Develop Reference

Is it a good practice to use a single fragment to display the UI instead of the activity?

As the title states, does it considered as a good practice to use a single fragment within an activity to display the content? I began to notice that more and more developers start to use the fragment as an insulation layer to separate the lifecycle logic from UI that activity (sorry, fragment) displays. The most recent example that I stumble on is the architecture blueprints provided by Google developers. They use just one single fragment for UI while the activity handles ViewModel and all the navigation between screens.
So, is this a good practice or just a personal preference? Would you care to share your opinion on the subject?

Using Fragments as your UI is a good practice.
Activity can hold all common logic, while you can use different fragments to show different UI for mobile vs tablet.
If the UI is in Fragment, you can reuse it in multiple activities.
If you have a workflow scenario like Registration flow, using a single activity with multiple fragments will help you out a lot.
Manipulation of fragment backstack is a lot easier than trying to do the same with activities.

Use fragment for display the UI is good,Reason is listed below:-
1.You can create one activity,and display multiple fragment inside that activity.
2.Handling back press is easy as you can override onBackpress() in main activity class and handle back key event from fragment(s) as check its(fragment) visibility and handle event.
3. Reusability of layout.
4. Reusability of fragment.
5. Handling different action menu is very easy for different fragment(s).

What are the advantages of creating a Fragment vs Extending a View or ViewGroup?

We recently converted an app from a multiple activity based one, to one with a single activity with multiple fragments. The activities that became Fragments used to contain fragments themselves, so we use child fragment managers to host the Fragments in the Fragments (these child fragments I should add, are small and there can be 4 or 5 these on the screen at one time).
This has caused a few issues, namely having to create, and keep track of Unique IDs for the Fragment holders. (Which cause headaches when dealing with the Backstack as well as if any are in any sort of AdapterViews).
We're thinking of just rewriting these components to extend some sort of ViewGroup, likely FrameLayout or LinearLayout. We already do that anyway in some cases, but I was wondering if there are any disadvantages to doing it that way? (I must admit, I don't really see the big deal about Fragments... anything you can do with Fragments, you can do by creating a Custom View. Is this wrong?).

This is a bit of a reverse answer, explaining fragment use. In general you can do most things with Activities, but as the SDK supports fragments, many things will become more cumbersome (ViewPager is an example where fragment use is great).
The advantages (of fragments): code encapsulation, reusable chunks of UI.
The disadvantages (of fragments): more code (e.g. FragmentManager, FragmentTransaction).
Your original use of activities was good, it's not a case where I would have switched to fragments.
Let's say I designed a mobile phone app with two activities: ContactList and ContactDetails. So far so good, no reason to use fragments yet. If I wanted to support a larger device, it would behoove me to display both screens side by side. This is where fragments come in handy. In terms of exactly how to structure your activities/fragments, there's some good advice here:
https://developer.android.com/guide/practices/tablets-and-handsets.html#Fragments
Here are the important bits:
Multiple fragments, one activity:
Use one activity regardless of the
device size, but decide at runtime whether to combine fragments in the
layout (to create a multiple-pane design) or swap fragments (to create
a single-pane design).
Or...
Multiple fragments, multiple activities:
On a tablet, place multiple fragments in one activity; on a handset,
use separate activities to host each fragment. For example, when the
tablet design uses two fragments in an activity, use the same activity
for handsets, but supply an alternative layout that includes just the
first fragment. When running on a handset and you need to switch
fragments (such as when the user selects an item), start another
activity that hosts the second fragment.
.
The approach you choose
depends on your design and personal preferences. The first option (one
activity; swapping fragments) requires that you determine the screen
size at runtime and dynamically add each fragment as
appropriate—rather than declare the fragments in your activity's XML
layout—because you cannot remove a fragment from an activity if it's
been declared in the XML layout. When using the first technique, you
might also need to update the action bar each time the fragments
change, depending on what actions or navigation modes are available
for each fragment. In some cases, these factors might not affect your
design, so using one activity and swapping fragments might work well
(especially if your tablet design requires that you add fragments
dynamically anyway). Other times, however, dynamically swapping
fragments for your handset design can make your code more complicated,
because you must manage all the fragment combinations in the
activity's code (rather than use alternative layout resources to
define fragment combinations) and manage the back stack of fragments
yourself (rather than allow the normal activity stack to handle
back-navigation).

Is is better to add fragments to an activity by <fragment> tags in xml or by FragmentManager in a code?

