feat_sdk library

A thin Dart/Flutter client for the feat feature-flag platform.

feat evaluates flags server side for client keys and returns resolved values only. This SDK is a thin client over those remote-evaluation endpoints: it polls for values and, optionally, holds a Server-Sent Events stream for near-instant updates. It performs no local evaluation and needs no bucketing engine.

The core library is pure Dart (no Flutter dependency) so it runs equally in Flutter apps and pure-Dart backends. Flutter-only glue (lifecycle observer, shared_preferences-backed storage) lives outside the core - see the example.

Classes

EvalContext
The evaluation context sent to the server on every poll and stream connection. It mirrors the multi-context model used by the other feat SDKs: an optional top-level targetingKey (shorthand for the user key) plus zero or more per-kind objects, each of which must carry its own key.
EvalContextBuilder
Fluent builder for an EvalContext.
EvalReason
Well-known evaluation reasons. The set is open (the server may introduce new reasons), so treat EvaluatedFlag.reason as an opaque string and use these constants only for convenient comparisons.
EvalStream
Manages a single logical SSE subscription with automatic reconnection and exponential backoff. It is transport-agnostic via SseConnector.
EvaluatedFlag
A single flag as resolved by the server for the current context.
FeatClient
A thin client over the feat remote-evaluation endpoints.
FeatClientConfig
Configuration for a FeatClient.
FeatStorage
Pluggable key/value persistence used by the SDK for two things: the stable anonymous key and the last-evaluated flag snapshot (so the next start can serve cached values before the network responds).
FlagChange
Emitted on FeatClient.changes whenever a flag's resolved value flips, after either a context change, a poll, or a streamed update.
InMemoryStorage
The default, non-persistent FeatStorage. Values live only for the lifetime of the process. Suitable for pure-Dart backends and tests.
SseConnection
An open SSE connection: a stream of parsed events plus a teardown hook.
SseEvent
A parsed Server-Sent Event.

Enums

FeatAppLifecycleState
App lifecycle states, mirroring Flutter's AppLifecycleState so a Flutter WidgetsBindingObserver can forward states 1:1 without importing Flutter into the core package. See the example for the glue.
StreamingMode
Streaming policy for the client.

Constants

defaultFeatBaseUrl → const String
The default feat data-plane endpoint.
featSdkVersion → const String
The published version of this SDK. Sent to the server in the X-Feat-Sdk header as dart/<version> so usage can be attributed.

Functions

buildAnonymousContext(FeatStorage storage) → Future<EvalContext>
Build a stable anonymous EvalContext. A UUID is minted on first use and persisted through storage so the same install lands in the same targeting bucket across restarts. When storage is an InMemoryStorage (the default), the key is stable only for the process lifetime.
httpSseConnector(Client client) → SseConnector
Default SseConnector built on package:http. Unlike a browser EventSource, request headers can be set here, so the SDK version travels in X-Feat-Sdk; the client key and context still travel in the query string per the stream wire contract.
parseSse(Stream<String> lines) → Stream<SseEvent>
Transform a stream of already-split text lines into SseEvents.
randomUuidV4() → String
Generate an RFC 4122 version 4 UUID string.

Typedefs

SseConnector = Future<SseConnection> Function(Uri uri, Map<String, String> headers)
Opens an SSE connection to uri with the given request headers and resolves once connected. Injectable so tests can supply a fake transport.

Exceptions / Errors

FeatHttpException
Thrown for unexpected, non-recoverable HTTP responses from the evaluate endpoint. The documented soft statuses (404 no-datafile, 403, 429) are treated as "no change" and never surface as this exception.
SseConnectException
Thrown by the default HTTP connector when the stream endpoint answers with a non-200 status.