作用
lifecycle是一个生命周期感知型组件可执行操作来响应另一个组件(如 Activity 和 Fragment)的生命周期状态的变化。可以让将一部分Activity的业务逻辑放到Lifecycle中让代码看起来更条理和简洁。
基本使用
MainActivity.kt
class MainActivity : AppCompatActivity() {
private val TAG:String = "MainLifecycle"
private val life:MainLifecycle = MainLifecycle()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
Log.d(TAG,"onCreate")
lifecycle.addObserver(life)
}
override fun onStart() {
super.onStart()
Log.d(TAG,"onStart")
}
override fun onResume() {
super.onResume()
Log.d(TAG,"onResume")
}
override fun onPause() {
super.onPause()
Log.d(TAG,"onPause")
}
override fun onStop() {
super.onStop()
Log.d(TAG,"onStop")
}
override fun onDestroy() {
super.onDestroy()
Log.d(TAG,"onDestroy")
}
}
MainLifecycle.kt
class MainLifecycle : LifecycleObserver{
private val TAG:String = "MainLifecycle"
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
private fun create(){
Log.i(TAG,"create")
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
private fun start(){
Log.i(TAG,"start")
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
private fun resume(){
Log.i(TAG,"resume")
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
private fun pause(){
Log.i(TAG,"pause")
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private fun stop(){
Log.i(TAG,"stop")
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
private fun destroy(){
Log.i(TAG,"destroy")
}
}
Log输出
2021-07-20 10:29:52.046 15488-15488/com.example.jetpacklearn D/MainLifecycle: onCreate
2021-07-20 10:29:52.049 15488-15488/com.example.jetpacklearn I/MainLifecycle: create
2021-07-20 10:29:52.062 15488-15488/com.example.jetpacklearn D/MainLifecycle: onStart
2021-07-20 10:29:52.063 15488-15488/com.example.jetpacklearn I/MainLifecycle: start
2021-07-20 10:29:52.064 15488-15488/com.example.jetpacklearn D/MainLifecycle: onResume
2021-07-20 10:29:52.067 15488-15488/com.example.jetpacklearn I/MainLifecycle: resume
2021-07-20 10:29:55.947 15488-15488/com.example.jetpacklearn I/MainLifecycle: pause
2021-07-20 10:29:55.948 15488-15488/com.example.jetpacklearn D/MainLifecycle: onPause
2021-07-20 10:29:56.366 15488-15488/com.example.jetpacklearn I/MainLifecycle: stop
2021-07-20 10:29:56.370 15488-15488/com.example.jetpacklearn D/MainLifecycle: onStop
需要注意一点,Resume之前都是先执行Activity的,Resume之后都是先执行Lifecycle的生命周期。
将自定义的Class继承LifecycleObserver,将要关联生命周期的方法前加上注解
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
在Activity中调用
getLifecycle.addObserver(lifecycle)
与之关联就可以了
原理解析
从getLifecycle.addObserver(lifecycle)方法进入到Activity继承自ComponentActivity
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
LifecycleRegistry这个类负责LifecycleOwner和LifecycleObserver之间时间的传递
在它的OnCreate方法中
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
ReportFragment.injectIfNeededIn(this)
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
activity.registerActivityLifecycleCallbacks(
new LifecycleCallbacks());
}
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
API大于29会注册LifecycleCallbacks。点进去这个方法看一下
public void registerActivityLifecycleCallbacks(
@NonNull Application.ActivityLifecycleCallbacks callback) {
synchronized (mActivityLifecycleCallbacks) {
mActivityLifecycleCallbacks.add(callback);
}
}
查看mActivityLifecycleCallbacks这个变量会发现
private void dispatchActivityPreCreated(@Nullable Bundle savedInstanceState) {
getApplication().dispatchActivityPreCreated(this, savedInstanceState);
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i = 0; i < callbacks.length; i++) {
((Application.ActivityLifecycleCallbacks) callbacks[i]).onActivityPreCreated(this,
savedInstanceState);
}
}
}
private void dispatchActivityPreStarted() {
getApplication().dispatchActivityPreStarted(this);
Object[] callbacks = collectActivityLifecycleCallbacks();
if (callbacks != null) {
for (int i = 0; i < callbacks.length; i++) {
((Application.ActivityLifecycleCallbacks) callbacks[i]).onActivityPreStarted(this);
}
}
}
有很多类似生命周期的方法会循环调用存入的callback来通知生命周期的改变。collectActivityLifecycleCallbacks这个方法只是将mActivityLifecycleCallbacks变量从List变成Array方便循环调用。
再回过头去看ReportFragment
@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);
}
都是通过dispatch()方法将生命周期传递出去。
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);
}
}
}
将Activity强制转换为LifecycleRegistryOwner后通过调用getLifecycle()来获取LifecycleRegistry去调用它的 handleLifecycleEvent。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
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.");
}
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;
}
判断状态后会分别调用backwardPass(lifecycleOwner),forwardPass(lifecycleOwner)。
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();
}
}
}
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();
}
}
}
先看一下ObserverWithState,忘了说addObserver方法了,顺便看一下
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--;
}
创建了ObserverWithState对象,将LifecycleObserver与ObserverWithState存入mObserverMap,经过判断后最后调用了ObserverWithState的dispatchEvent方法。这下来看ObserverWithState
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;
}
}
它的构造方法将LifecycleObserver包装成LifecycleEventObserver,通过LifecycleEventObserver.onStateChanged来发送生命周期的变化。 上面的backwardPass(lifecycleOwner),forwardPass(lifecycleOwner)。最后也会调用dispatchEvent方法来完成事件的发送。