详解Android JetPack之LiveData的工作原理

public class UserProfileViewModel extends ViewModel {
 private String userId;
 private MutableLiveData<User> user;
 private UserRepository userRepo;

 public void init(String userId) {
  this.userId = userId;
  userRepo = new UserRepository();
  user = userRepo.getUser(userId);
 }

 public void refresh(String userId) {
  user = userRepo.getUser(userId);
 }

 public MutableLiveData<User> getUser() {
  return user;
 }

}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
 super.onActivityCreated(savedInstanceState);
 String userId = getArguments().getString(UID_KEY);
 viewModel = ViewModelProviders.of(this).get(UserProfileViewModel.class);
 viewModel.init(userId);
 //标注1
 viewModel.getUser().observe(UserProfileFragment.this, new Observer<User>() {
  @Override
  public void onChanged(@Nullable User user) {
   if (user != null) {
    tvUser.setText(user.toString());
   }
  }
 });
}

@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
 //标注1
 if (owner.getLifecycle().getCurrentState() == DESTROYED) {
  // ignore
  return;
 }
 //标注2
 LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
 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;
 }
 owner.getLifecycle().addObserver(wrapper);
}

class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
 @NonNull final LifecycleOwner mOwner;

 LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<T> observer) {
  super(observer);
  mOwner = owner;
 }

 @Override
 boolean shouldBeActive() {
  return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
 }

 @Override
 public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
  if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
   removeObserver(mObserver);
   return;
  }
  activeStateChanged(shouldBeActive());
 }

 @Override
 boolean isAttachedTo(LifecycleOwner owner) {
  return mOwner == owner;
 }

 @Override
 void detachObserver() {
  mOwner.getLifecycle().removeObserver(this);
 }
}

public class UserRepository {
 final MutableLiveData<User> data = new MutableLiveData<>();

 public MutableLiveData<User> getUser(final String userId) {
  if ("xiasm".equals(userId)) {
   data.setValue(new User(userId, "夏胜明"));
  } else if ("123456".equals(userId)) {
   data.setValue(new User(userId, "哈哈哈"));
  } else {
   data.setValue(new User(userId, "unknow"));
  }
  return data;
 }
}

public class MutableLiveData<T> extends LiveData<T> {
 @Override
 public void postValue(T value) {
  super.postValue(value);
 }

 @Override
 public void setValue(T value) {
  super.setValue(value);
 }
}

@MainThread
protected void setValue(T value) {
 assertMainThread("setValue");
 mVersion++;
 mData = value;
 dispatchingValue(null);
}

private void dispatchingValue(@Nullable ObserverWrapper initiator) {
 if (mDispatchingValue) {
  mDispatchInvalidated = true;
  return;
 }
 mDispatchingValue = true;
 do {
  mDispatchInvalidated = false;
  //标注1
  if (initiator != null) {
   considerNotify(initiator);
   initiator = null;
  } else {
   //标注2
   for (Iterator<Map.Entry<Observer<T>, ObserverWrapper>> iterator =
     mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
    considerNotify(iterator.next().getValue());
    if (mDispatchInvalidated) {
     break;
    }
   }
  }
 } while (mDispatchInvalidated);
 mDispatchingValue = false;
}

private void considerNotify(ObserverWrapper observer) {
 if (!observer.Mactive) {
  return;
 }
 // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
 //
 // we still first check observer.active to keep it as the entrance for events. So even if
 // the observer moved to an active state, if we've not received that event, we better not
 // notify for a more predictable notification order.
 if (!observer.shouldBeActive()) {
  observer.activeStateChanged(false);
  return;
 }
 if (observer.mLastVersion >= mVersion) {
  return;
 }
 observer.mLastVersion = mVersion;
 //noinspection unchecked
 observer.mObserver.onChanged((T) mData);
}

viewModel.getUser().observe(UserProfileFragment.this, new Observer<User>() {
 @Override
 public void onChanged(@Nullable User user) {
  if (user != null) {
   tvUser.setText(user.toString());
  }
 }
});