I've started developing Android apps and I am wondering which way is better in case of adding fragments to activity. Let's assume that view for activity contain always three fragments. They won't changed. Always be the same. Thus is it better to add them by tags or include them in the activity code?
And a second question issue:
Let's say I have activity with fragment which is a list. Then when I clicked on item I want to show new view. Can I then replace the list fragment with new completely different fragment? Even if the answer is yes then is it better than creating new activity?
Thanks for all replies

which way is better in case of adding fragments to activity
One approach is not necessarily 'better' than the other - they both serve their own purposes, as with any static vs. dynamic comparison.
For example, fragments declared in a layout cannot be given arguments using setArguments(). Such a fragment can also not be replaced by another fragment: if it's part of the layout, it'll always be there. Of course you can still show/hide the instance, but attempting to actually remove it through a FragmentTransaction will simply not work. Static elements are usually easier to work with though, because they have a well-defined lifetime and behaviour.
Regarding your second question: yes, that's very possible. Some developers build their app around a single Activity container, swapping out fragments as the user navigates its way through the content. In most cases, from a user's point of view, there is little difference between doing this or having multiple activities. The important thing to keep in mind is to choose an approach you're comfortable with, doesn't overly complicate things and takes advantage of the patterns explained in Implementing Effective Navigation.

Why use Fragments? [duplicate]

This question already has answers here:
What is the benefit of using Fragments in Android, rather than Views?
(6 answers)
Closed 10 years ago.
What is the advantage to using Fragments over using custom Views that are reused in different layouts?
In the original blog post introducing fragments, Dianne Hackborn says that
[Fragments] make it easier for developers to write applications that can scale
across a variety of screen sizes, beyond the facilities already
available in the platform.
and she goes on to explain Fragments in the context of making a tablet layout for an app that combines the UI of two activities from the phone version of the same app.
But it seems that the same reuse could be achieved using custom Views. The main different between Fragments and Views seems to be that they have differing lifecycles...
The Fragment lifecycle is:
onAttach(), onCreate(), onCreateView(), onActivityCreated(), onStart(), onResume(), onPause(), onStop(), onDestroyView(), onDestroy(), onDetatch().
The View lifecycle is:
ctor, onFinishInflate(), onAttachedToWindow(), onMeasure(), onLayout(), onDetatchedFromWindow()
I'd like to hear from developers with experience writing large apps about what benefits (if any) they've seen in using Fragments vs custom Views to divide up the UI into reusable pieces.

The main reason is that fragments are more reusable than custom views.
Sometimes you can't create a fully encapsulated UI component relying on views alone. This is because there are things you would want to put into your view but can't because only an Activity can handle them, thus forcing tight coupling between an Activity and a View.
Here is one such example. Lets say you want to create a reusable UI component that, among many things, want to capture a photo and do something with it. Traditionally you would fire an intent that starts the camera and returns with the captured image.
Notice that your custom UI component can't fully encapsulate this functionality because it will have to rely on hosting Activity's startActivityForResult because views don't accept activity results (they can indirectly fire an intent through context).
Now if you wanted to reuse your custom UI component in different activities you would be repeating the code for Activity.startActivityForResult.
Fragment on the other hand cleanly solve this problem.
Similarly your fragment can contribute items to your options menu, something traditionally only an Activity could do. Again this could be important if the state of your custom view dictates what goes in the menu.

A fragment is way more than just a view. In fact it can even be totally without a view. It can have all sorts of stuff in it including AsyncTasks, various Listeners, file and database access and so on and so on.
Think of it as a small activity, but you can have multiple of them on the screen and work with them all including communicating with each other while they are visible.
E.g. you could have a list of shopping cart displayed in one fragment and the currently selected cart in detail in another fragment. You then e.g. change the quantity of an item in the detail view and the list view could be notified about it and update the total price in the list view. You can totally orchestrate interactions like that nicely while e.g. still having only one of them visible on a smaller screen device.
I have refactored a large business app (>15 activities) from activities to fragments to get good tablet support and I would never start a new app without fragments.
Update Feb 2016: While the above still holds true, there are complexities with fragments that caused many people to entirely avoid using them. Newer patterns such as usage of MVC approaches and more powerful views provide alternatives. As they say .. YMMV.

