Lifecycle是什么
- Lifecycle可以有效的避免内存泄漏和解决android生命周期的常见难题
- Livecycle 是一个表示android生命周期及状态的对象
- LivecycleOwner 用于连接有生命周期的对象，如activity,fragment
- LivecycleObserver 用于观察查LifecycleOwner
- Lifecycle框架使用观察者模式实现观察者监听被观察者的生命周期的变化
LifeCycle的各种使用方式
不使用lifecycle对生命周期的处理：
1.使用监听器
// TODO 第一个版本 监听器监听生命周期
class MainActivity : AppCompatActivity() {
private var myListener: MyListener ? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
myListener = MyListener()
}
override fun onStart() {
super.onStart()
myListener ?.start() // 会出现人为失误，一旦出现人为失误 造成不一致性功能问题
}
override fun onStop() {
super.onStop()
myListener ?.stop() // 会出现人为失误
}
}
class MyListener {
private val TAG = "MyListener"
fun start() = Log.d(TAG, "start run ...")
fun stop() = Log.d(TAG, "stop run ...")
}
2.MVP的P层
// TODO 第二个版本 MVP 的 P层
class MainActivity : AppCompatActivity() {
private var myPresenter: MyPresenter ? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
myPresenter = MyPresenter()
}
override fun onResume() {
super.onResume()
myPresenter ?.onResume() // 会出现人为失误
}
override fun onPause() {
super.onPause()
myPresenter ?.onPause() // 会出现人为失误
}
}
class MyPresenter {
private val TAG = "MyPresenter"
fun onResume() = Log.d(TAG, "onResume run ...")
fun onPause() = Log.d(TAG, "onPause run ...")
}
使用lifecycle:
1.
// TODO 第二个版本 MVP 的 P层
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// 一般是在BaseActivity 关联注册的
// 观察者 与 被观察者 关联的环节
lifecycle.addObserver(MyObserver())
lifecycle.addObserver(MyObserver2())
}
}
// 观察者 眼睛
class MyObserver : LifecycleObserver {
private val TAG = "MyObserver"
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME) // 画面可见 就连接
fun connectListener() = Log.d(TAG,"connectListener run ...")
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) // 画面不可见 就断开连接
fun disconnectListener() = Log.d(TAG,"disconnectListener run ...")
}
// DefaultLifecycleObserver 就是对 LifecycleObserver 二次封装 为了用户好用
class MyObserver2 : DefaultLifecycleObserver {
private val TAG = "MyObserver2"
override fun onCreate(owner: LifecycleOwner) {
super.onCreate(owner)
Log.d(TAG,"onCreate run ...")
// 项目实战 给大家演示好处 可以拿到 Activity Fragment 所有环境 Toast
}
override fun onResume(owner: LifecycleOwner) {
super.onResume(owner)
Log.d(TAG,"onResume run ...")
}
override fun onPause(owner: LifecycleOwner) {
super.onPause(owner)
Log.d(TAG,"onPause run ...")
}
}
2.使用内部类监听
// TODO 第四个版本 会内部类 监听
class MainActivity : AppCompatActivity() {
private val TAG = "MainActivity"
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// 一般是在BaseActivity 关联注册的
lifecycle.addObserver(MyObserver())
}
inner class MyObserver : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume() {
Log.d(TAG, "Lifecycle call onResume");
// 逻辑
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
fun onPause() {
Log.d(TAG, "Lifecycle call onPause");
}
}
/*override fun onResume() {
super.onResume()
// 逻辑
}*/
}
3.使用接口继承observer，再去实现接口
// TODO 第五个版本 接口监听法，设计模式的设计环节，你会见到这种写法
class MainActivity : AppCompatActivity() {
private val TAG = "MainActivity"
private var myPresenter : IPresenter ? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// 一般是在BaseActivity 关联注册的 (省略：工厂设计模式)
myPresenter = MyPresenter3()
lifecycle.addObserver(myPresenter !!)
