本文目标
理解并掌握Lifecycle组件用法和原理
1.什么是Lifecycle
具备宿主生命周期感知能力的组件,它能持有组件(如Activity 和 Fragment)生命周期状态的信息,并且运行其他观察者监听宿主的状态
2.Lifecycle怎么用
首先需要在app里的gradle中引入
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlin-kapt'
dependencies {
def lifecycle_version = "2.2.0"
// ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
// LiveData
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
// Lifecycles only (without ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
//扫描所有编辑了 OnLifecycleEvent 注解的类,并生成实现类(可选)
kapt "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
}
一般情况下ViewModel,LiveData,Lifecycle都是混合使用,当然也可以只是引入Lifecycle
2.1第一种用法LifecycleObserver
该用法如果没有在gradle中引入 kapt "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
则是采用的 注解+反射去实现的,否则就是扫描所有编辑了 OnLifecycleEvent 注解的类,并生成实现类
第1步,自定义MyLifeObserver观察者,用注解声明每个方法观察宿主的声明周期状态
/**
* Author: 信仰年轻
* Date: 2020-11-25 16:16
* Email: [email protected]
* Des:lifecycle组件库的使用
* 我们用这个观查者来盯好需要感知生命周期的对象
*/
public class MyLifeObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void onCreateYd() {
Log.i("yd", "onCreateYd");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
void onStartYd() {
Log.i("yd", "onStartYd");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void onResumeYd() {
Log.i("yd", "onResumeYd");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
void onPauseYd() {
Log.i("yd", "onPauseYd");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
void onStopYd() {
Log.i("yd", "onStopYd");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
void onDestroyYd() {
Log.i("yd", "onDestroyYd");
}
}
第2步,在宿主中注册观察者,观察宿主生命周期状态变化,而且也不用实现反注册
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//注册观察者,监听Activity的生命周期
getLifecycle().addObserver(new MyLifeObserver());
}
}
这就可以完全监听MainActivity的生命周期了
2.2第二种用法 LifecycleEventObserver
该用法不会用到反射+注解的实现(建议使用这种)
第1步,自定义MyLifecycleEventObserver观察者实现LifecycleEventObserver接口,重写onStateChanged()方法,方法内根据event自行判断
/**
* Author: 信仰年轻
* Date: 2020-11-25 16:16
* Email: [email protected]
* Des:lifecycle组件库的使用 第2种用法
* 我们用这个观查者来盯好需要感知生命周期的对象
*/
public class MyLifecycleEventObserver implements LifecycleEventObserver {
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
//需要自行判断 life-event 是 onstart,还是onstop
switch (event){
case ON_CREATE:
Log.i("yd","ON_CREATE");
break;
case ON_START:
Log.i("yd","ON_START");
break;
case ON_RESUME:
Log.i("yd","ON_RESUME");
break;
case ON_PAUSE:
Log.i("yd","ON_PAUSE");
break;
case ON_STOP:
Log.i("yd","ON_STOP");
break;
case ON_DESTROY:
Log.i("yd","ON_DESTROY");
break;
default:
break;
}
}
}
第2步,在宿主中注册观察者,观察宿主生命周期状态变化,而且也不用实现反注册
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//注册观察者,监听Activity的生命周期
getLifecycle().addObserver(new MyLifecycleEventObserver());
}
}
这就可以完全监听MainActivity的生命周期了
2.3第三种用法 FullLifecycleObserver
但是目前在2.2.0这个版本,FullLifecycleObserver该接口 不是public的了
interface FullLifecycleObserver extends LifecycleObserver {
void onCreate(LifecycleOwner owner);
void onStart(LifecycleOwner owner);
void onResume(LifecycleOwner owner);
void onPause(LifecycleOwner owner);
void onStop(LifecycleOwner owner);
void onDestroy(LifecycleOwner owner);
}
实现该接口也是可以去监听生命周期的,但是目前该接口并不public,所以还是用前面两种吧
3.源码分析
我们从MainActivity中注册观察者这行代码开始分析
getLifecycle().addObserver(new MyLifecycleEventObserver());
通过一顿找,getLifecycle()这行代码是在MainActivity的顶层父类ComponentActivity中,
可以到在onCreate()方法中有一行ReportFragment.injectIfNeededIn(this),该行代码就是往Activity上添加一个fragment,用以报告生命周期的变化,
然后LifecycleRegistry对象是成员变量直接就创建出来了
public class ComponentActivity implements LifecycleOwner{
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//往Activity上添加一个fragment,用以报告生命周期的变化
//目的是为了兼顾不是继承自AppCompactActivity的场景
ReportFragment.injectIfNeededIn(this);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
来我们看下ReportFragment
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public class ReportFragment extends Fragment {
public static void injectIfNeededIn(Activity activity) {
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
@Override
public void onStart() {
super.onStart();
dispatch(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
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);
}
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
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);
}
}
}
}
可以看到生命周期事件发送是通过LifecycleRegistry的andleLifecycleEvent(event);执行的,来我们继续看addObserver(new MyLifecycleEventObserver());这行代码,是在LifecycleRegistry类中
