Fragment vs. Custom View in Android

Fragment can be used in different scenarios but most used are:

• wrapper around a view
• headless fragment - i.e. no view => not very helpful in general but can be used
• retainable fragment - can be any of above. By using Fragment.setRetainInstance(true) you can bypass Fragment.onDestroy(), i.e. can keep fragment data on configuration changes but fragment view structure is still destroyed/recreated
• can be added to activity back stack, i.e. easy Back button previous state restore

There are cases where fragment are complete pain in the neck, then there are cases where they can achieve results quicker.

For some custom and more flexible situations fragments can get cluttered and managing them would be difficult. So dealing with views directly can be really handy and more helpful for some cases. But everything is based on requirements.

Note View has its own life cycle too and can store/recreate saved instance state. A little bit more work but it has the option too.

Custom Views have the advantage of simplicity and their primary purpose is to display a piece of data on the screen. They must rely on other components in order to do more.

Think of Fragments as a functional unit, a way to display a portion of UI that has a specific purpose, using one or more Views. Fragments are connected to the Activity lifecycle and they can include and control Loaders to populate the Views with data. They can also include sub-fragments. Finally, they can also be added to a synthetic back stack. They can do many things and are somewhat complex to learn.

As you can see, Fragments have much more in common with Activities than they have with custom views.

As a side note, Fragments can also be headless (with no UI). Headless fragments provide a way to encapsulate non-visual functionality relying on the Activity lifecycle in a separate component.

Fragments come with their own lifecycle, which can be a hinderance or a bonus, depending on what you need.

Fragments get lifecycle methods like onResume or onSavedInstanceState, which can help you deal with state transitions in your application. If you're using custom views, you need to handle that kind of things on your own.

There are people who advocate against using fragments, I suggest reading https://corner.squareup.com/2014/10/advocating-against-android-fragments.html