1、概述
Lifecycle 是一个生命周期感知型组件,它可以执行操作来响应其他组件(如 Activity 和 Fragment)的生命周期状态变化,进而帮助我们编写出更有条理、更易维护且往往更精简的代码,同时还能一定程度上避免内存泄漏。Lifecycle 基于观察者模式,有三个重要角色:
- Lifecycle:存储有关组件(如 Activity 或 Fragment)生命周期状态的抽象类,默认实现为 LifecycleRegistry
- LifecycleOwner:生命周期的拥有者,是一个接口,如 Activity、Fragment 等。是观察者模式中被观察者的角色
- LifecycleObserver:生命周期的观察者,也是一个接口,在与 LifecycleOwner 建立了绑定关系之后,可以接收到 LifecycleObserver 生命周期的变化
2、基础使用
先引入 Lifecycle 的依赖:
dependencies {
def lifecycle_version = "2.4.0"
// Lifecycles only (without ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
// ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
// LiveData
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
// optional - ProcessLifecycleOwner provides a lifecycle for the whole application process
implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"
...
}
假如我们要监听 Activity 的生命周期,由于 AppCompatActivity 的父类 ComponentActivity 已经实现了 LifecycleOwner,因此我们创建的 Activity 无需再次实现 LifecycleOwner。监听者可以通过实现 LifecycleEventObserver、DefaultLifecycleObserver 或 LifecycleObserver 三个接口之一实现对 Activity 生命周期的监听。
2.1 实现 LifecycleEventObserver
LifecycleEventObserver 是 LifecycleObserver 的子接口,观察者可以通过该接口的 onStateChanged() 接收到 Lifecycle.Event,从而获取到 Activity 的生命周期事件:
public class TestPresenter implements LifecycleEventObserver {
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
switch (event) {
case ON_CREATE:
Log.d("TestPresenter", "onCreate");
break;
case ON_START:
Log.d("TestPresenter", "onStart");
break;
case ON_RESUME:
Log.d("TestPresenter", "onResume");
break;
case ON_PAUSE:
Log.d("TestPresenter", "onPause");
break;
case ON_STOP:
Log.d("TestPresenter", "onStop");
break;
case ON_DESTROY:
Log.d("TestPresenter", "onDestroy");
break;
case ON_ANY:
Log.d("TestPresenter", "onAny");
break;
}
}
}
2.2 实现 DefaultLifecycleObserver
DefaultLifecycleObserver 是 FullLifecycleObserver 的子接口,内部定义的 6 个生命周期方法都是默认方法:
public interface DefaultLifecycleObserver extends FullLifecycleObserver {
@Override
default void onCreate(@NonNull LifecycleOwner owner) {
}
@Override
default void onStart(@NonNull LifecycleOwner owner) {
}
@Override
default void onResume(@NonNull LifecycleOwner owner) {
}
@Override
default void onPause(@NonNull LifecycleOwner owner) {
}
@Override
default void onStop(@NonNull LifecycleOwner owner) {
}
@Override
default void onDestroy(@NonNull LifecycleOwner owner) {
}
}
可以根据需要实现 DefaultLifecycleObserver 内的部分方法以完成生命周期的监听。
2.3 实现 LifecycleObserver
如果你所使用的 Lifecycle 版本低于 2.4.0,也可以通过实现 LifecycleObserver 接口并配合 @OnLifecycleEvent 注解实现生命周期监听,但是不推荐使用这种方法。原因是使用注解时,内部可能会通过反射回调,使性能受到影响,所以官方从 Lifecycle 的 2.4.0 版本开始已经将 LifecycleObserver 标记为 @Deprecated,不再推荐使用。
如果你使用这种方式需要做两件事:
public class TestPresenter implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate() {
Log.d("TestPresenter", "onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart() {
Log.d("TestPresenter", "onStart");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume() {
Log.d("TestPresenter", "onResume");
}
@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
public void onAny() {
Log.d("TestPresenter", "onAny");
}
// 其余生命周期方法,省略...