4.LifecycleRegistry
addObserver方法
public class LifecycleRegistry extends Lifecycle {
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
//只要不是在宿主的DESTROYED状态注册的,这里都是INITIALIZED状态
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//包装成拥有宿主状态的观察者
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//添加到mObserverMap这个map中
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//1.先通过`calculateTargetState()`方法计算出来宿主应该到达的状态,实际上就是当前宿主的状态
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
//2.把观察者的状态和targetState的状态做一个比较,如果 小于0,则说明观察者的状态还没有到达宿主的状态,既然没有,就让宿主的生命周期前进
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
//3.分发事件,里面会upEvent
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--;
}
//根据宿主状态来判断生命周期该接受哪个事件
private static Event upEvent(State state) {
switch (state) {
case INITIALIZED:
case DESTROYED:
return ON_CREATE;
case CREATED:
return ON_START;
case STARTED:
return ON_RESUME;
case RESUMED:
throw new IllegalArgumentException();
}
throw new IllegalArgumentException("Unexpected state value " + state);
}
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
//根据event反推出新的宿主状态
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
//根据event反推出新的宿主状态
static State getStateAfter(Event event) {
switch (event) {
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);
}
}
该addObserver方法中
- 1.先通过
calculateTargetState()方法计算出来宿主应该到达的状态,实际上就是当前宿主的状态 - 2.把观察者的状态和targetState的状态做一个比较,如果 小于0,则说明观察者的状态还没有到达宿主的状态,既然没有,就让宿主的生命周期前进
- 3.根据宿主状态来判断生命周期该接受哪个事件
- 4.接受完事件来到
dispatchEvent方法中去分发事件 - 5.分发完之后还会再次比较
statefulObserver.mState.compareTo(targetState) < 0直到不小于0(观察者的状态和宿主的状态到达一致)
while循环的目的是为了让ObserverWithState这个观察者状态从INITIALIZED状态前进到宿主当前的状态(可能是CREATED,STARTED,也可能是RESUMED)
假设现在在宿主的onResume()方法中去注册一个观察者,我们的观察者会接受到哪几个生命周期的事件呢?
首先会有onResume(),还会有onCreate(),onStart()等事件,这个逻辑就在这个while循环里面
5.宿主生命周期与宿主状态模型图
宿主的状态和生命周期不是一个概念,
- 宿主在创建之初是
INITIALIZED状态,当执行了ON_CREATE方法之后就会前进到CREATED状态, - 接着当宿主执行了
ON_START方法就会前进到STARTED状态, - 然后执行了
ON_RESUME方法就会前进到RESUMED状态,
以上是生命周期前进的过程,从INITIALIZED初始化到RESUMED
接着是生命周期倒退的过程 - 从
RESUMED状态执行了ON_PAUSE方法就会倒退到STARTED状态 - 在之后宿主执行了
ON_STOP方法倒退到CREATED状态 - 执行了
ON_DESTROY方法倒退到DESTROYED状态
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
}
6. 宿主的生命周期变化之后是如何分发给观察者的
在Fragment中会在每个生命周期的方法中调用 mLifecycleRegistry.handleLifecycleEvent(EVENT),如下代码
public class Fragment implements LifecycleOwner{
LifecycleRegistry mLifecycleRegistry;
private void initLifecycle() {
mLifecycleRegistry = new LifecycleRegistry(this);
mSavedStateRegistryController = SavedStateRegistryController.create(this);
}
@Override
@NonNull
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
void performCreate(Bundle savedInstanceState) {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
void performStart() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
}
......
void performResume() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
}
}
来看下handleLifecycleEvent方法的具体实现
public class LifecycleRegistry extends Lifecycle {
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//根据当前的事件推导出下一个事件
State next = getStateAfter(event);
moveToState(next);
}
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.");
}
//判断的是往这个集合中注册的Observer的所有的观察者状态都已经同步到了跟宿主一致的状态
//如果还没有就进
while (!isSynced()) {
mNewEventOccurred = false;
//宿主当前的状态和集合中每个观察者的状态做比较
//如果宿主的状态 < 观察者的状态(发生在生命周期倒退的阶段)
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
//集合中所有观察者的状态倒退到和宿主一样的状态
backwardPass(lifecycleOwner);
}
Entry newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//生命周期前进
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
//集合中所有观察者的状态倒退到和宿主一样的状态,并且分发相应的事件
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
//生命周期倒退
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
//集合中所有观察者的状态前进到和宿主一样的状态,并且分发相应的事件
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
//生命周期前进
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
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;
}
}
}
总结一下,宿主的生命周期和宿主的状态的关系,每个生命周期变化的时候都会分发相应的事件,并且会根据这个事件推导出宿主的状态,然后遍历所有的观察者,让他们的状态也随之升级或者降级,并且把本次事件分发给观察者
7.LifecycleOwner,Lifecycle,LifecycleRegistry的关系
- 1.
LifecycleOwner,宿主的意思,代表这Activity和Fragment,只要是实现该接口就必须要重写接口中的Lifecycle getLifecycle();方法,返回Lifecycle对象,其实该对象就是LifecycleRegistry对象 - 2.
Lifecycle,是个抽象类 - 3.
LifecycleRegistry继承自Lifecycle
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}