLifecycleOwner; public LifecycleRegistry(@NonNull LifecycleOwner provider) { mLifecycleOwner = new WeakReference<>(provider); mState = INITIALIZED; } /** * 添加LifecycleObserver观察者,并将以前的状态分发给这个Observer,例如咱们在onResume以后注册这个Observer, * 该Observer依然能收到ON_CREATE事件 */ public void addObserver(@NonNull LifecycleObserver observer) { State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); ...... // 例如:Observer初始状态是INITIALIZED,当前状态是RESUMED,须要将INITIALIZED到RESUMED之间的 // 全部事件分发给Observer // while ((statefulObserver.mState.compareTo(targetState) < 0 && mObserverMap.contains(observer))) { pushParentState(statefulObserver.mState); statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState)); popParentState(); targetState = calculateTargetState(observer); } ...... } /** * 处理生命周期事件 */ public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { State next = getStateAfter(event); moveToState(next); } /** * 改变状态 */ private void moveToState(State next) { if (mState == next) { return; } mState = next; ...... sync(); ...... } /** * 同步Observer状态,并分发事件 */ private void sync() { LifecycleOwner lfecycleOwner = mLifecycleOwner.get(); if (lifecycleOwner == null) { Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch " + "new events from it."); return; } while (!isSynced()) { mNewEventOccurred = false; // State中,状态值是从DESTROYED-INITIALIZED-CREATED-STARTED-RESUMED增大 // 若是当前状态值 < Observer状态值,须要通知Observer减少状态值,直到等于当前状态值 if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) { backwardPass(lifecycleOwner); } Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest(); // 若是当前状态值 > Observer状态值,须要通知Observer增大状态值,直到等于当前状态值 if (!mNewEventOccurred && newest != null && mState.compareTo(newest.getValue().mState) > 0) { forwardPass(lifecycleOwner); } } mNewEventOccurred = false; } /** * 向前传递事件,对应图中的INITIALIZED -> RESUMED * 增长Observer的状态值,直到状态值等于当前状态值 */ 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); // 分发状态改变事件 observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState)); popParentState(); } } } /** * 向后传递事件,对应图中的RESUMED -> DESTROYED * 减少Observer的状态值,直到状态值等于当前状态值 */ 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); // 分发状态改变事件 pushParentState(getStateAfter(event)); observer.dispatchEvent(lifecycleOwner, event); popParentState(); } } } }
咱们再看看新版AppCompatActivity是如何将事件分发给LifecycleRegistry的,还记得SupportActivity中有这么一段逻辑吗:
public class SupportActivity extends Activity implements LifecycleOwner { @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this); } }
咱们看看ReportFragment是什么
public class ReportFragment extends Fragment { 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(); } } @Override public void onStart() { super.onStart(); dispatchStart(mProcessListener); dispatch(Lifecycle.Event.ON_START); } // ...... 其余生命周期也是一样调用了dispatch(Lifecycle.Event.xxx)分发事件 @Override public void onPause() { super.onPause(); dispatch(Lifecycle.Event.ON_PAUSE); } 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); } } } }
能够看到,SupportActivity添加了一个没有页面的ReportFragment,在ReportFragment的生命周期函数中,调用了LifecycleRegistry.handleLifecycleEvent()方法来分发生命周期事件。用一张图总结一下就是:
AppCompatActivity中添加了一个ReportFragment,其生命周期变化时,调用LifecycleRegistry.handleLifecycleEvent()方法通知LifecycleRegistry改变状态,LifecycleRegistry内部调用moveToState()改变状态,并调用每一个LifecycleObserver.onStateChange()方法通知生命周期变化。
为何不直接在SupportActivity的生命周期函数中给Lifecycle分发生命周期事件,而是要加一个Fragment呢?
