android源码（1）LiveData源码
2、LiveData如何感知生命周期？
总结：首先看数据：LiveData中的 mObservers 和 LifecycleRegistry 中的 mObserverMap。
LiveData中：
// key: observer; value: new LifecycleBoundObserver(owner, observer)
SafeIterableMap<Observer<T>, ObserverWrapper> mObservers = new SafeIterableMap<>();
public void observe(LifecycleOwner owner, Observer<T> observer) {
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
mObservers.putIfAbsent(observer, wrapper);
owner.getLifecycle().addObserver(wrapper);
}
LifecycleRegistry中：
// key：LifecycleBoundObserver(owner, observer)
// value : ObserverWithState( LifecycleBoundObserver(owner, observer), initialState)
FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>();
public void addObserver(LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
}
说白了就这两条线：
1、从 livedata 的 setValue()触发：使用mObservers中存储的observer更新数据；
2、从 fragment 的 onCreat/onStart/…等触发： 使用LifecycleRegistry中存储的observer（不用mObservers）更新数据。
关键对象：
1、LiveData: 作为数据入口，维护观察者和被观察者的关系
2、Observer：数据改变后最终触发的函数，回调接口，调用端实现
3、LifecycleOwner: Activity, Fragment 提供 Lifecycle
4、Lifecycle, LifecycleRegistry
5、LifecycleObserver，GenericLifecycleObserver, LifecycleBoundObserver: 封装Observer, LifecycleOwner 对象
SupportActivity中：
public class SupportActivity extends Activity implements LifecycleOwner {
@Override
@SuppressWarnings("RestrictedApi")
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
}
上面关键的一句ReportFragment.injectIfNeededIn(this);下面看ReportFragment各生命周期：
public class ReportFragment extends Fragment {
@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;
}
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) {
// 让LifecycleRegistry去处理生命周期事件,把生命周期 Event 传递给 LifecycleObserver
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
}
LifecycleRegistry就是LifeCycle的登记处，它以弱引用的方式持有了当前的Activity。
至此：当lifecycle发生变化时，handleLifecycleEvent 会通过 getStateAfter()方法获取当前应处的状态并修改mState值，紧接着遍历所有 ObserverWithState并调用他们的sync方法来同步且通知LifecycleObserver状态发生变化。
public class LifecycleRegistry extends Lifecycle {
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
//设置当前状态并通知观察者
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
// 根据生命周期事件得到对应的状态
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;
// 同步状态给各个观察者
sync();
mHandlingEvent = false;
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
+ "new events from it.");
return;
}
while (!isSynced()) {
mNewEventOccurred = false;
//比较当前的生命周期和Map中第一个的观察者的生命周期
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
// 倒序遍历，通知LifecycleObserver，去修改状态
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
// 正序遍历，通知观察者，去修改状态
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
// key：LifecycleBoundObserver(owner, observer)
// value : ObserverWithState( LifecycleBoundObserver(owner, observer), initialState)
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>();
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));
//生命周期变化后
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);
//生命周期变化后
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
}
ObserverWithState中：
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//到这里
//mLifecycleObserver ： LifecycleBoundObserver(owner, observer)
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
LifecycleBoundObserver是LiveData的内部类：
LiveData 和 生命周期的关系基本操作：
生命周期正常的时候， addObserver 注册观察者；生命周期结束的时候 removeObserver 取消观察者
class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
.........
@Override
boolean shouldBeActive() {
return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
}
//该方法会在生命周期变化后触发
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
//根据事件判断是否要解除订阅
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
removeObserver(mObserver);
return;
}
//看这里
activeStateChanged(shouldBeActive());
}
}
LiveData中：
@MainThread
public void removeObserver(@NonNull final Observer<T> observer) {
assertMainThread("removeObserver");
ObserverWrapper removed = mObservers.remove(observer);
if (removed == null) {
return;
}
removed.detachObserver();
removed.activeStateChanged(false);
}
ObserverWrapper中:
private abstract class ObserverWrapper {
void activeStateChanged(boolean newActive) {
if (newActive == mActive) {
return;
}
........
