Support sizing of Compose hosting views on iOS by svastven · Pull Request #2984 · JetBrains/compose-multiplatform-core

svastven · 2026-04-19T23:00:30Z

This PR adds a sizing integration that lets a Compose-hosting UIView participate in UIKit/SwiftUI sizing loops by reporting Compose’s preferred size for a given sizing proposal.

Fixes CMP-5788 iOS investigate intrinsic sizing of ComposeUIViewController
Fixes CMP-5873 [Epic] iOS intrinsic sizing of interop elements

How it works

UIKit/SwiftUI calls sizeThatFits(...) with a proposal (including UIViewNoIntrinsicMetric for unbounded axes).
The container converts the proposal into Compose Constraints.
After each Compose layout completes, we probe-measure the scene under the latest proposal constraints and compute the preferred size.
If the preferred size changes, the hosting view invalidates its intrinsic content size, allowing UIKit/SwiftUI to re-run layout and apply the new size.

Fallback handling uses super.sizeThatFits for non-intrinsic axes to avoid 0 sizes before Compose has produced a measurement.

Public API changes

Add useSelfSizing to ComposeContainerConfiguration. Default is false to preserve existing behavior. When enabled, the container can size itself to fit Compose content under UIKit/SwiftUI size proposals.

Example usage

(SwiftUI, iOS 16+): sizeThatFits

On the Kotlin side, enable the feature (defaults to false):

val view = ComposeUIView(
    configure = { useSelfSizing = true },
) { FooComposable() }

On the SwiftUI side, wrap the returned UIView/UIViewController and forward SwiftUI’s proposal to UIKit sizeThatFits:

import SwiftUI
import UIKit

struct ComposeView: UIViewRepresentable {
    // Provided by the Kotlin/Swift bridge
    let makeComposeUIView: () -> UIView

    func makeUIView(context: Context) -> UIView {
        makeComposeUIView()
    }

    func updateUIView(_ uiView: UIView, context: Context) {}

    @available(iOS 16.0, *)
    func sizeThatFits(_ proposal: ProposedViewSize, uiView: UIView, context: Context) -> CGSize? {
        let proposed = CGSize(
            width: proposal.width ?? UIView.noIntrinsicMetric,
            height: proposal.height ?? UIView.noIntrinsicMetric
        )
        // Calls into ComposeHostingView/ComposeContainerView.sizeThatFits, which uses the latest
        // proposal constraints to produce Compose’s preferred size.
        return uiView.sizeThatFits(proposed)
    }
}

(SwiftUI, iOS <16): sizeThatFits

The same mechanism works via intrinsic sizing alone: SwiftUI reacts to invalidateIntrinsicContentSize() and re-queries layout.

Testing

Adds instrumented tests that simulate the SwiftUI sizing loop (proposal → sizeThatFits → apply frame → wait for intrinsic invalidation → repeat) and validate that Compose scene size and hosting view frame converge as Compose content or size proposed by SwiftUI changes.

This should be tested by QA

Release Notes

Features - iOS

Support automatic sizing of Compose hosting views (ComposeHostingView / ComposeHostingViewController) used on the UIKit / SwiftUI side. Enabled by default; opt out via ComposeContainerConfiguration.useSelfSizing.

Breaking Changes - iOS

Compose hosting views now automatically adjust their size to match their content by default. This may result in different layout behavior than before. A new flag ComposeContainerConfiguration.useSelfSizing allows opting out. Set it to false to preserve the previous behavior. This flag is temporary and will be removed in version 1.13.0.

svastven · 2026-04-20T09:09:56Z

Marked as draft as I still need to add tests for the UIKit/SwiftUI intrinsic sizing path.

svastven · 2026-04-20T11:14:16Z

@m-sasha Adding you as reviewer, too because desktop sources are involved and there is overlap with #2938

m-sasha · 2026-04-21T16:08:01Z

+/**
+ * Returns the current content size (in pixels) measured with the provided [constraints].
+ *
+ * @throws IllegalStateException when [ComposeScene] content has lazy layouts without maximum
+ * size bounds (e.g. LazyColumn without maximum height) and the constraints are unbounded.
+ */


This comment is only correct if the relevant dimension is unconstrained (e.g. height=Infinity).

so "the constraints are unbounded" doesn't reflect the behavior correctly? It seems to me that the doc is the same as for unconstrainedSize() only specifies the unbounded constraints in addition which is exactly what happens in unconstrainedSize()

Sorry, you're right. I didn't read the comment to the end, though it was just copy/pasted from unconstrainedSize.

