Why fragments and when to use fragments instead of activities

Question

In Android API 11   Google has released a new class called Fragment   In the videos  Google suggests that whenever possible  link1  link2   we should use fragments instead of activities  but they didn t explain exactly why   What s the purpose of fragments and some possible uses of them  other than some UI examples that can be easily be achieved by simple views layouts    My question is about fragments    What are the purposes of using a fragment  What are the advantages and disadvantages of using fragments compared to using activities views layouts    Bonus questions    Can you give some really interesting uses for fragments  Things that Google didn t mention in their videos  What s the best way to communicate between fragments and the activities that contain them  What are the most important things to remember when you use fragments  Any tips and warnings from your experience

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A fragment lives inside an activity  while an activity lives by itself

User · Answer

Not sure what video s  you are referring to  but I doubt they are saying you should use fragments instead of activities  because they are not directly interchangeable  There is actually a fairly detailed entry in the Dev Guide  consider reading it for details    In short  fragments live inside activities  and each activity can host many fragments  Like activities  they have a specific lifecycle  unlike activities  they are not top-level application components  Advantages of fragments include code reuse and modularity  e g   using the same list view in many activities   including the ability to build multi-pane interfaces  mostly useful on tablets   The main disadvantage is  some  added complexity  You can generally achieve the same thing with  custom  views in a non-standard and less robust way

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A Fragment is a piece of an application s user interface or behavior that can be placed in an Activity which enable a more modular activity design  It will not be wrong if we say a fragment is a kind of subactivity   Following are important points about a fragment    A fragment has its own layout and its own behavior with its own lifecycle callbacks  You can add or remove fragments in an activity while the activity is running  You can combine multiple fragments in a single activity to build a multi-pane UI  A fragment can be used in multiple activities  The fragment life cycle is closely related to the lifecycle of its host activity  When the activity is paused  all the fragments available in the acivity will also be stopped  A fragment can implement a behavior that has no user interface component  Fragments were added to the Android API in Android nbsp 3  Honeycomb  with API version 11    For more details  please visit the official site  Fragments

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Adding to above answers  I shall tell using example of an app I released on playstore  This was the first app I developed when learning android there fore I worked only with activities There are multiple activity pages I think about 12  Most of these had content that could be reused in other pages yet I ended up with a separate activity page for almost every single click on the app  Once I learnt fragments I realised how all reusables could just be implemented and separate fragments and just be used with very few activities  My user may not see any difference  but the same can be done with lesser code besides fragments are light weight  apart from the reusability and modularity they offer

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Activities are the full screen components in the app with the toolbar  everything else are preferably Fragments   One full screen parent activity with a toolbar can have multiple panes  scrollable pages  dialogs  etc   all fragments   all of which can be accessed from the parent and communicate via the parent   Example   Activity A  Activity B  Activity C     All activities need to have same code repeated  to show a basic toolbar for example  or inherit from a parent activity  becomes cumbersome to manage   To move from one activity to the other  either all of them need to be in memory  overhead  or one needs to be destroyed for the other to open  Communication between activities can be done via Intents    vs  Activity A  Fragment 1  Fragment 2  Fragment 3    No code repetition  all screens have toolbars etc  from that one activity  Several ways to move from one fragment to next - view pager  multi pane etc  Activity has most data  so minimal inter-fragment communication needed  If still necessary  can be done via interfaces easily  Fragments do not need to be full screen  lots of flexibility in designing them  Fragments do not need to inflate layout if views are not necessary  Several activities can use the same fragment

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This is important information that I found on fragments      Historically each screen in an Android app was implemented as a separate Activity  This creates a challenge in passing information between screens because the Android Intent mechanism does not allow passing a reference type  i e  object  directly between Activities  Instead the object must be serialized or a globally accessible reference made available       By making each screen a separate Fragment  this data passing headache   is completely avoided  Fragments always exist within the context of a   given Activity and can always access that Activity  By storing the   information of interest within the Activity  the Fragment for each   screen can simply access the object reference through the Activity    Source  https   www pluralsight com blog software-development android-fragments

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Fragment can be thought of as non-root components in a composite tree of ui elements while activities sit at the top in the forest of composites ui trees    A rule of thumb on when not to use Fragment is when as a child the fragment has a conflicting attribute  e g   it may be immersive or may be using a different style all together or has some other architectural   logical difference and doesn t fit in the existing tree homogeneously   A rule of thumb on when to prefer Activity over Fragment is when the task  or set of coherent task  is fully independent and reusable and does some heavy weight lifting and should not be burdened further to conform to another parent-child composite  SRP violation  second responsibility would be to conform to the composite   For e g   a MediaCaptureActivity that captures audio  video  photos etc and allows for edits  noise removal  annotations on photos etc and so on  This activity module may have child fragments that do more granular work and conform to a common display theme

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I know this was already discussed to death  but I d like to add some more points    Frags can be used to populate Menus and can handle MenuItem clicks on their own  Thus giving futher modulation options for your Activities  You can do ContextualActionBar stuff and so on without your Activity knowing about it and can basically decouple it from the basic stuff your Activity handles  Navigation Settings About   A parent Frag with child Frags can give you further options to modulize your components  E g  you can easily swap Frags around  put new Frags inside a Pager or remove them  rearrange them  All without your Activity knowing anything about it just focusing on the higher level stuff

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1 Purposes of using a fragment    Ans     Dealing with device form-factor differences  Passing information between app screens  User interface organization  Advanced UI metaphors

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1  amp   2 what are the purposes of using a fragment  amp  what are the   advantages and disadvantages of using fragments compared to using   activities views layouts    Fragments are Android s solution to creating reusable user interfaces  You can achieve some of the same things using activities and layouts  for example by using includes   However  fragments are wired in to the Android API  from HoneyComb  and up  Let me elaborate    The ActionBar  If you want tabs up there to navigate your app  you quickly see that ActionBar TabListener interface gives you a FragmentTransaction as an input argument to the onTabSelected method  You could probably ignore this  and do something else and clever  but you d be working against the API  not with it  The FragmentManager handles   back   for you in a very clever way  Back does not mean back to the last activity  like for regular activities  It means back to the previous fragment state   You can use the cool ViewPager with a FragmentPagerAdapter to create swipe interfaces  The FragmentPagerAdapter code is much cleaner than a regular adapter  and it controls instantiations of the individual fragments  Your life will be a lot easier if you use Fragments when you try to create applications for both phones and tablets  Since the fragments are so tied in with the Honeycomb  APIs  you will want to use them on phones as well to reuse code  That s where the compatibility library comes in handy   You even could and should use fragments for apps meant for phones only  If you have portability in mind  I use ActionBarSherlock and the compatibility libraries to create  ICS looking  apps  that look the same all the way back to version 1 6  You get the latest features like the ActionBar  with tabs  overflow  split action bar  viewpager etc       Bonus 2   The best way to communicate between fragments are intents  When you press something in a Fragment you would typically call StartActivity   with data on it  The intent is passed on to all fragments of the activity you launch

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Fragments lives within the Activity and has    its own lifecycle its own layout its own child fragments and etc    Think of Fragments as a sub activity of the main activity it belongs to  it cannot exist of its own and it can be called reused again and again  Hope this helps

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Fragments are of particular use in some cases like where we want to keep a navigation drawer in all our pages  You can inflate a frame layout with whatever fragment you want and still have access to the navigation drawer   If you had used an activity  you would have had to keep the drawer in all activities which makes for redundant code  This is one interesting use of a fragment   I m new to Android and still think a fragment is helpful this way

[android] Why fragments, and when to use fragments instead of activities?

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