}
首先观察者要实现 LifecycleObserver,然后用 @OnLifecycleEvent 注解标记一个方法(方法名可任取,不用像示例那样),注解的值为关注的生命周期事件。
无论你使用上述三种方法中的哪一种定义观察者,最后都需要与被观察者进行绑定才能顺利收到生命周期的变化通知:
// Activity 中通过 getLifecycle() 拿到的是 LifecycleRegistry 对象
getLifecycle().addObserver(new TestPresenter());
2.4 自定义 LifecycleOwner
Android 系统中的 LifecycleOwner 除了 Activity、Fragment 还有一个 ProcessLifecycleOwner(在 lifecycle-process 依赖中),它会在应用级别监听 Activity 生命周期。
除此之外,如果你想自定义 LifecycleOwner,需要在类内持有一个 LifecycleRegistry 对象,并通过调用 LifecycleRegistry 的 handleLifecycleEvent() 传入相应的生命周期事件以向观察者分发生命周期:
public class MyLifecycleOwner implements LifecycleOwner {
private LifecycleRegistry lifecycleRegistry = new LifecycleRegistry(this);
public void create() {
lifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
public void start() {
lifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
}
...
@NonNull
@Override
public Lifecycle getLifecycle() {
return lifecycleRegistry;
}
}
3、实现原理
主要关注 LifecycleObserver(后简称观察者)的注册以及 LifecycleOwner(后简称被观察者)生命周期变化时如何通知观察者这两个过程。大体思路是:
- 被观察者持有一个 LifecycleRegistry,观察者向 LifecycleRegistry 注册,后者会保存这些观察者
- 被观察者生命周期发生变化时,会更新 LifecycleRegistry 内维护的被观察者的生命周期状态,然后由 LifecycleRegistry 通知观察者生命周期状态的变化
3.1 注册观察者
在 Activity 中通过 getLifecycle().addObserver(LifecycleObserver) 的方式注册观察者,这是因为 AppCompatActivity 的父类 ComponentActivity 实现了 LifecycleOwner 并且指定 getLifecycle() 返回的是 LifecycleRegistry 对象:
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner {
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
/**
* 获取 Lifecycle 对象,返回的是其唯一实现类 LifecycleRegistry 的实例
*/
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
所以 addObserver() 实际上是向 LifecycleRegistry 中注册观察者,确切的说是添加到 LifecycleRegistry 内的 mObserverMap 中:
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
new FastSafeIterableMap<>();
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
/**
* 根据 observer 实现观察者的方式返回一个对应类型的 LifecycleEventObserver,
* 详见【3.2 区分不同类型的观察者】
*/
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
// 观察者状态初始化为传入的初始状态 initialState
mState = initialState;
}
// LifecycleRegistry 会通过这个方法告诉观察者生命周期发生变化
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
// 将生命周期的变化回调给观察者
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
ObserverWithState 封装了观察者的状态,并且将观察者又封装成 LifecycleEventObserver,当被观察者生命周期发生变化时,会调用 ObserverWithState.dispatchEvent(),执行 LifecycleEventObserver.onStateChanged() 回调给观察者。
接下来再看 addObserver() 如何注册观察者:
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
// 设置 observer 的初始状态,并将其和 observer 封装到 ObserverWithState 中
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
// 如果 observer 还没在 mObserverMap 中,可以将其存入 mObserverMap 中并返回 null,否则返回已有的 observer
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
// 正在添加观察者或者正在执行 sync() 时 isReentrance = true。
// mHandlingEvent 在 moveToState() 内执行 sync() 前被置为 true,执行后复位
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
// 计算出 observer 的目标状态
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
// 如果当前 statefulObserver 的状态数值小于目标状态,且 observer 已经存入
// mObserverMap,那就进行状态转移,通过 dispatchEvent() 进行状态分发
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
// 将被观察者的生命周期状态同步给观察者,这使得刚刚注册的观察者能立即
// 获得被观察者的生命周期状态
sync();
}
mAddingObserverCounter--;
}
注册观察者主要做了三件事:
- 将观察者对象 observer 存入 mObserverMap
- 计算出 observer 的目标状态,如果满足状态转移条件,则进行状态转移,通过 ObserverWithState 的 dispatchEvent() 将状态分发给 observer,这使得刚刚注册的观察者能立即获得被观察者的生命周期状态
- 如果当前没有处于添加观察者或执行 sync() 的状态,会执行一次 sync() 将被观察者的生命周期同步给 mObserverMap 中的所有观察者
3.2 生命周期变化的感知与通知
感知 Activity 生命周期变化的任务交给了 ReportFragment,在 ComponentActivity 的 onCreate() 中:
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
// ReportFragment 进行注入
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
一个没有 UI 的 ReportFragment 将自己依附到 ComponentActivity 上:
public static void injectIfNeededIn(Activity activity) {