I would just clarify in the comment that this applies on each axis separately.

m-sasha · 2026-04-21T16:13:30Z

+     * platform integrations to avoid retain cycles between a hosting container and
+     * the scene/root.
+     */
+    fun registerOnLayoutCompletedListener(listener: () -> Unit): AutoCloseable


Not a blocker, but I'd slightly prefer a dedicated class instead of AutoCloseable.
Existing examples:

PinnableContainer.PinnedHandle

DelegatableNode.RegistrationHandle (maybe even just use this)

added a custom OnLayoutCompletedListenerHandle

MatkovIvan

I don't think it's the right way to support it.
The root problem now is that compose stages are not aligned with the platform framework.
We already have an item to do it properly, let's combine the efforts to do it right instead of redoing things again later

MatkovIvan · 2026-04-22T08:19:19Z

+     * the listener. This is important for platform integrations to avoid retain cycles between
+     * a hosting container and the scene/root.
+     */
+    fun registerOnLayoutCompletedListener(listener: () -> Unit): OnLayoutCompletedListenerHandle


It's supposed to be a separate "layout" call from the platform (TBD). The scene itself shouldn't trigger layout independently, so such event shouldn't exist/be required

Could you please clarify for me: We already have an item to do it properly ?

Do you mean

we already have a way to do this properly?

we have a TODO item to do it properly?

If the second, do we have some more description as to what is missing and what we require?

See #3012 for phases’ split

ASalavei · 2026-04-22T11:00:37Z

+     * `sizeThatFits` / intrinsic sizing).
+     */
+    @ExperimentalComposeUiApi
+    var useSelfSizing: Boolean = false


Do we need this option? I assumed that self-sizing (like, providing intrinsicContentSize) should be always ON.

I added this flag so we don't change behavior for existing users. If we turn it on by default, it will affect what they already have. Do we want that, or keep it opt-in for now?

Hm.. Previously we didn't have any constraints there - so users must configure them explicitly. I expect that transition should go smooth to the new state in most cases.
It looks like a safe option here is to make useSelfSizing = true by default, with "Breaking Change" release note and also with intent to remove the flag in the next version (1.13).

ASalavei · 2026-04-22T11:43:18Z

+
+            rootViewController.view.let {
+                if (configuration.useSelfSizing) {
+                    it.addSubview(hostingView)


Not sure I get the point. If we're adding view with the translatesAutoresizingMaskIntoConstraints == true, it means the frame of this view must be manually adjusted somewhere (btw, I didn't find the place where it happened). The idea behind the "Self Sizing" Compose views was to make it work correctly inside Autolayout and in the SwiftUI system. No sure we should manually update the frame.

ASalavei · 2026-04-22T12:54:01Z

+                invalidateComposeSceneContainerSize = {
+                    // SwiftUI observes the hosting view’s intrinsic size, not the internal Compose
+                    // container view.
+                    view.superview?.invalidateIntrinsicContentSize()


The comment is true, but in case of ComposeHostingViewController, the view.superview will refer the user's view, not a compose-related one.

svastven force-pushed the svastven/sizing-compose-view branch 2 times, most recently from c3fcdd8 to 9d38020 Compare April 20, 2026 00:12

svastven changed the title ~~Auto-sizing of Compose Hosting on iOS~~ Sizing of Compose Hosting Views on iOS Apr 20, 2026

svastven changed the title ~~Sizing of Compose Hosting Views on iOS~~ Support sizing of Compose Hosting Views on iOS Apr 20, 2026

svastven changed the title ~~Support sizing of Compose Hosting Views on iOS~~ Support sizing of Compose hosting views on iOS Apr 20, 2026

svastven requested review from ASalavei, MatkovIvan and mazunin-v-jb April 20, 2026 09:05

svastven requested a review from m-sasha April 20, 2026 11:13

m-sasha mentioned this pull request Apr 20, 2026

Expose ComposeScene.measurableContent to allow querying the scene for its size properties #2986

Merged

svastven force-pushed the svastven/sizing-compose-view branch 4 times, most recently from 28c50d6 to 5b3ada9 Compare April 21, 2026 14:40

svastven added 5 commits April 21, 2026 16:52

skiko: add constrained sizing + persistent layout-complete listener

102de25

ios: reconcile sizeThatFits proposals with Compose preferred size

08dc210

uikitInstrumentedTest: add ComposeUIView sizing reconciliation tests

94b9b3a

ios: honor usePreferredSizeSizing in ComposeContainer

f3e476c

ios: simplify usePreferredSizeSizing documentation

0ae5bce

svastven force-pushed the svastven/sizing-compose-view branch from 5b3ada9 to 0ae5bce Compare April 21, 2026 14:54

m-sasha reviewed Apr 21, 2026

View reviewed changes

skiko: return unregister handle from registerOnLayoutCompletedListener

c90ac46

svastven requested a review from m-sasha April 21, 2026 22:21

MatkovIvan reviewed Apr 22, 2026

View reviewed changes

ASalavei reviewed Apr 22, 2026

View reviewed changes

Change useSelfSizing default value to true

189077d

MatkovIvan added the changes in API label Apr 27, 2026

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How it works

Public API changes

Example usage

(SwiftUI, iOS 16+): sizeThatFits

(SwiftUI, iOS <16): sizeThatFits

Testing

Release Notes

Features - iOS

Breaking Changes - iOS

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svastven commented Apr 19, 2026 •

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