Lifecycle 使用及原理解析一文搞懂

Myth丶恋晨 2022-03-08 02:06 133阅读 0赞

\*本篇文章已授权微信公众号 guolin\_blog （郭霖）独家发布

Lifecycle是Android Architecture Components的一员,这玩意儿挺方便的,它是能感知Activity和Fragment的生命周期的.

## 一、使用Lifecycle ##

#### 1. 引入Lifecycle ####

我们来看一下如何引入:

1.  非androidX项目引入:

//运行时
    implementation "android.arch.lifecycle:runtime:1.1.1"
    // 编译期
    annotationProcessor "android.arch.lifecycle:compiler:1.1.1"

1.  androidX项目引入:

> androidX是support库的新时代,Google正在将support迁移到androidx中.

implementation "androidx.lifecycle:lifecycle-runtime:2.0.0"
    implementation "androidx.lifecycle:lifecycle-extensions:2.0.0"
    implementation "androidx.lifecycle:lifecycle-common-java8:2.0.0"
    annotationProcessor  "androidx.lifecycle:lifecycle-compiler:2.0.0"

#### 2. 创建生命周期观察者 ####

public class MyObserver implements LifecycleObserver { 
    
        private static final String TAG = "MyObserver";
    
        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)  
        public void onCreate() { 
            Log.w(TAG, "onCreate: ");
        }
    
        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        public void onStart() { 
            Log.w(TAG, "onStart: ");
        }
    
        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        public void onResume() { 
            Log.w(TAG, "onResume: ");
        }
    
        @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        public void onPause() { 
            Log.w(TAG, "onPause: ");
        }
    
        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        public void onStop() { 
            Log.w(TAG, "onStop: ");
        }
    
        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        public void onDestroy() { 
            Log.w(TAG, "onDestroy: ");
        }
    
    }

我们首先创建了一个类,它实现了`LifecycleObserver`接口,并且我写了几个模拟生命周期的方法,并在每个方法上加上了注解.

#### 3. 观察生命周期 ####

然后我在Activity中这样写:

public class MainActivity extends AppCompatActivity { 
    
        @Override
        protected void onCreate(Bundle savedInstanceState) { 
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    
            //添加一个生命周期观察者 getLifecycle()是FragmentActivity中的方法
            MyObserver observer = new MyObserver();
            getLifecycle().addObserver(observer);
        }
    }

我们将项目运行起来,观察结果:

2019-03-12 22:14:26.672 15790-15790/? W/MyObserver: onCreate: 
    2019-03-12 22:14:26.676 15790-15790/? W/MyObserver: onStart: 
    2019-03-12 22:14:26.679 15790-15790/? W/MyObserver: onResume: 
    2019-03-12 22:15:13.054 15790-15790/? W/MyObserver: onPause: 
    2019-03-12 22:15:13.234 15790-15790/? W/MyObserver: onStop: 
    2019-03-12 22:15:13.241 15790-15790/? W/MyObserver: onDestroy:

我们发现,不管Activity的生命周期如何变化,我创建的观察者总是能够监听到响应的生命周期变化,并且变化时还会回调我写的生命周期方法(比如:`public void onDestroy()`).

方不方便? 你可能会问,这有啥用? 用处大了,比如我现在Presenter中就可以很方便的监听Activity中的生命周期,从而进行一些相应的操作和处理.

## 二、Lifecycle原理解析 ##

#### 1. 从使用处入手 ####

我们从使用的地方入手

MyObserver observer = new MyObserver();
    getLifecycle().addObserver(observer);

`getLifecycle()`方法点进去是FragmentActivity,看注释意思是返回生命周期提供者的Lifecycle

/** * Returns the Lifecycle of the provider. */
    @Override
    public Lifecycle getLifecycle() { 
        return super.getLifecycle();
    }

再跟着`super.getLifecycle();`进入,来到了`androidx.core.app.ComponentActivity`,可以看到,ComponentActivity是继承自Activity并实现了LifecycleOwner(该接口的作用是标记类有Android的生命周期的,比如Activity和Fragment)接口.

public class ComponentActivity extends Activity
            implements LifecycleOwner, KeyEventDispatcher.Component { 
        private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
        @Override
        public Lifecycle getLifecycle() { 
            return mLifecycleRegistry;
        }   
    }
    
    /** * A class that has an Android lifecycle */
    public interface LifecycleOwner { 
        @NonNull
        Lifecycle getLifecycle();
    }

那么其实最终是返回的LifecycleRegistry,它是Lifecycle的子类

Lifecycle是一个抽象类,里面有3个方法(添加观察者和移除观察者,获取当前的状态),还有一些状态的枚举定义.