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();
}
}
API 大于等于 29 会使用 LifecycleCallbacks 向观察者分发 Activity 的生命周期状态事件:
@RequiresApi(29)
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
static void registerIn(Activity activity) {
activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
}
@Override
public void onActivityPostCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
dispatch(activity, Lifecycle.Event.ON_CREATE);
}
@Override
public void onActivityPostStarted(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_START);
}
@Override
public void onActivityPostResumed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_RESUME);
}
@Override
public void onActivityPrePaused(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_PAUSE);
}
@Override
public void onActivityPreStopped(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_STOP);
}
@Override
public void onActivityPreDestroyed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_DESTROY);
}
...
}
API 低于 29 的版本会在 ReportFragment 的生命周期方法中对外分发状态事件:
@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);
}
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
@Override
public void onStop() {
super.onStop();
dispatch(Lifecycle.Event.ON_STOP);
}
@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
// just want to be sure that we won't leak reference to an activity
mProcessListener = null;
}
dispatch() 内部会通过 LifecycleRegistry.handleLifecycleEvent() 分发生命周期事件:
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);
}
}
}
LifecycleRegistry.handleLifecycleEvent() 会根据生命周期事件计算出生命周期状态,并进行状态转移:
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
// 根据 event 事件获取下一个状态
State next = getStateAfter(event);
// 移动到下一个状态
moveToState(next);
}
getStateAfter() 根据传入的生命周期事件计算出 Activity 处于哪一种生命周期状态:
static State getStateAfter(Event event) {
switch (event) {
// 在 Activity 发生了 onCreate() 和 onStop() 后,观察者应该进入 CREATED 状态
case ON_CREATE:
case ON_STOP:
return CREATED;
case ON_START:
case ON_PAUSE:
return STARTED;
case ON_RESUME:
return RESUMED;
case ON_DESTROY:
return DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException("Unexpected event value " + event);
}
这个状态转移的过程可以用下图表示:
上图中能看出在 ON_PAUSE 和 ON_STOP 两个事件后并没有进入 PAUSED 和 STOPED 状态,而是复用了 STARTED 和 CREATED 状态,这样生命周期的发展就有两个方向,这两个方向体现在 moveToState() 的 sync() 内:
// 表示 LifecycleOwner 当前的状态,在 sync() 内所有观察者会同步到 mState
private State mState;
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}
// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
// 将 Activity 的状态同步给观察者
private void sync() {
// private final WeakReference<LifecycleOwner> mLifecycleOwner;
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.");
}
// mObserverMap 中的所有观察者没有同步完成前会一直循环
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) {
// 将当前状态值与最早加入 mObserverMap 中的 Observer 的状态值相比,如果小于
// 就要逆向(即上图中向左,由可见到销毁的方向)移动
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
// 正向(上图向右,由不可见向可见的方向)移动
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
生命周期正向移动与方向移动的处理内容是类似的,以正向为例:
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);
final Event event = Event.upFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + observer.mState);
}
// 将最新状态分发给观察者
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
两层 while 循环,外层是遍历观察者的,内层是不断的对单个观察者分发生命周期事件的(因为状态转移是从当前状态到目标状态之间的每个状态都要触发,而不是直接跳转到目标状态)。upFrom() 会根据观察者的状态计算出从当前状态转移到下一个状态需要哪个事件触发:
@Nullable
public static Event upFrom(@NonNull State state) {
switch (state) {
case INITIALIZED:
return ON_CREATE;
case CREATED:
return ON_START;
case STARTED:
return ON_RESUME;
default:
return null;
}
}
最终由 ObserverWithState.dispatchEvent() 执行 LifecycleEventObserver.onStateChanged() 将这个事件传给观察者:
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
...
