使用Lifecycle
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。
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"
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.
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。
Lifecycle原理解析
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。
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。
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。
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。
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。