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();
    
        @SuppressWarnings("WeakerAccess")
        public enum Event { 
            /** * Constant for onCreate event of the {@link LifecycleOwner}. */
            ON_CREATE,
            /** * Constant for onStart event of the {@link LifecycleOwner}. */
            ON_START,
            /** * Constant for onResume event of the {@link LifecycleOwner}. */
            ON_RESUME,
            /** * Constant for onPause event of the {@link LifecycleOwner}. */
            ON_PAUSE,
            /** * Constant for onStop event of the {@link LifecycleOwner}. */
            ON_STOP,
            /** * Constant for onDestroy event of the {@link LifecycleOwner}. */
            ON_DESTROY,
            /** * An {@link Event Event} constant that can be used to match all events. */
            ON_ANY
        }
    
    
        @SuppressWarnings("WeakerAccess")
        public enum State { 
    
            DESTROYED,
    
    
            INITIALIZED,
    
    
            CREATED,
    
    
            STARTED,
    
    
            RESUMED;
    
    
            public boolean isAtLeast(@NonNull State state) { 
                return compareTo(state) >= 0;
            }
        }
    }

LifecycleRegistry是Lifecycle的一个实现,它是用在Fragment和Activity上的,它可以处理多个生命周期观察者. 具体它有什么作用,后面再讲.

#### 2. ReportFragment的由来 ####

下面是ComponentActivity的onCreate()方法.

protected void onCreate(@Nullable Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
    }

可以看到,在上面搞了一句,注入一个ReportFragment(报告Fragment? 纪检委员? 打小报告的? 当然,我只是猜测).

有一点眉目了,其实就是在Activity中搞了一个Fragment,Fragment的生命周期我们知道了,当然就知道了Activity的生命周期,接着通知相关的观察者即可.当然,这个Fragment是没有界面的. 我们来看看,这个注入的方法干了啥.

public class ReportFragment extends Fragment { 
        private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
                + ".LifecycleDispatcher.report_fragment_tag";
    
        public static void injectIfNeededIn(Activity activity) { 
            // ProcessLifecycleOwner should always correctly work and some activities may not extend
            // FragmentActivity from support lib, so we use framework fragments for activities
            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();
            }
    }

其实这个injectIfNeededIn()看起来像是注入的方法干的就是将Fragment添加到Activity中,

来看看这个ReportFragment的生命周期方法都干了些啥,

@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);
    }
    
    private void dispatchCreate(ActivityInitializationListener listener) { 
        if (listener != null) { 
            listener.onCreate();
        }
    }

1.  通过调用dispatchCreate(mProcessListener)方法,感觉从命名上(是不是有点像`dispatchTouchEvent()`)看就知道是在干啥了: 分发当前的生命周期事件.
2.  dispatch(Lifecycle.Event.ON\_START); 感觉这个方法也像是在分发事件.

我们跟着这个mProcessListener来看看是在哪里设置的

/** * Class that provides lifecycle for the whole application process. */
    public class ProcessLifecycleOwner implements LifecycleOwner { 
        
        //注意,我是一个单例
        private static final ProcessLifecycleOwner sInstance = new ProcessLifecycleOwner();
    
        static void init(Context context) { 
            sInstance.attach(context);
        }
    
        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();
                }
            });
        }
    }
    
    //Activity的监听器
    ActivityInitializationListener mInitializationListener =
                new ActivityInitializationListener() { 
                    @Override
                    public void onCreate() { 
                    }
    
                    @Override
                    public void onStart() { 
                        activityStarted();
                    }
    
                    @Override
                    public void onResume() { 
                        activityResumed();
                    }
    
    private final LifecycleRegistry mRegistry = new LifecycleRegistry(this);
    
    //Activity创建的时候,分发Lifecycle.Event.ON_START事件
    void activityStarted() { 
        mStartedCounter++;
        if (mStartedCounter == 1 && mStopSent) { 
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
            mStopSent = false;
        }
    }

ReportFragment.java
    static ReportFragment get(Activity activity) { 
        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                REPORT_FRAGMENT_TAG);
    }

ProcessLifecycleOwner看起来就像是老大哥,给整个APP提供lifecycle的,也就是说通过它我们可以观察到整个应用程序的生命周期. 如何做到的? ProcessLifecycleOwner的attach()中registerActivityLifecycleCallbacks()注册了一个监听器,一旦有Activity创建就给它设置一个Listener.这样就保证了每个ReportFragment都有Listener.

