Android JetPack组件之Lifecycle
官方文档地址：https://developer.android.google.cn/topic/libraries/architecture/lifecycle
定义
Lifecycle是Jetpack提供的一个组件，可以感知Activity,Fragment的生命周期变化。这些组件可以根据 Activity 或 Fragment 的当前生命周期状态自动调整其行为。
相关概念
LifecycleOwner：我们的 Activity/Fragment 都实现了该接口，用以声明它是一个能够提供生命周期事件的宿主。同时必须复写getLifecycle()方法提供一个Lifecycle对象；
Lifecycle：是一个抽象类，里面定义了枚举State宿主的状态，Event 需要分发的事件的类型；
LifecycleRegistry: 是 Lifecycle 的唯一实现类，主要用来负责注册 Observer，以及分发宿主状态事件给它们。
基本使用
//添加以下两个依赖
implementation "androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
通过LifecycleOwner来获取到Lifecycler，View层已经实现了LifecyclerOnwer接口。比如说AppCompatActivity、Fragment已经实现了LifecycleOwner接口。
在activity中，或者fragment中
直接
this.lifecycle
kotlin的写法，如果是java get即可。
拿到以后，往里面添加监听
this.lifecycle.addObserver()
一般来说，传接口都是要实现xxx接口的
这个是我们addObserver的方法
void addObserver(@NonNull LifecycleObserver observer)
也就是说，你需要传一个实现了LifecycleObserver 的类，或者实现了LifecycleObserver的子类才可以。
比如说我们的Presenter需要知道View层的生命周期变化。
我们把Owner传给Presneter
MusicPresenter(owner: LifecycleOwner)
初始化的时候，添加观察者
init {
owner.lifecycle.addObserver(viewLifeImpl)
}
这个观察者的实现类
private val viewLifeImpl by lazy {
ViewLifeImpl()
}
LifecycleEventObserver实现类，LifecycleEventObserver是LifecycleObserver的子类
inner class ViewLifeImpl : LifecycleEventObserver {
/**
* 被动通知View层的生命周期变化
*/
override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
when (event) {
Lifecycle.Event.ON_START -> {
//监听GPS信号变化
println("监听GPS信号变化")
//开始监听网络变化
}
Lifecycle.Event.ON_PAUSE -> {
//停止GPS信号变化监听
println("停止GPS信号变化监听")
//停止网络状态变化更新
}
else -> {
}
}
}
}
当View层的生命周期发生变化的时候，就会通知到我们添加的监听了。
也就是会通知到onStateChange里头。
另外一种方式则是通过注解的方式。
同样也是实现LifecycleObserver接口
里面的方法添加注解，比如说：
inner class ValueObserverWrapper(private val valueObserver: (T?) -> Unit) :
LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
fun removeValueObserver() {
println("removeValueObserver...")
//当监听到当前的View，生命周期为Destroy的时候，就把LifecycleProvider从集合中删除
viewLifecycleProviders.remove(valueObserver)
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
fun onStop() {
}
@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
fun onAny() {
}
}
然后创建这个类，添加到Lifecycle里
以上两种方式，是被动的通知。也就是生命周期发生变化的时候，会通知你View层的生命周期发生变化了。
其实除了被动通知以外，还可以主动获取。
我们拿到Lifecycle以后，直接获取currentState，就知道当前View是一个什么状态了。
lifecycle.currentState
如果是java的话，就get
源码分析：
- View层生命周期变化如何通知Lifecycle
- Lifecycler如何通知观察者View层生命周期变化
View层生命周期变化如何通知Lifecycle?
首先在View层中（比如FragmentActivity）创建一个
final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this);
在View走onCreate方法的时候
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
比如说Destroy的时候
@Override
protected void onDestroy() {
super.onDestroy();
...
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
}
到这里，就通知到了Lifecycle生命周期发生变化了。
Lifecycle如何通知观察者View层生命周期变化?
我们在注册观察者的时候开始向Lifecycle挂上关系的
owner.lifecycle.addObserver(viewLifeImpl)
下面是addObserver的方法
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
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;
}
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
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.
sync();
}
mAddingObserverCounter--;
}
其中 mObserverMap.putIfAbsent方法会将观察者添加到集合中
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
接着，我们回去看生命周期发生变化的地方
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
这是moveToState方法
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;
}
这里其实就是事件分发了
// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
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;
}
再到这里的遍历
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();
}
}
}
里面有一个 observer.dispatchEvent(lifecycleOwner, event);方法
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
onStateChange()方法
mLifecycleObserver.onStateChanged(owner, event);
走到这里，就会通知到我们的实现代码了，因为mLifecycleObserver就是我们传进来保存到mObserverMap集合里的实例。
inner class ViewLifeImpl:LifecycleEventObserver{
/**
* 被动通知View层的生命周期变化
*/
override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
when(event){
Lifecycle.Event.ON_START->{
println("监听GPS信号变化")
//开始监听网络变化
}
Lifecycle.Event.ON_PAUSE->{
println("停止GPS的信号变化")
//停止网络状态信息变化更新
}
}
}
}