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
由于 LifecycleEventObserver 接口的实现类众多,下面我们来看看不同的实现类是如何处理生命周期事件的。
3.3 区分不同类型的观察者
在 ObserverWithState 的构造方法中通过 Lifecycling.lifecycleEventObserver() 可以确定 LifecycleEventObserver 的具体类型。 由于在【2.1~2.3】节介绍了多种实现观察者的方法,不同的实现方法决定了处理的方法也不同,因此需要加以区分:
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
// 1.观察者将两个接口都实现了
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
// 2.观察者仅实现了 FullLifecycleObserver 接口
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
// 3.观察者仅实现了 LifecycleEventObserver 接口
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}
// 4.获取观察者构造方法的类型,如果是 GENERATED_CALLBACK 则通过 GeneratedAdapter 回调
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);
}
// 5.如果 type 是 REFLECTIVE_CALLBACK,则使用反射回调生命周期变化
return new ReflectiveGenericLifecycleObserver(object);
}
无论返回哪一种实现类对象,最终都是要通过 LifecycleEventObserver 接口的 onStateChanged() 通知观察者生命周期状态发生了变化,只不过不同实现类的具体处理方式不同罢了。
3.3.1 观察者实现接口
当观察者将 FullLifecycleObserver、LifecycleEventObserver 两个接口都实现时,会被包装成 FullLifecycleObserverAdapter:
class FullLifecycleObserverAdapter implements LifecycleEventObserver {
private final FullLifecycleObserver mFullLifecycleObserver;
private final LifecycleEventObserver mLifecycleEventObserver;
FullLifecycleObserverAdapter(FullLifecycleObserver fullLifecycleObserver,
LifecycleEventObserver lifecycleEventObserver) {
mFullLifecycleObserver = fullLifecycleObserver;
mLifecycleEventObserver = lifecycleEventObserver;
}
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
// switch 是按照 FullLifecycleObserver 的方式通知观察者
switch (event) {
case ON_CREATE:
mFullLifecycleObserver.onCreate(source);
break;
case ON_START:
mFullLifecycleObserver.onStart(source);
break;
case ON_RESUME:
mFullLifecycleObserver.onResume(source);
break;
case ON_PAUSE:
mFullLifecycleObserver.onPause(source);
break;
case ON_STOP:
mFullLifecycleObserver.onStop(source);
break;
case ON_DESTROY:
mFullLifecycleObserver.onDestroy(source);
break;
case ON_ANY:
throw new IllegalArgumentException("ON_ANY must not been send by anybody");
}
// 以 LifecycleEventObserver 的方式通知观察者
if (mLifecycleEventObserver != null) {
mLifecycleEventObserver.onStateChanged(source, event);
}
}
}
分别以两种接口规定的处理方式通知观察者。
若观察者只实现了 FullLifecycleObserver,也会被包装成 FullLifecycleObserverAdapter,只不过其内部的 mLifecycleEventObserver 是 null,不会调用 mLifecycleEventObserver 的 onStateChanged()。
3.3.2 观察者使用注解与反射
回到 Lifecycling 继续看 lifecycleEventObserver(),如果观察者没有实现上一小节中提到的任何一个接口,那么就只能通过 GeneratedAdapter 或反射的方式向观察者分发生命周期了。
先根据观察者的 Class 对象,通过 getObserverConstructorType() 拿到观察者的回调类型:
// 缓存,key 是观察者的 Class 对象,value 是回调类型
private static Map<Class, Integer> sCallbackCache = new HashMap<>();
private static int getObserverConstructorType(Class<?> klass) {
Integer callbackCache = sCallbackCache.get(klass);
if (callbackCache != null) {
return callbackCache;
}
// 生成回调类型
int type = resolveObserverCallbackType(klass);
sCallbackCache.put(klass, type);
return type;
}
既然对 callbackCache 做了缓存,根据经验,resolveObserverCallbackType() 里应该是用到反射了:
/**
* 回调类型分为 REFLECTIVE_CALLBACK(=1) 和 GENERATED_CALLBACK(=2) 两种,