由于不是全部的页面都继承AppCompatActivity,为了兼容非AppCompatActivity,因此封装一个一样具备生命周期的Fragment来给Lifecycle分发生命周期事件。
这里咱们直接使用AndroidStudio强大的搜索功能,alt+F7搜索ReportFragment的调用者,咱们发现:除了SupportActivity之外,还有两个地方使用到了ReportFragment:LifecycleDispatcher和ProcessLifecycleOwner
字面意思:生命周期分发者
class LifecycleDispatcher { static void init(Context context) { ((Application) context.getApplicationContext()) .registerActivityLifecycleCallbacks(new DispatcherActivityCallback()); } /** * 给全部子Fragment设置State */ private static void markState(FragmentManager manager, State state) { Collection<Fragment> fragments = manager.getFragments(); if (fragments == null) { return; } for (Fragment fragment : fragments) { if (fragment == null) { continue; } markStateIn(fragment, state); if (fragment.isAdded()) { markState(fragment.getChildFragmentManager(), state); } } } private static void markStateIn(Object object, State state) { if (object instanceof LifecycleRegistryOwner) { LifecycleRegistry registry = ((LifecycleRegistryOwner) object).getLifecycle(); registry.markState(state); } } // 经过注册Application.registerActivityLifecycleCallbacks来获取Activity的生命周期回调 static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks { @Override public void onActivityCreated(Activity activity, Bundle savedInstanceState) { if (activity instanceof FragmentActivity) { ((FragmentActivity) activity).getSupportFragmentManager() .registerFragmentLifecycleCallbacks(mFragmentCallback, true); } // 给每一个Activity添加ReportFragment ReportFragment.injectIfNeededIn(activity); } @Override public void onActivityStopped(Activity activity) { if (activity instanceof FragmentActivity) { markState((FragmentActivity) activity, CREATED); } } @Override public void onActivitySaveInstanceState(Activity activity, Bundle outState) { if (activity instanceof FragmentActivity) { markState((FragmentActivity) activity, CREATED); } } } }
能够看到LifecycleDispatcher是经过注册Application.registerActivityLifecycleCallbacks来监听Activity的生命周期回调的。
字面意思:线程生命周期拥有者
public class ProcessLifecycleOwner implements LifecycleOwner { public static LifecycleOwner get() { return sInstance; } static void init(Context context) { sInstance.attach(context); } void activityResumed() { mResumedCounter++; if (mResumedCounter == 1) { if (mPauseSent) { mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME); mPauseSent = false; } else { mHandler.removeCallbacks(mDelayedPauseRunnable); } } } void activityPaused() { mResumedCounter--; if (mResumedCounter == 0) { mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS); } } private ActivityInitializationListener mInitializationListener = new ActivityInitializationListener() { @Override public void onCreate() {} @Override public void onStart() { activityStarted(); } @Override public void onResume() { activityResumed(); } }; void attach(Context context) { mHandler = new Handler(); mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); Application app = (Application) context.getApplicationContext(); app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() { @Override public void onActivityCreated(Activity activity, Bundle savedInstanceState) { ReportFragment.get(activity).setProcessListener(mInitializationListener); } @Override public void onActivityPaused(Activity activity) { activityPaused(); } @Override public void onActivityStopped(Activity activity) { activityStopped(); } }); } }
根据官方注释,咱们能够了解到:
那么问题又来了,ProcessLifecycleOwner和LifecycleDispatcher两个类是在哪里初始化呢?
从源码中咱们看到,他们的入口都是init(Context),继续搜索源码,咱们发现:
public class ProcessLifecycleOwnerInitializer extends ContentProvider { @Override public boolean onCreate() { LifecycleDispatcher.init(getContext()); ProcessLifecycleOwner.init(getContext()); return true; } // ...... } AndroidManifest.xml <manifest xmlns:android="http://schemas.android.com/apk/res/android"> ...... <provider android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer" android:authorities="me.baron.achitecturelearning.lifecycle-trojan" android:exported="false" android:multiprocess="true" /> </manifest>
了然,Lifecycle自动在咱们的AndroidManifest.xml中添加了一个ContentProvider,用于初始化ProcessLifecycleOwner和LifecycleDispatcher,这么作的好处是,不须要咱们在Application中显示调用,不须要咱们写一行代码。
回到咱们最初的两个问题:
在Activity中添加一个ReportFragment(若是你的Activity继承AppCompatActivity,会在父类的onCreate()中添加ReportFragment,不然由LifecycleDispatcher添加),在ReportFragment生命周期函数中调用LifecycleRegistry.handleLifecycleEvent()方法改变State。
LifecycleRegistry在收到handleLifecycleEvent()后,内部调用moveToState()方法,改变State值,每一次State值改变,都会调用LifecycleObserver.onStateChanged()方法将Event分发到LifecycleObserver
最后,但愿每一个单身的程序员,都能找到属于本身的她,注入你的生命周期
喜欢这篇文章的话,记得给我一个当心心哦
做者:XBaron 连接:https://www.jianshu.com/p/2c9bcbf092bc 来源:简书 简书著做权归做者全部,任何形式的转载都请联系做者得到受权并注明出处。