*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:
- 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"
- 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();
}
}
- 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. - 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;
}
}
- 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.
- 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?
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.
辣鸡.png
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