}
}
interface IPresenter : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume()
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
fun onPause()
}
class MyPresenter3 : IPresenter {
private val TAG = "test"
override fun onResume() {
Log.d(TAG, "Lifecycle call onResume")
}
override fun onPause() {
Log.d(TAG, "Lifecycle call onPause")
}
}
LifeCycle的源码解析
通过追溯源码可以发现，当前Activity继承的是androidx.core.app.Component-Activity，而ComponentActivity实现了LifecycleOwner接口，所以开发者可以直接调用getLifecycle方法。
/**
* A class that has an Android lifecycle. These events can be used by custom components to
* handle lifecycle changes without implementing any code inside the Activity or the Fragment.
*
* @see Lifecycle
*/
@SuppressWarnings({"WeakerAccess", "unused"})
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
除了ComponentActivity之外，在ComponentActivity的子类androidx.fragment.app.FragmentActivity、androidx.appcompat.app.AppCompatActivity以及androidx.fragment.app.Fragment中都是可以直接使用Lifecycle的，这是AndroidX帮助开发者完成的。
在ComponentActivity的onCreate的函数中，使用了一个ReportFragment，ReportFragment是一个无页面的Fragment，它是用来协助Activity处理任务的。代码如下：
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
（这里可以看到高于29的版本做了单独的处理，后面会提到）
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
activity.registerActivityLifecycleCallbacks(
new LifecycleCallbacks());
}
// Prior to API 29 and to maintain compatibility with older versions of
// ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
// need to support activities that don't extend from FragmentActivity from support lib),
// use a framework fragment to get the correct timing of Lifecycle events
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();
}
}
通过源码注释可以看出，引入ReportFragment是为了兼容那些并不是直接继承自FragmentActivity的页面，这样它们就可以正常使用Lifecycle了。在Report-Fragment中可以看到对应生命周期的方法中都会执行dispatch方法，代码如下：
@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;
}
接下来看看dispatch方法的主要代码，具体如下：
private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
// Only dispatch events from ReportFragment on API levels prior
// to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
// added in ReportFragment.injectIfNeededIn
dispatch(getActivity(), event);
}
}
对于高于29的版本：
// this class isn't inlined only because we need to add a proguard rule for it. (b/142778206)
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
@Override
public void onActivityCreated(@NonNull Activity activity,
@Nullable Bundle bundle) {
}
@Override
public void onActivityPostCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
dispatch(activity, Lifecycle.Event.ON_CREATE);
}
@Override
public void onActivityStarted(@NonNull Activity activity) {
}
@Override
public void onActivityPostStarted(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_START);
}
@Override
public void onActivityResumed(@NonNull Activity activity) {
}
@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 onActivityPaused(@NonNull Activity activity) {
}
@Override
public void onActivityPreStopped(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_STOP);
}
@Override
public void onActivityStopped(@NonNull Activity activity) {
}
@Override
public void onActivitySaveInstanceState(@NonNull Activity activity,
@NonNull Bundle bundle) {
}
@Override
public void onActivityPreDestroyed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_DESTROY);
}
@Override
public void onActivityDestroyed(@NonNull Activity activity) {
}
}
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);
}
}
}
dispatch方法最终都会进入handleLifecycleEvent方法中，通过handleLifecycle-Event设置状态并通知观察者，Activity便能监听到生命周期的变化了。
dispatch()方法则会判断Activity是否实现了LifecycleOwner接口，如果实现了该接口就调用LifecycleRegister的handleLifecycleEvent()
这样生命周期的状态就会借由LifecycleRegistry通知给各个LifecycleObserver从而调用其中对应Lifecycle.Event的方法。这种通过Fragment来感知Activity生命周期的方法其实在Glide的中也是有体现的。
接下来我们看一下handleLifecycleEvent方法：
/**
* Sets the current state and notifies the observers.
* <p>
* Note that if the {@code currentState} is the same state as the last call to this method,
* calling this method has no effect.