* 分别表示通过反射方式回调和通过生成注解代码的方式回调。
*/
private static int resolveObserverCallbackType(Class<?> klass) {
// 1.klass 是匿名类,只能通过反射
// anonymous class bug:35073837
if (klass.getCanonicalName() == null) {
return REFLECTIVE_CALLBACK;
}
// 2.能拿到 klass 的构造方法,就通过生成的代码回调
Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
if (constructor != null) {
sClassToAdapters.put(klass, Collections
.<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
return GENERATED_CALLBACK;
}
// 3.klass 中有方法使用了 @OnLifecycleEvent 注解
boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
if (hasLifecycleMethods) {
return REFLECTIVE_CALLBACK;
}
Class<?> superclass = klass.getSuperclass();
List<Constructor<? extends GeneratedAdapter>> adapterConstructors = null;
// 4.klass 的父类中有实现了 LifecycleObserver 的类
if (isLifecycleParent(superclass)) {
if (getObserverConstructorType(superclass) == REFLECTIVE_CALLBACK) {
return REFLECTIVE_CALLBACK;
}
adapterConstructors = new ArrayList<>(sClassToAdapters.get(superclass));
}
// 5.klass 实现的接口中,有构造方法类型是 REFLECTIVE_CALLBACK
for (Class<?> intrface : klass.getInterfaces()) {
if (!isLifecycleParent(intrface)) {
continue;
}
if (getObserverConstructorType(intrface) == REFLECTIVE_CALLBACK) {
return REFLECTIVE_CALLBACK;
}
if (adapterConstructors == null) {
adapterConstructors = new ArrayList<>();
}
adapterConstructors.addAll(sClassToAdapters.get(intrface));
}
// 6.adapterConstructors 不为空
if (adapterConstructors != null) {
sClassToAdapters.put(klass, adapterConstructors);
return GENERATED_CALLBACK;
}
// 7.默认情况
return REFLECTIVE_CALLBACK;
}
当从 getObserverConstructorType() 获取的类型是 GENERATED_CALLBACK 时,会根据观察者构造方法的个数生成 SingleGeneratedAdapterObserver(持有一个 GeneratedAdapter)或 CompositeGeneratedAdaptersObserver(持有多个 GeneratedAdapter),在回调 onStateChanged() 时会通过 GeneratedAdapter 接口的 callMethods() 通知观察者生命周期的变化。
而当 getObserverConstructorType() 返回 REFLECTIVE_CALLBACK 时,就会使用反射方式来通知观察者生命周期发生变化,也就是在【3.2】节中的 lifecycleEventObserver() 最终会得到一个 ReflectiveGenericLifecycleObserver 对象:
/**
* 一个基于反射的 LifecycleObserver 的内部实现类
*/
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
// 被封装的观察者
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
ReflectiveGenericLifecycleObserver 中有两个成员,mWrapped 是被封装的观察者对象,而 mInfo 是一个 CallbackInfo 对象,CallbackInfo 是 ClassesInfoCache 的内部类,内部有两个 Map 保存着生命周期事件和与该事件绑定的方法之间的映射关系:
static class CallbackInfo {
// Key 是生命周期事件,Value 是绑定了该事件的方法列表
final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
// Key 是处理生命周期事件的方法,Value 是与该方法绑定的生命周期事件
final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
/**
* 构造方法传入 handlerToEvent 并保存为 mHandlerToEvent ,并由此生成 mEventToHandlers
*/
CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
mHandlerToEvent = handlerToEvent;
mEventToHandlers = new HashMap<>();
for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {
Lifecycle.Event event = entry.getValue();
List<MethodReference> methodReferences = mEventToHandlers.get(event);
if (methodReferences == null) {
methodReferences = new ArrayList<>();
mEventToHandlers.put(event, methodReferences);
}
methodReferences.add(entry.getKey());
}
}
...
