Notes

Hosting Compose UI in an Activity: setContent

3 min read

In modern Android development, Jetpack Compose provides a declarative, Kotlin-first approach to UI. Under the hood, Compose still relies on Android’s classic View system. The ComponentActivity.setContent extension functions as the bridge between these two worlds. This article walks through its implementation, design rationale, and practical use.

Why ComponentActivity.setContent Exists

  • Offers control over the composition context by accepting an optional CompositionContext
  • Enables reuse of an existing ComposeView, preserving UI state and animations across configuration changes or process recovery

1. Lookup of an Existing ComposeView

val container = window.decorView
    .findViewById<ViewGroup>(android.R.id.content)
val existing = container.getChildAt(0) as? ComposeView

This step checks the activity’s content container for a pre-existing ComposeView. If present, it avoids recreating the view and losing state.

2. Reuse vs. Recreate: Two Execution Paths

Reusing the View

existing.apply {
    setParentCompositionContext(parent)
    setContent { content() }
}
  • Maintains the previous composition hierarchy
  • Retains animations, scroll positions, and remember-based state
  • Ideal for configuration changes or rapid content swaps

Creating a New View

ComposeView(this).apply {
    setParentCompositionContext(parent)
    setContent { content() }
    setOwners()
    setContentView(this, DefaultActivityContentLayoutParams)
}
  • Constructs a fresh ComposeView
  • Wires up the passed composable content
  • Attaches to the activity’s view hierarchy with full-screen layout parameters

3. Role of setOwners()

Prior to attaching the new view, setOwners() installs three essential owners on the view tree:

  1. ViewTreeLifecycleOwner

    • Synchronizes composition recomposition with the activity (or fragment) lifecycle

  2. ViewTreeSavedStateRegistryOwner

    • Enables saved-state APIs such as rememberSavedInstanceState and SavedStateHandle

  3. ViewTreeOnBackPressedDispatcherOwner

    • Routes back-button events into Compose’s BackHandler composable

Installing these owners before view inflation and listener registration ensures correct lifecycle, state restoration, and back-navigation behavior.

4. Full-Screen Layout by Default

The use of DefaultActivityContentLayoutParams (which corresponds to MATCH_PARENT × MATCH_PARENT) guarantees that the Compose UI fills the entire screen without additional layout configuration.

5. Typical Invocation in an Activity

class MyActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyAppTheme {
                Surface {
                    Greeting("Compose World")
                }
            }
        }
    }
}

Each call to setContent triggers the logic that:

  • Determines whether to reuse or recreate the ComposeView
  • Installs owners if needed
  • Attaches the composable tree

6. When to Supply a Parent CompositionContext

  • Sharing composition scopes across multiple hosts, such as nested windows or dialogs
  • Migrating an existing composition into a new host while preserving state

Conclusion

The ComponentActivity.setContent extension orchestrates a careful integration of Compose within the Android View framework. By leveraging view reuse, owner installation, and full-screen layout parameters, it provides a robust foundation for declarative UIs that benefit from both Compose’s flexibility and the established Android view lifecycle.

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