如何在 ViewPager2 中按标签获取片段

Question

我被告知要使用 FragmentStateAdapter 的 createFragment 覆盖来按索引获取当前片段。

这就是我的适配器的样子。

class AddRestaurantPagerAdapter(fragmentManager: FragmentManager, lifecycle: Lifecycle) :
FragmentStateAdapter(fragmentManager, lifecycle) {
private var fragments = listOf<Fragment>(
FragmentA.newInstance(),
FragmentB.newInstance(),
FragmentC.newInstance(),
FragmentD.newInstance()
)
override fun getItemCount() = 4
override fun createFragment(position: Int) =
fragments[position]
}

这工作正常，直到我们强制在此活动/片段上使用 onSaveInstance/OnRestoreInstance。

ViewPager2 重新加载了它上面已经存在的片段，我们需要一种方法来访问它。FragmentStateAdapter 类中的以下 LOC 证明了这一点

。

private void ensureFragment(int position) {
long itemId = getItemId(position);
if (!mFragments.containsKey(itemId)) {
// TODO(133419201): check if a Fragment provided here is a new Fragment
Fragment newFragment = createFragment(position);
newFragment.setInitialSavedState(mSavedStates.get(itemId));
mFragments.put(itemId, newFragment);
}
}

在重新创建活动/片段后，您会发现 this If 块永远不会执行，因为 mFragment 已经包含在收集活动之前添加的片段。

所以我想知道应该如何提取视图页面中已有的片段2

Answer 1

如果你看看这个方法，

/**
* @param holder that has been bound to a Fragment in the {@link #onBindViewHolder} stage.
*/
@SuppressWarnings("WeakerAccess") // to avoid creation of a synthetic accessor
void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {
Fragment fragment = mFragments.get(holder.getItemId());
if (fragment == null) {
throw new IllegalStateException("Design assumption violated.");
}
FrameLayout container = holder.getContainer();
View view = fragment.getView();
/*
possible states:
- fragment: { added, notAdded }
- view: { created, notCreated }
- view: { attached, notAttached }
combinations:
- { f:added, v:created, v:attached } -> check if attached to the right container
- { f:added, v:created, v:notAttached} -> attach view to container
- { f:added, v:notCreated, v:attached } -> impossible
- { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
- { f:notAdded, v:created, v:attached } -> illegal state
- { f:notAdded, v:created, v:notAttached } -> illegal state
- { f:notAdded, v:notCreated, v:attached } -> impossible
- { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
*/
// { f:notAdded, v:created, v:attached } -> illegal state
// { f:notAdded, v:created, v:notAttached } -> illegal state
if (!fragment.isAdded() && view != null) {
throw new IllegalStateException("Design assumption violated.");
}
// { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
if (fragment.isAdded() && view == null) {
scheduleViewAttach(fragment, container);
return;
}
// { f:added, v:created, v:attached } -> check if attached to the right container
if (fragment.isAdded() && view.getParent() != null) {
if (view.getParent() != container) {
addViewToContainer(view, container);
}
return;
}
// { f:added, v:created, v:notAttached} -> attach view to container
if (fragment.isAdded()) {
addViewToContainer(view, container);
return;
}
// { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
if (!shouldDelayFragmentTransactions()) {
scheduleViewAttach(fragment, container);
mFragmentManager.beginTransaction()
.add(fragment, "f" + holder.getItemId())
.setMaxLifecycle(fragment, STARTED)
.commitNow();
mFragmentMaxLifecycleEnforcer.updateFragmentMaxLifecycle(false);
} else {
if (mFragmentManager.isDestroyed()) {
return; // nothing we can do
}
mLifecycle.addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (shouldDelayFragmentTransactions()) {
return;
}
source.getLifecycle().removeObserver(this);
if (ViewCompat.isAttachedToWindow(holder.getContainer())) {
placeFragmentInViewHolder(holder);
}
}
});
}
}

这个tag"f" + holder.getItemId().

@Override
public long getItemId(int position) {
return position;
}

默认情况下，getItemId(int postion)返回位置。 但是，如果您覆盖此方法。

int postionOfTheFragmentYouWantToGet=0;
Fragment page = getSupportFragmentManager().findFragmentByTag("f" + postionOfTheFragmentYouWantToGet);