我正在尝试在视图分页器的第一个选项卡中实现片段的切换。我尝试了此处link中找到的方法，该方法在第一个选项卡中使用根片段，然后切换子片段。但是，当在第二个子片段中发生屏幕旋转时，应用程序会切换回第一个子片段，此外，这看起来像是黑客。
我尝试搜索其他方法，但发现了几十个看起来都很混乱的答案。 here的解释似乎是正确的，但我不完全理解。以下是该链接的解释和代码：
此方法是通过覆盖List<ClioFolder> test = allData.FirstOrDefault()?.data;
方法和每次调用FragmentPagerAdapter都会调用的getItem(...)来切换getItemPosition(Object object)内部显示的片段。例如：
viewPager.getAdapter().notifyDataSetChanged()假设我有public static class MyPagerAdapter extends FragmentStatePagerAdapter {
// Return a different fragment for position based on additional state tracked in a member variable
@Override
public Fragment getItem(int position) {
// For a given position, return two different potential fragments based on a condition
}
// Force a refresh of the page when a different fragment is displayed
@Override
public int getItemPosition(Object object) {
// this method will be called for every fragment in the ViewPager
if (object instanceof SomePermanantCachedFragment) {
return POSITION_UNCHANGED; // don't force a reload
} else {
// POSITION_NONE means something like: this fragment is no longer valid
// triggering the ViewPager to re-build the instance of this fragment.
return POSITION_NONE;
}
}
}
和ListFragment应该在第一个标签页中切换。
DetailFragment？viewPager.getAdapter().notifyDataSetChanged()是指SomePermanantCachedFragment？例如，我也尝试搜索github，但没有成功...
答案 0 :(得分：1)
如果PagerAdapter中的片段是动态的，则应使用Dynamic FragmentPagerAdapter。
public class DynamicFragmentPagerAdapter extends PagerAdapter {
private static final String TAG = "DynamicFragmentPagerAdapter";
private final FragmentManager fragmentManager;
public static abstract class FragmentIdentifier implements Parcelable {
private final String fragmentTag;
private final Bundle args;
public FragmentIdentifier(@NonNull String fragmentTag, @Nullable Bundle args) {
this.fragmentTag = fragmentTag;
this.args = args;
}
protected FragmentIdentifier(Parcel in) {
fragmentTag = in.readString();
args = in.readBundle(getClass().getClassLoader());
}
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeString(fragmentTag);
dest.writeBundle(args);
}
protected final Fragment newFragment() {
Fragment fragment = createFragment();
Bundle oldArgs = fragment.getArguments();
Bundle newArgs = new Bundle();
if(oldArgs != null) {
newArgs.putAll(oldArgs);
}
if(args != null) {
newArgs.putAll(args);
}
fragment.setArguments(newArgs);
return fragment;
}
protected abstract Fragment createFragment();
}
private ArrayList<FragmentIdentifier> fragmentIdentifiers = new ArrayList<>();
private FragmentTransaction currentTransaction = null;
private Fragment currentPrimaryItem = null;
public DynamicFragmentPagerAdapter(FragmentManager fragmentManager) {
this.fragmentManager = fragmentManager;
}
private int findIndexIfAdded(FragmentIdentifier fragmentIdentifier) {
for (int i = 0, size = fragmentIdentifiers.size(); i < size; i++) {
FragmentIdentifier identifier = fragmentIdentifiers.get(i);
if (identifier.fragmentTag.equals(fragmentIdentifier.fragmentTag)) {
return i;
}
}
return -1;
}
public void addFragment(FragmentIdentifier fragmentIdentifier) {
if (findIndexIfAdded(fragmentIdentifier) < 0) {
fragmentIdentifiers.add(fragmentIdentifier);
notifyDataSetChanged();
}
}
public void removeFragment(FragmentIdentifier fragmentIdentifier) {
int index = findIndexIfAdded(fragmentIdentifier);
if (index >= 0) {
fragmentIdentifiers.remove(index);
notifyDataSetChanged();
}
}
@Override
public int getCount() {
return fragmentIdentifiers.size();
}
@Override
public void startUpdate(@NonNull ViewGroup container) {
if (container.getId() == View.NO_ID) {
throw new IllegalStateException("ViewPager with adapter " + this
+ " requires a view id");
}
}
@SuppressWarnings("ReferenceEquality")
@NonNull
@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {
if (currentTransaction == null) {
currentTransaction = fragmentManager.beginTransaction();
}
final FragmentIdentifier fragmentIdentifier = fragmentIdentifiers.get(position);
// Do we already have this fragment?
final String name = fragmentIdentifier.fragmentTag;
Fragment fragment = fragmentManager.findFragmentByTag(name);
if (fragment != null) {
currentTransaction.attach(fragment);
} else {
fragment = fragmentIdentifier.newFragment();
currentTransaction.add(container.getId(), fragment, fragmentIdentifier.fragmentTag);
}
if (fragment != currentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
return fragment;
}
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
if (currentTransaction == null) {
currentTransaction = fragmentManager.beginTransaction();
}
currentTransaction.detach((Fragment) object);
}
@SuppressWarnings("ReferenceEquality")
@Override
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment) object;
if (fragment != currentPrimaryItem) {
if (currentPrimaryItem != null) {
currentPrimaryItem.setMenuVisibility(false);
currentPrimaryItem.setUserVisibleHint(false);
}
fragment.setMenuVisibility(true);
fragment.setUserVisibleHint(true);
currentPrimaryItem = fragment;
}
}
@Override
public void finishUpdate(@NonNull ViewGroup container) {
if (currentTransaction != null) {
currentTransaction.commitNowAllowingStateLoss();
currentTransaction = null;
}
}
@Override
public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
return ((Fragment) object).getView() == view;
}
@Override
public Parcelable saveState() {
Bundle bundle = new Bundle();
bundle.putParcelableArrayList("fragmentIdentifiers", fragmentIdentifiers);
return bundle;
}
@Override
public void restoreState(Parcelable state, ClassLoader loader) {
Bundle bundle = ((Bundle)state);
bundle.setClassLoader(loader);
fragmentIdentifiers = bundle.getParcelableArrayList("fragmentIdentifiers");
}
}
有了这些，就可以使用removeFragment和addFragment来更改ViewPager中的片段。
从技术上讲，您可以添加一个setFragment(int index)方法来专门将其替换为索引0。
如果片段似乎没有更新，则刷新ViewPager的诀窍总是setAdapter(null); setAdapter(adapter);。