Some description:
Imagine Activity as a plate that hold one big cake.
Fragment would be a container that slices the same cake into pieces.
Each slice contains it own logics (listeners, etc).
And in total they are almost no different with the one big cake.
The benefit:
When you plate can't hold a big cake. (Screen is small) You can easily use a a few plates (Activity) to hold each of them WITHOUT the need to move your logics into the new activity.
Better re-usability. I have some instances where I could reuse a fragment entirely in another App. You might claim that a custom view could does that too. But refer to point 1, I could reuse it with just few lines of layout changes but for a custom view, it have to find a way to plug it into both layout and code.
It is, in some sense, a more OO ways of organising your UI logics in Android programming. When you have a feature (A new partition on the screen for example), you create a new Fragment class, with minor modification to existing activity class. However if you are programming only with activity, you will need to add logics and make big modification on tested class.
Just my 2 cents. :)

The lifecycle methods are probably your biggest hint. If you think about it, they correlate closely to the activity lifecycle (with some hooks into the activity and views). In fact, in the article you linked, Hackborn says:
In some ways you can think of a Fragment as a mini-Activity
As with many things in software design/development, there are a multitude of ways to do things. There are many different places you could put your code. Yes, you could probably put a lot into a view, but keeping different concerns separated in different classes is a good thing though. The classic pattern of this is MVC and it applies in this scenario. You don't want to bake in too much controller logic into your view. It's better to keep it in controller-like classes which are the activity and now the fragment. This is why the fragment's lifecycle is more like the activity's than the view's--it was made to facilitate this kind of organization.

I touched Fragments once and found them not very useful (see this post). From what I have read, A Fragment is really a fancy word for an Object with access to Activity Context. I like to ignore Fragments in my work, and just create these Objects myself. I have created very large, very demanding apps by passing an Activity to constructors, instead of Context. One major benefit, however, for using Fragments is that they are supported by the View layout system - so you can easily add them to Android xml (if you use it for your layouts).

Custom views are much more work than just using fragments in place of your activities. if you decide to use Activities and custom Views, you have to create your custom view, and then you have to implement the same activity lifecycle methods in your activity (a very similar lifecycle is used for fragments).
Using Fragments also allows you to separate components into their own classes (Fragments), rather than having too much logic in a single Activity. Let me ground that with an example:
Say you're implementing a magazine reader application. using fragments, you could create a fragment: ArticleList, which displays a list of articles, and another fragment: ArticleDisplay, which handles the logic for displaying content. you can then specify how these fragments should interact using the fragments tools, so that on a handset, you can use the full screen real-estate for ArticleDisplay, while on a tablet, you can display the fragments side by side.
If you were to attempt this with an Activity/custom view, you'd have the logic for both Fragments in your monolithic Activity, you'd have to write a custom view, and you'd have to debug this unwieldy monster.
Fragments are, in general, a more sophisticated and powerful way to write your applications. They can do everything an Activity can do, and more. If you don't need the extra functionality, the defaults will probably get you where you need to go, and with less work.

Converting Multiple Activites into a Single Fragment

I've recently decided to update my app to support the new fragments feature in honeycomb 3.0.
My Application currently works on a list view that opens different activities depending on which list item is clicked.
Using an adaptation of the code in this tutorial I have created an app that consists of only two activities, but depending on which list item is clicked the second "viewer" activity launches using a different layout xml.
Unfortunately I haven't been able to figure out how to call the old methods that had all the functionality. Should I Import all of my old activities and then call the methods into the viewer activity (I may need some advice on how exactly to do this) or should I just put all the methods directly into the same viewer activity (please consider the size of these methods(which is very large by the way)).
Once everything is working with two activities upfront then it will be a pretty simple task of "fragmenting" the app as demonstrated here
Although I haven't considered that there might be a way to allow multiple fragments to occupy the same space in an activity(If this is the case then please let me know how it's done)
Thanks

As James has pointed out you will have to move the business logic from your Activities to your Fragments.
To handle events you can create a listener Interface. The CONTAINER activity/ies will implement this interface. As fragments has access to the container activity you will be able to delegate to the container Activity the "logic" for the desired events. For this events the activity will decide whether to launch a new activity, show/hide new fragments or whatever.
I had a similar question, take a look to the question and answer: here
Although I haven't considered that there might be a way to allow multiple fragments to occupy the same space in an activity(If this is the case then please let me know how it's done)
I think its possible to allow multiple fragments to occupy the same space in an activity. Again, take a look to the answer here ... I think the concept/scope of Activity has change a bit and now an Activity can contain different Fragments which every one will allow user to do a single focused thing.

I'm not sure what you mean by "call the old methods that had all the functionality". You'll want to rewrite all of your activity classes as fragments. Check out this tutorial here (it's very concise). Basically, you'll want an activity that consists of a ListFragment and a FrameLayout. Your ListFragment will update the FrameLayout by changing to the appropriate Fragment based on which row was selected.