少し改善され、マネージャーソリューションに組み込まれました。
覚えておくべきこと。FragmentManagerはシングルトンではなく、アクティビティ内のフラグメントのみを管理するため、すべてのアクティビティで新しいものになります。また、これまでのところ、このソリューションでは、フラグメントの表示を制御するのに役立つsetUserVisibleHint（）メソッドを呼び出すViewPagerは考慮されていません。
この問題に対処するときは、次のクラスを自由に使用してください（Dagger2インジェクションを使用）。活動の呼び出し：
//inject FragmentBackstackStateManager instance to myFragmentBackstackStateManager
FragmentManager fragmentManager = getSupportFragmentManager();
myFragmentBackstackStateManager.apply(fragmentManager);
FragmentBackstackStateManager.java：
@Singleton
public class FragmentBackstackStateManager {
private FragmentManager fragmentManager;
@Inject
public FragmentBackstackStateManager() {
}
private BackstackCallback backstackCallbackImpl = new BackstackCallback() {
@Override
public void onFragmentPushed(Fragment parentFragment) {
parentFragment.onPause();
}
@Override
public void onFragmentPopped(Fragment parentFragment) {
parentFragment.onResume();
}
};
public FragmentBackstackChangeListenerImpl getListener() {
return new FragmentBackstackChangeListenerImpl(fragmentManager, backstackCallbackImpl);
}
public void apply(FragmentManager fragmentManager) {
this.fragmentManager = fragmentManager;
fragmentManager.addOnBackStackChangedListener(getListener());
}
}
FragmentBackstackChangeListenerImpl.java：
public class FragmentBackstackChangeListenerImpl implements FragmentManager.OnBackStackChangedListener {
private int lastBackStackEntryCount = 0;
private final FragmentManager fragmentManager;
private final BackstackCallback backstackChangeListener;
public FragmentBackstackChangeListenerImpl(FragmentManager fragmentManager, BackstackCallback backstackChangeListener) {
this.fragmentManager = fragmentManager;
this.backstackChangeListener = backstackChangeListener;
lastBackStackEntryCount = fragmentManager.getBackStackEntryCount();
}
private boolean wasPushed(int backStackEntryCount) {
return lastBackStackEntryCount < backStackEntryCount;
}
private boolean wasPopped(int backStackEntryCount) {
return lastBackStackEntryCount > backStackEntryCount;
}
private boolean haveFragments() {
List<Fragment> fragmentList = fragmentManager.getFragments();
return fragmentList != null && !fragmentList.isEmpty();
}
/**
* If we push a fragment to backstack then parent would be the one before => size - 2
* If we pop a fragment from backstack logically it should be the last fragment in the list, but in Android popping a fragment just makes list entry null keeping list size intact, thus it's also size - 2
*
* @return fragment that is parent to the one that is pushed to or popped from back stack
*/
private Fragment getParentFragment() {
List<Fragment> fragmentList = fragmentManager.getFragments();
return fragmentList.get(Math.max(0, fragmentList.size() - 2));
}
@Override
public void onBackStackChanged() {
int currentBackStackEntryCount = fragmentManager.getBackStackEntryCount();
if (haveFragments()) {
Fragment parentFragment = getParentFragment();
//will be null if was just popped and was last in the stack
if (parentFragment != null) {
if (wasPushed(currentBackStackEntryCount)) {
backstackChangeListener.onFragmentPushed(parentFragment);
} else if (wasPopped(currentBackStackEntryCount)) {
backstackChangeListener.onFragmentPopped(parentFragment);
}
}
}
lastBackStackEntryCount = currentBackStackEntryCount;
}
}
BackstackCallback.java：
public interface BackstackCallback {
void onFragmentPushed(Fragment parentFragment);
void onFragmentPopped(Fragment parentFragment);
}