Composition gives you maximum control over the visual and
interaction design of your widgets while also allowing a
large amount of code reuse. In the framework, we’ve decomposed
complex widgets to pieces that separately implement
the visual, interaction, and motion design. You can remix
these widgets however you like to make your own custom
widgets that have full range of expression.

Why would I want to share layout code across iOS and Android?

You can choose to implement different app layouts for
iOS and Android. Developers are free to check the mobile OS
at runtime and render different layouts,
though we find this practice to be rare.

More and more, we see mobile app layouts and designs evolving
to be more brand-driven and unified across platforms.
This implies a strong motivation to share layout and UI
code across iOS and Android.