}
按照代码的执行顺序,当创建 ReflectiveGenericLifecycleObserver 对象时,在构造方法中通过 ClassesInfoCache 的 getInfo() 获取到 mInfo:
/**
* 如已有 klass 缓存则直接从 mCallbackMap 获取对应 CallbackInfo 后返回,
* 否则通过 createInfo() 创建一个新的 CallbackInfo 对象。
*/
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
如果没有缓存,需要通过 createInfo() 创建新的 CallbackInfo 对象:
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
// 1.klass 父类中如果有 mHandlerToEvent 要添加到 handlerToEvent 中
Class superclass = klass.getSuperclass();
// <生命周期变化时要执行的方法,该方法绑定的生命周期事件>
Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
handlerToEvent.putAll(superInfo.mHandlerToEvent);
}
}
// 2.klass 接口中的 <MethodReference, Lifecycle.Event> 也要加入 handlerToEvent
Class[] interfaces = klass.getInterfaces();
for (Class intrfc : interfaces) {
for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
intrfc).mHandlerToEvent.entrySet()) {
verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
}
}
// 3.getInfo() 给 declaredMethods 传的 null,所以这里走 getDeclaredMethods(),获取本类的所有方法
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
// 获取方法上的 OnLifecycleEvent 注解
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = method.getParameterTypes();
// 调用类型,根据参数个数变化,会被封装到 MethodReference 中
int callType = CALL_TYPE_NO_ARG;
// 先处理 OnLifecycleEvent 注解中有一个参数的情况
if (params.length > 0) {
callType = CALL_TYPE_PROVIDER;
if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
throw new IllegalArgumentException(
"invalid parameter type. Must be one and instanceof LifecycleOwner");
}
}
// OnLifecycleEvent 注解的值是Lifecycle 的事件类型,交给 event 保存
Lifecycle.Event event = annotation.value();
// 再看注解有两个参数的情况,看 if 条件,第二个参数如果有的话只能是 Lifecycle.Event.ON_ANY
if (params.length > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
if (event != Lifecycle.Event.ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
// 参数不能多于两个
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
// 将方法与对应的 callType 封装到 MethodReference 中
MethodReference methodReference = new MethodReference(callType, method);
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
// 将方法及其关注的生命周期事件封装到 CallbackInfo 中
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
所以 createInfo() 主要就是找到使用了 @OnLifecycleEvent 注解的方法,封装到 MethodReference 中,并将 MethodReference 添加到 handlerToEvent 集合,最后被封装到 CallbackInfo 对象中。
以上是 ReflectiveGenericLifecycleObserver 在构造方法中获取 CallbackInfo 的过程。接下来,当生命周期状态发生变化时,会回调 ReflectiveGenericLifecycleObserver 的 onStateChanged(),进而调用 CallbackInfo 的 invokeCallbacks():
/**
* ReflectiveGenericLifecycleObserver 的 onStateChanged() 调用此方法通知观察者
*/
@SuppressWarnings("ConstantConditions")
void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target);
}
/**
* 遍历 handlers 中的 MethodReference,调用其 invokeCallback()
*/
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
if (handlers != null) {
for (int i = handlers.size() - 1; i >= 0; i--) {
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
最终会调用到 MethodReference 的 invokeCallback(),以反射的形式调用方法:
static class MethodReference {
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
try {
switch (mCallType) {
// mMethod 上的 @OnLifecycleEvent 没有参数
case CALL_TYPE_NO_ARG:
mMethod.invoke(target);
break;
// mMethod 上的 @OnLifecycleEvent 有一个参数
case CALL_TYPE_PROVIDER:
mMethod.invoke(target, source);
break;
// mMethod 上的 @OnLifecycleEvent 有两个参数
case CALL_TYPE_PROVIDER_WITH_EVENT:
mMethod.invoke(target, source, event);
break;
}
} catch ...
}
}
至此,Lifecycle 的原理分析完毕。Lifecycle 是整个 Jetpack 的基础,后续我们要介绍的很多组件都与 Lifecycle 有着千丝万缕的联系。