既然是一个全局的单例,并且可以监听整个应用程序的生命周期,那么,肯定一开始就需要初始化.  
既然没有让我们在Application里面初始化,那么肯定就是在ContentProvider里面初始化的.

#### 3. 初始化 ####

> ps: 这里穿插一个小知识点: ContentProvider的onCreate()方法执行时间比Application的onCreate()执行时间还要早,而且肯定会执行.所以在ContentProvider的onCreate()方法里面初始化几个特殊的小东西是没啥问题的.

我们跟着ProcessLifecycleOwner的init()方法的调用处,来到了ProcessLifecycleOwnerInitializer,果不其然,它是一个ContentProvider.并且,在这里,真的就初始化了2个小东西.

public class ProcessLifecycleOwnerInitializer extends ContentProvider { 
        @Override
        public boolean onCreate() { 
            LifecycleDispatcher.init(getContext());
            ProcessLifecycleOwner.init(getContext());
            return true;
        }
    }

1.  ProcessLifecycleOwner初始化就不说了,是拿来观察整个应用的生命周期的,其原理就是利用ReportFragment,我们稍后详细到来.
2.  LifecycleDispatcher尤其重要.

class LifecycleDispatcher { 
        static void init(Context context) { 
            ...
            //registerActivityLifecycleCallbacks 注册一个监听器
            ((Application) context.getApplicationContext())
                    .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
        }
    }
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks { 
        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) { 
            //又来注入咯
            ReportFragment.injectIfNeededIn(activity);
        }
        @Override
        public void onActivityStopped(Activity activity) { 
        }
        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) { 
        }
    }

初始化的时候,就注册了一个监听器,每个创建的时候都给它注入一个ReportFragment.咦?这里又来注入一次,不是每个Activity都注册了一次么,在ComponentActivity中,搞啥玩意儿?

![u_3304691937_3517639434_fm_27_gp_0.jpg][]

我猜,可能是为了兼容吧.2次注入,确保万无一失.而且这个injectIfNeededIn()方法,内部实现是只会成功注入一次的,所以多调用一次,无所谓.

#### 4. 分发事件 ####

相当于,到了这里,应用程序里面的任何一个Activity都会被注入一个ReportFragment.而注入的这个无界面的ReportFragment是可以观察到当然Activity的生命周期的.

下面我们来仔细看一下,这个事件是如何一步步分发下去的.

ReportFragment.java
    @Override
    public void onActivityCreated(Bundle savedInstanceState) { 
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }
    
    private void dispatch(Lifecycle.Event event) { 
        Activity activity = getActivity();
        if (activity instanceof LifecycleRegistryOwner) { 
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }
    
        if (activity instanceof LifecycleOwner) { 
            //获取Activity中的LifecycleRegistry
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) { 
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }

不知道小伙伴儿们是否记得ComponentActivity是实现了LifecycleOwner的.

public class ComponentActivity extends Activity
            implements LifecycleOwner

下面我们获取到Activity中的LifecycleRegistry,下面的代码做了精简,只保留关键代码

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { 
        State next = getStateAfter(event);
        moveToState(next);
    }
    
    private void moveToState(State next) { 
        ......
        sync();
        ......
    }
    
    private void sync() { 
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        
        //循环 遍历所有观察者
        while (...) { 
            ....
            //分发事件
            forwardPass(lifecycleOwner);
        }
    }
    
    
    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);
                //分发事件
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }

上面的observer其实是一个ObserverWithState对象,

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);
            //生命周期变了.... 关键代码
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }

在ObserverWithState的构造方法中，通过 Lifecycling.getCallback(observer)根据传进来的 observer ，构造了一个 GenericLifecycleObserver 类型的 mLifecycleObserver ,我们跟进去看一下.

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);
    }

这个方法大概意思就是,根据传进的LifecycleObserver进行判断,构造一个GenericLifecycleObserver(目前是只有4个子类:FullLifecycleObserverAdapter、SingleGeneratedAdapterObserver、CompositeGeneratedAdaptersObserver、ReflectiveGenericLifecycleObserver)的对象.

