Does Compose UI get translated to Android Views un...

# compose

[Mehdi Haghgoo]

Does Compose UI get translated to Android Views under the hood to create native UIs and interact with the Android framework?

[jim]

Yes, but not a separate AndroidView for each widget (that would be too slow). You get an AndroidView at the root, and occasionally in some other extenuating circumstances.

[Orhan Tozan]

@jim so like one big canvas right?

[jim]

Yeah, "one big canvas" a sufficiently true approximation.

[romainguy]

To be more specific: it gets (sometimes) translated to Views, as in the base View class

[romainguy]

Not things like android.widget.Button for instance

[Mehdi Haghgoo]

So, for example, a box gets translated to a native FrameLayout? a LazyColumn to a native RecyclerView? and so on? Or has it nothing to do with framework Views?

[jim]

No, individual widgets are not converted to Views. Only one View at the root to bootstrap the framework.

[Mehdi Haghgoo]

Maybe, Compose UI and Android View system are similiar mostly in the fact that they have some measure pass and some layout (I mean draw) pass, but they are co-independent and self-sufficient; They don't usually use each other to achieve a piece of UI. Am I right?

[Mehdi Haghgoo]

Also, the fact that they are tree-based!

[jim]

All UI frameworks are ultimately tree based and have some sort of measure pass and some sort of draw pass.

[jim]

Compose is highly interoperable with Views (you can use Android Views in your Compose tree and vice versa). We need to create Views at the root and in a few other circumstances, and they share a drawing layer. But those are mostly implementation details related to interoperability, for all intents and purposes, you can consider Compose to be an entirely new UI framework.

[Mehdi Haghgoo]

The Compose documentation is already nice, but I think it would be great to add some section (or content to some existing section), on how Compose works under the hood, a little bit of what Compose compiler does and how it interacts with Kotlin compiler and compiler extension, scratching a bit on how UI trees are constructed. In case it is not already hidden somewhere in the docs, I guess that would serve to satisfy some curiosity from people interested in the other side of things. Not sure if that would be much developer-friendly though.

[jim]

There is an entire book on the topic: https://leanpub.com/composeinternals

(I am not affiliated with the author and haven't read the book yet so I can't endorse it, but minimum price is free and it might be a good place to start)

[Mehdi Haghgoo]

That's nice. Thank you @jim

[Mehdi Haghgoo]

The funny thing about my Compose journey is that when I was first introduced to compose in DevSummit 2019, I took the tutorial and said to myself "It's so easy." I got this! And now, after two years, I'm still learning Compose 😂
Nevertheless the Compose team did a great job delivering Compose to developers gradually from early alpha versions to stable. Developers kind of grew with the framework!