if (mActive) {
dispatchingValue(this);
}
}
liveData中dispatchingValue（）
// key: observer ; value: LifecycleBoundObserver(owner, observer)
private SafeIterableMap<Observer<T>, ObserverWrapper> mObservers = new SafeIterableMap<>();
private void dispatchingValue(@Nullable ObserverWrapper initiator) {
if (mDispatchingValue) {
mDispatchInvalidated = true;
return;
}
mDispatchingValue = true;
do {
mDispatchInvalidated = false;
//initiator ：LifecycleBoundObserver(owner, observer)
//class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver
if (initiator != null) {
considerNotify(initiator); //看这里
initiator = null;
} else {
for (Iterator<Map.Entry<Observer<T>, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
//mObservers看上面
considerNotify(iterator.next().getValue()); //看这里
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}
//class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver
//参数observer ：LifecycleBoundObserver(owner, observer)
private void considerNotify(ObserverWrapper observer) {
if (!observer.mActive) {
return;
}
if (!observer.shouldBeActive()) {
observer.activeStateChanged(false);
return;
}
if (observer.mLastVersion >= mVersion) {
return;
}
observer.mLastVersion = mVersion;
// 最终调用观察者的onChanged()回调
observer.mObserver.onChanged((T) mData); //最后到这里
}
3、LiveData注册Observer
添加观察者过程实际上是把 Observer 传递给 LiveData 对象 和 LifecycleRegistry 对象。
我们一般在fragment中这样用：
liveData.observe(this, new Observer<Object>() {
@Override
public void onChanged(@Nullable Object o) {
}
});
进入observe方法：
// key: observer ; value: new LifecycleBoundObserver(owner, observer)
private SafeIterableMap<Observer<T>, ObserverWrapper> mObservers = new SafeIterableMap<>();
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
return;
}
// 将LifecycleOwner和Observer包装了一层
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
// 判断Observer有没有被添加过，如果没有则添加到一个HashMap中mObservers
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && !existing.isAttachedTo(owner)) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
// 将包装了owner 和 observer 的 LifecycleBoundObserver添加到LifecycleRegistry
owner.getLifecycle().addObserver(wrapper);
}
继续进入LifecycleRegistry的addObserver()：LifecycleRegistry 中存储 observer:
// key：LifecycleBoundObserver(owner, observer)
// value : ObserverWithState( LifecycleBoundObserver(owner, observer), initialState)
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
new FastSafeIterableMap<>();
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
// 将LifecycleObserver包装为ObserverWithState
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
// 如果没有保存过就存入mObserverMap
// mObserverMap是一个可以在迭代过程中修改的HashMap
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(); //源码前面有
}
mAddingObserverCounter--;
}
4、LiveData数据设置
LiveData有两种方式设置数据，第一种setValue()，在主线程中使用
而 postValue在异步线程中使用：
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null); //前面已经有了dispatchingValue的源码
}
postValue方法:
通过一个以MainLooper作为Looper的Handler将mPostValueRunnable发送到主线程上，最终还是调用了setValue()。setValue()中调用了dispatchingValue()，进而调用了considerNotify()来通知数据的改变。
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
private final Runnable mPostValueRunnable = new Runnable() {
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
//noinspection unchecked
setValue((T) newValue);
}
};
ArchTaskExecutor 使用了代理模式, 实际函数执行类是 DefaultTaskExecutor：
public class DefaultTaskExecutor extends TaskExecutor {
private final Object mLock = new Object();
private ExecutorService mDiskIO = Executors.newFixedThreadPool(2);
@Nullable
private volatile Handler mMainHandler;
@Override
public void executeOnDiskIO(Runnable runnable) {
mDiskIO.execute(runnable);
}
@Override
public void postToMainThread(Runnable runnable) {
if (mMainHandler == null) {
synchronized (mLock) {
if (mMainHandler == null) {
mMainHandler = new Handler(Looper.getMainLooper());
}
}
}
//noinspection ConstantConditions
mMainHandler.post(runnable);
}
@Override
public boolean isMainThread() {
return Looper.getMainLooper().getThread() == Thread.currentThread();
}
}