Understanding Fragment s setRetainInstance boolean

Question

Starting with the documentation   public void setRetainInstance  boolean retain  Control whether a fragment instance is retained across Activity re-creation  such as from a configuration change   This can only be used with fragments not in the back stack  If set  the fragment lifecycle will be slightly different when an activity is recreated   onDestroy   will not be called  but onDetach   still will be  because the fragment is being detached from its current activity   onCreate Bundle  will not be called since the fragment is not being re-created  onAttach Activity  and onActivityCreated Bundle  will still be called    I have some questions   Does the fragment also retain its view  or will this be recreated on configuration change  What exactly does  quot retained quot  mean   Will the fragment be destroyed when the user leaves the activity   Why doesn t it work with fragments on the back stack   Which are the use cases where it makes sense to use this method

User · Answer

setRetaininstance is only useful when your activity is destroyed and recreated due to a configuration change because the instances are saved during a call to onRetainNonConfigurationInstance  That is  if you rotate the device  the retained fragments will remain there they re not destroyed and recreated   but when the runtime kills the activity to reclaim resources  nothing is left  When you press back button and exit the activity  everything is destroyed   Usually I use this function to saved orientation changing Time Say I have download a bunch of Bitmaps from server and each one is 1MB  when the user accidentally rotate his device  I certainly don t want to do all the download work again So I create a Fragment holding my bitmaps and add it to the manager and call setRetainInstance all the Bitmaps are still there even if the screen orientation changes

User · Answer

First of all  check out my post on retained Fragments  It might help   Now to answer your questions      Does the fragment also retain its view state  or will this be recreated on configuration change - what exactly is  retained     Yes  the Fragment s state will be retained across the configuration change  Specifically   retained  means that the fragment will not be destroyed on configuration changes  That is  the Fragment will be retained even if the configuration change causes the underlying Activity to be destroyed      Will the fragment be destroyed when the user leaves the activity    Just like Activitys  Fragments may be destroyed by the system when memory resources are low  Whether you have your fragments retain their instance state across configuration changes will have no effect on whether or not the system will destroy the Fragments once you leave the Activity  If you leave the Activity  i e  by pressing the home button   the Fragments may or may not be destroyed  If you leave the Activity by pressing the back button  thus  calling finish   and effectively destroying the Activity   all of the Activitys attached Fragments will also be destroyed      Why doesn t it work with fragments on the back stack    There are probably multiple reasons why it s not supported  but the most obvious reason to me is that the Activity holds a reference to the FragmentManager  and the FragmentManager manages the backstack  That is  no matter if you choose to retain your Fragments or not  the Activity  and thus the FragmentManager s backstack  will be destroyed on a configuration change  Another reason why it might not work is because things might get tricky if both retained fragments and non-retained fragments were allowed to exist on the same backstack      Which are the use cases where it makes sense to use this method    Retained fragments can be quite useful for propagating state information     especially thread management     across activity instances  For example  a fragment can serve as a host for an instance of Thread or AsyncTask  managing its operation  See my blog post on this topic for more information   In general  I would treat it similarly to using onConfigurationChanged with an Activity    don t use it as a bandaid just because you are too lazy to implement handle an orientation change correctly  Only use it when you need to

User · Answer

setRetainInstance   - Deprecated  As Fragments Version 1 3 0-alpha01     The setRetainInstance   method on Fragments has been deprecated  With   the introduction of ViewModels  developers have a specific API for   retaining state that can be associated with Activities  Fragments  and   Navigation graphs  This allows developers to use a normal  not   retained Fragment and keep the specific state they want retained   separate  avoiding a common source of leaks while maintaining the   useful properties of a single creation and destruction of the retained   state  namely  the constructor of the ViewModel and the onCleared     callback it receives

User · Answer

setRetainInstance boolean  is useful when you want to have some component which is not tied to Activity lifecycle  This technique is used for example by rxloader to  handle Android s activity lifecyle for rxjava s Observable   which I ve found here

User · Answer

SetRetainInstance true  allows the fragment sort of survive  Its members will be retained during configuration change like rotation  But it still may be killed when the activity is killed in the background  If the containing activity in the background is killed by the system  it s instanceState should be saved by the system you handled onSaveInstanceState  properly  In another word the onSaveInstanceState will always be called  Though onCreateView won t be called if SetRetainInstance is true and fragment activity is not killed yet  it still will be called if it s killed and being tried to be brought back   Here are some analysis of the android activity fragment hope it helps  http   ideaventure blogspot com au 2014 01 android-activityfragment-life-cycle html

[android] Understanding Fragment's setRetainInstance(boolean)

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setRetainInstance() - Deprecated

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