Sync API for feature flags

[gokberkbar]

Problem Statement

Hi PostHog team,
I'm using the Flutter SDK and noticed that all feature flag functions (isFeatureEnabled(), getFeatureFlag(), getFeatureFlagPayload()) return Future — requiring async/await.
This is problematic in Flutter because widget build() methods are synchronous and cannot await async calls.
Looking at your other SDKs (Web, Node, ReactNative etc.), feature flag checks are synchronous. It would be great if the Flutter SDK could offer the same.

We are fetching and caching all the feature flags in the app launch as a workaround but its unsustainable and iOS SDK already caches it. We end up with double cache.

Thanks!

Solution Brainstorm

I believe this happens because the Flutter SDK wraps the native iOS/Android SDKs via platform channels, which are async by nature. A pure Flutter/Dart implementation would solve this.

[marandaneto]

I believe this happens because the Flutter SDK wraps the native iOS/Android SDKs via platform channels, which are async by nature

correct.

Hello @gokberkbar can you use a FutureBuilder instead?
Or have a flag set with setState, so you can watch its state change and react to it?

[gokberkbar]

I believe this happens because the Flutter SDK wraps the native iOS/Android SDKs via platform channels, which are async by nature

correct.

Hello @gokberkbar can you use a FutureBuilder instead?
Or have a flag set with setState, so you can watch its state change and react to it?

Thanks for the response! I understand the technical reason (platform channels being async), but I think there's a fundamental UX issue here that makes the current implementation problematic:
Why async feature flags are problematic for UI:

Feature flags often control UI elements directly. Using FutureBuilder or setState means the UI will visibly change mid-screen, which creates a jarring experience for users
This is exactly why none of the other PostHog SDKs (iOS, Android, Web, etc.) expose an async API for reading flags - they all fetch in the background and provide sync access to cached values

The core issue: The Flutter SDK fetches config via native platform channels, but ideally, the config fetching could happen directly in Dart/Flutter with a local cache. This would allow sync access to flags without blocking the UI or causing mid-render changes.
Our situation (100+ flags across multiple apps) isn't unique - any app using feature flags for UI control will face this same problem. The current workarounds essentially require building our own caching layer on top of the SDK, which defeats the purpose of using the SDK.
Would it be feasible to move the config fetching to the Flutter/Dart layer instead of relying on platform channels? This would bring the Flutter SDK in line with how the other SDKs work.

[marandaneto]

I agree 100% with you.

This SDK started as a wrapper that covers most use cases, sometimes with a bit of boilerplate or side effects (caching flags again, screens changing midway, etc.).

FutureBuilder would work without changing screens midway; maybe it'd just drop a frame until the flag is resolved, which should be extremely fast anyway because it's just a method channel call.

We'd change this after doing #51, since we'd need a native Dart SDK anyway.
The other option is to use Dart FFI (https://dart.dev/interop/c-interop), but it's not yet stable on all platforms.