如何通过旋转正确地保留对话框片段？

Inside your DialogFragment, call Fragment.setRetainInstance(boolean) with the value true. You don't need to save the fragment manually, the framework already takes care of all of this. Calling this will prevent your fragment from being destroyed on rotation and your network requests will be unaffected.

You may have to add this code to stop your dialog from being dismissed on rotation, due to a bug with the compatibility library:

@Override
public void onDestroyView() {
Dialog dialog = getDialog();
// handles https://code.google.com/p/android/issues/detail?id=17423
if (dialog != null && getRetainInstance()) {
dialog.setDismissMessage(null);
}
super.onDestroyView();
}

One of the advantages of using dialogFragment compared to just using alertDialogBuilder is exactly because dialogfragment can automatically recreate itself upon rotation without user intervention.

However, when the dialogfragment does not recreate itself, it is possible that you overwrite onSaveInstanceState but didn't to call super:

@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState); // <-- must call this if you want to retain dialogFragment upon rotation
...
}

This is a convenience method using the fix from antonyt's answer:

public class RetainableDialogFragment extends DialogFragment {


public RetainableDialogFragment() {
setRetainInstance(true);
}


@Override
public void onDestroyView() {
Dialog dialog = getDialog();
// handles https://code.google.com/p/android/issues/detail?id=17423
if (dialog != null && getRetainInstance()) {
dialog.setDismissMessage(null);
}
super.onDestroyView();
}
}

Just let your DialogFragment extend this class and everything will be fine. This becomes especially handy, if you have multiple DialogFragments in your project which all need this fix.

In case nothing helps, and you need a solution that works, you can go on the safe side, and each time you open a dialog save its basic info to the activity ViewModel (and remove it from this list when you dismiss dialog). This basic info could be dialog type and some id (the information you need in order to open this dialog). This ViewModel is not destroyed during changes of Activity lifecycle. Let's say user opens a dialog to leave a reference to a restaurant. So dialog type would be LeaveReferenceDialog and the id would be the restaurant id. When opening this dialog, you save this information in an Object that you can call DialogInfo, and add this object to the ViewModel of the Activity. This information will allow you to reopen the dialog when the activity onResume() is being called:

// On resume in Activity
override fun onResume() {
super.onResume()
    

// Restore dialogs that were open before activity went to background
restoreDialogs()
}

Which calls:

    fun restoreDialogs() {
mainActivityViewModel.setIsRestoringDialogs(true) // lock list in view model


for (dialogInfo in mainActivityViewModel.openDialogs)
openDialog(dialogInfo)


mainActivityViewModel.setIsRestoringDialogs(false) // open lock
}

When IsRestoringDialogs in ViewModel is set to true, dialog info will not be added to the list in view model, and it's important because we're now restoring dialogs which are already in that list. Otherwise, changing the list while using it would cause an exception. So:

// Create new dialog
override fun openLeaveReferenceDialog(restaurantId: String) {
var dialog = LeaveReferenceDialog()
// Add id to dialog in bundle
val bundle = Bundle()
bundle.putString(Constants.RESTAURANT_ID, restaurantId)
dialog.arguments = bundle
dialog.show(supportFragmentManager, "")
        

// Add dialog info to list of open dialogs
addOpenDialogInfo(DialogInfo(LEAVE_REFERENCE_DIALOG, restaurantId))
}

Then remove dialog info when dismissing it:

// Dismiss dialog
override fun dismissLeaveReferenceDialog(Dialog dialog, id: String) {
if (dialog?.isAdded()){
dialog.dismiss()
mainActivityViewModel.removeOpenDialog(LEAVE_REFERENCE_DIALOG, id)
}
}

And in the ViewModel of the Activity:

fun addOpenDialogInfo(dialogInfo: DialogInfo){
if (!isRestoringDialogs){
val dialogWasInList = removeOpenDialog(dialogInfo.type, dialogInfo.id)
openDialogs.add(dialogInfo)
}
}




fun removeOpenDialog(type: Int, id: String) {
if (!isRestoringDialogs)
for (dialogInfo in openDialogs)
if (dialogInfo.type == type && dialogInfo.id == id)
openDialogs.remove(dialogInfo)
}

You actually reopen all the dialogs that were open before, in the same order. But how do they retain their information? Each dialog has a ViewModel of its own, which is also not destroyed during the activity lifecycle. So when you open the dialog, you get the ViewModel and init the UI using this ViewModel of the dialog as always.

Most of the answers here are incorrect because they use setRetainInstance(true), but this is now deprecated as of API 28. Here is the solution I am using:

fun isDialogVisible(fm: FragmentManager): Boolean {
val dialog = fm.findFragmentByTag("<FRAGMENT_TAG>")
return dialog?.isResumed ?: false
}

If the function returns false, then simply call dialog.show(fm, "<FRAGMENT_TAG>") to show it again.