*
* @param event The event that was received
*/
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
@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
}
/**
* Lifecycle states. You can consider the states as the nodes in a graph and
* {@link Event}s as the edges between these nodes.
*/
@SuppressWarnings("WeakerAccess")
public enum State {
/**
* Destroyed state for a LifecycleOwner. After this event, this Lifecycle will not dispatch
* any more events. For instance, for an {@link android.app.Activity}, this state is reached
* <b>right before</b> Activity's {@link android.app.Activity#onDestroy() onDestroy} call.
*/
DESTROYED,
/**
* Initialized state for a LifecycleOwner. For an {@link android.app.Activity}, this is
* the state when it is constructed but has not received
* {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} yet.
*/
INITIALIZED,
/**
* Created state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached in two cases:
* <ul>
* <li>after {@link android.app.Activity#onCreate(android.os.Bundle) onCreate} call;
* <li><b>right before</b> {@link android.app.Activity#onStop() onStop} call.
* </ul>
*/
CREATED,
/**
* Started state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached in two cases:
* <ul>
* <li>after {@link android.app.Activity#onStart() onStart} call;
* <li><b>right before</b> {@link android.app.Activity#onPause() onPause} call.
* </ul>
*/
STARTED,
/**
* Resumed state for a LifecycleOwner. For an {@link android.app.Activity}, this state
* is reached after {@link android.app.Activity#onResume() onResume} is called.
*/
RESUMED;
/**
* Compares if this State is greater or equal to the given {@code state}.
*
* @param state State to compare with
* @return true if this State is greater or equal to the given {@code state}
*/
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
handleLifecycleEvent方法中State next = getStateAfter(event);事件发生的时候，先得到当前activity应该出现的下一个状态。moveToState(next);mState = next;更新现在的状态sync();backwardPass(lifecycleOwner);逆推forwardPass(lifecycleOwner);正推，从图表上分析 ：
//如果状态和原本的一致 就直接return了
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;
}
private void sync() {
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.");
}
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) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
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);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
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);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
forwardPass(lifecycleOwner)方法中的细节ObserverWithState observer = entry.getValue();找到ObserverWithState 类。调用mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);生成观察者适配器。接下来就是调用onStateChanged(),来通知 实现了 LifecycleObserver的类，生命周期发生了变化；
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
/**
* Class that can receive any lifecycle change and dispatch it to the receiver.
* <p>
* If a class implements both this interface and
* {@link androidx.lifecycle.DefaultLifecycleObserver}, then
* methods of {@code DefaultLifecycleObserver} will be called first, and then followed by the call
* of {@link LifecycleEventObserver#onStateChanged(LifecycleOwner, Lifecycle.Event)}
* <p>
* If a class implements this interface and in the same time uses {@link OnLifecycleEvent}, then
* annotations will be ignored.
*/
public interface LifecycleEventObserver extends LifecycleObserver {
/**
* Called when a state transition event happens.
*
* @param source The source of the event
* @param event The event
*/
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}
查看实现类ReflectiveGenericLifecycleObserver。onStateChanged()
ReflectiveGenericLifecycleObserver的构造方法中就把presenter中的方法和注解保存了下来，再通过onStateChanged()进行生命周期的方法的调用
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
ClassInfoCache中使用map来保存我们的反射拿到的类信息，这样第一次通过反射创建，第二次可以直接从缓存里面拿：
class ClassesInfoCache {
private final Map<Class<?>, CallbackInfo> mCallbackMap = new HashMap<>();
private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) {
Class<?> superclass = klass.getSuperclass();
Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
handlerToEvent.putAll(superInfo.mHandlerToEvent);
}
}
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);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = method.getParameterTypes();
int callType = CALL_TYPE_NO_ARG;
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");
}
}
Lifecycle.Event event = annotation.value();
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");
}
}
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
MethodReference methodReference = new MethodReference(callType, method);
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
}
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);
}
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);
}
}
}
LifecycleRegistry中有带有状态的观察者：
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
返回的就是ReflectiveGenericLifecycleObserver
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) 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);
}