#### 5. 依赖注入 ####

首先,这里穿插一点.我们在引入lifecycle时添加了语句`annotationProcessor "android.arch.lifecycle:compiler:1.1.1"`,这个其实是注解处理器的依赖.

引入这个之后,会自动生成`xxx_LifecycleAdapter`的文件,上面的demo中生成的是`MyObserver_LifecycleAdapter`文件,其内容如下:

public class MyObserver_LifecycleAdapter implements GeneratedAdapter { 
      final MyObserver mReceiver;
    
      MyObserver_LifecycleAdapter(MyObserver receiver) { 
        this.mReceiver = receiver;
      }
    
      @Override
      public void callMethods(LifecycleOwner owner, Lifecycle.Event event, boolean onAny,
          MethodCallsLogger logger) { 
        boolean hasLogger = logger != null;
        if (onAny) { 
          return;
        }
        if (event == Lifecycle.Event.ON_CREATE) { 
          if (!hasLogger || logger.approveCall("onCreate", 1)) { 
            mReceiver.onCreate();
          }
          return;
        }
        if (event == Lifecycle.Event.ON_START) { 
          if (!hasLogger || logger.approveCall("onStart", 1)) { 
            mReceiver.onStart();
          }
          return;
        }
        if (event == Lifecycle.Event.ON_RESUME) { 
          if (!hasLogger || logger.approveCall("onResume", 1)) { 
            mReceiver.onResume();
          }
          return;
        }
        if (event == Lifecycle.Event.ON_PAUSE) { 
          if (!hasLogger || logger.approveCall("onPause", 1)) { 
            mReceiver.onPause();
          }
          return;
        }
        if (event == Lifecycle.Event.ON_STOP) { 
          if (!hasLogger || logger.approveCall("onStop", 1)) { 
            mReceiver.onStop();
          }
          return;
        }
        if (event == Lifecycle.Event.ON_DESTROY) { 
          if (!hasLogger || logger.approveCall("onDestroy", 1)) { 
            mReceiver.onDestroy();
          }
          return;
        }
      }
    }

因为我们的事件是声明在MyObserver的方法注解上面的,每次去反射取这些东西,比较耗性能.那么我们通过该依赖库,把这些标注了的方法进行预处理,然后直接回调这些方法,避免反射,进行提高性能.666,佩服.

有了上面的知识之后,分析getCallback()方法,不难发现,因为MyObserver\_LifecycleAdapter只有一个构造方法,那么就会构造出SingleGeneratedAdapterObserver.而SingleGeneratedAdapterObserver内部其实就是调用一下方法而已.

public class SingleGeneratedAdapterObserver implements GenericLifecycleObserver { 
    
        private final GeneratedAdapter mGeneratedAdapter;
    
        SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) { 
            mGeneratedAdapter = generatedAdapter;
        }
    
        @Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) { 
            mGeneratedAdapter.callMethods(source, event, false, null);
            mGeneratedAdapter.callMethods(source, event, true, null);
        }
    }

上面的mGeneratedAdapter其实就是我们的MyObserver\_LifecycleAdapter.好了,结束了. 生命周期事件从Activity开始,然后到打小报告的ReportFragment那里出来,辗转发侧,终于到了我们定义的观察者,不容易啊.谷歌工程师写的代码就是牛逼.

![timg_image_quality_80_size_b9999_10000_sec_1552503196369_di_22d956465159008c3bfce40b4b2fa2e8_imgtype_0_src_http_3A_2F_2Fpic.962.net_2Fup_2F2018-4_2F15248137333417097.jpg][]

[u_3304691937_3517639434_fm_27_gp_0.jpg]: /images/20220308/1e00fb25b10d4ce0a862c247959b9a1b.png
[timg_image_quality_80_size_b9999_10000_sec_1552503196369_di_22d956465159008c3bfce40b4b2fa2e8_imgtype_0_src_http_3A_2F_2Fpic.962.net_2Fup_2F2018-4_2F15248137333417097.jpg]: https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1552503196369&di=22d956465159008c3bfce40b4b2fa2e8&imgtype=0&src=http%3A%2F%2Fpic.962.net%2Fup%2F2018-4%2F15248137333417097.jpg