Why Not Always Use Fragments With One Activity

Why would someone creates activities as opposed to one activity with many fragments? I see that with fragments, it is easier to share data and plus you have better use of onpause

Solution 1:

(apologies for the long answer in advance)

I don't think anything is stopping you from doing that.

Having said that, How I look at activities is as a place where I can define how the entire UI screen for the current transition should look: which fragments it should include and how these fragments should sit side-by-side and more importantly how the application flow works.

Fragments within my activity should together make a meaningful screen with functionalities I expect and should be re-usable but they shouldn't know of other fragments presence. That's the job of the activity. In fact, I strongly abide by the rule that a fragment should never know of another fragment's existence (although nowadays you can include fragments inside fragments which kind of defeats this point). If I need to transition to a completely different set of use-cases, I prefer to use activities to handle the transition and the new layout and sets of fragments to use.

Do I have to do it? No. I can do it all in one main activity and do the transitions between these "quasi-activity fragments" myself but why would I want to do that when there's already a framework in place that does it for me? Using intents to initialize activities (to transition) is a lot simpler and cleaner to achieve than manually writing my own engine that keeps track of the state of all my fragments and decides which ones to initiate and which ones to get rid of.

Similarly, handling different orientations and device sizes is easier to do using built-in functionalities in activities than doing it yourself but again, nothing is stopping you from doing so.

Much easier to explain this in an example:

Imaging this use-case: I need to write an app that shows a list of songs and movies. When I click on one of the songs, it should open a media player and play it in a separate screen and when I click on a movie, it should open a video player and play the video. There are clear transitions here: From the main list to an audio player or a video player and back.

Doing this using only one activity and 3 fragments, I would have to replace the list fragment with an appropriate media player fragment (video/audio) and get rid of the list fragment myself (to preserve memory) when the user clicks on a list item. This is all fine and easy to do but what happens if I have to also add a way to add comments to songs/movies? Now I have to modify my main (and only) activity to understand the behavior that if someone is playing a song and clicks on the comment button, I need to transition again to yet another fragment and allow that fragment to capture/show comments.

As you can probably imagine, every time I add a new functionality, I have to modify my main activity to handle the transition since fragments are not meant to know about each other and are meant to be independent of each other.

What I would end up doing, if I only had one activity, would be to create a God class (my main activity) where the entire flow of my application is defined. Maintaining this flow will become more and more complicated as the app becomes complicated. Again, this is going on the principle of having independent fragments. If you don't care about your fragments knowing about each other, that's entirely different but then, in my opinion, you're doing it wrong :)

How would I do it using activities? I would still have 3 fragments but also would have 3 activities whose only job is to maintain the transitions to/from other activities and defining which fragment should be used. Transitioning between list activity to song activity is a simple intent and the freeing up memory is done for me. Now if the song activity needs to transition to a comment activity, it's simply another intent to a comment activity from the song activity. I don't have to change the list activity at all and in fact, the list activity will never know about the existence of this comment activity.

I know it's a very simplified example but as your app gets more and more complex and as you add more and more use-cases, you'll find that having multiple activities makes sense. You don't want to maintain the transitions between fragments yourself, otherwise you'll end up with a very complex framework which would slowly become unmaintainable.

Same logic would apply to having different fragments/layout depending on the orientation and the size of your screen. Again, by all means, you can handle these yourself in your only activity but the question becomes: why?

Solution 2:

I think your question can be answered by two other questions:

1. What can Fragments do that Activities can't?

   • You can reuse fragments in layouts for phones and tablets, where the content that is presented in multiple screens on the phone appears as multiple columns in tablets.

   • Fragments can better retain instance state across configuration changes, with setRetainInstanceState(true).

   • You can nest Fragments inside other Fragments, each of which can have a different layout.

   • You can create Fragments without a user interface, unlike Activities, which should always have a content view.

2. What can Activities do that Fragment's can't?

   • Launcher Fragments don't exist, only launcher Activities.

   • Activities can be started from other apps with Intents.

   • Activities can be started as new tasks.

   • You can use Activities in older versions of Android, which is no longer that big of a deal since the usage rates of these older versions of android are pretty low nowadays.

While this list is certainly not comprehensive, there are many cases in which you could use either an Activity or a Fragment for the same content. Pick whichever one helps you keep your code organized. Fragments are good for reusable components where Activities are good for standalone capabilities. Personally, I tend to prefer using Activities unless there is a reason to use Fragments. I find that managing too many Fragments within a single Activity is tedious and error prone.