EventSource

A lightweight, spec-compliant Server-Sent Events (SSE) client for Swift.

Features

• [x] Full implementation of the [Server-Sent Events specification][spec]
• [x] Automatic connection management and reconnection with configurable retry intervals
• [x] Event parsing that handles all standard fields (id, event, data, retry)
• [x] Support for different line break formats (LF, CR, CRLF)
• [x] Multi-line data aggregation
• [x] AsyncSequence support for streaming events
• [x] Works on all Apple platforms and Linux

Requirements

• Swift 6.0+ / Xcode 16+

Installation

Swift Package Manager

Add the following to your Package.swift file:

dependencies: [
    .package(url: "https://github.com/mattt/EventSource.git", from: "1.4.0")
]

AsyncHTTPClient support

EventSource can optionally integrate with AsyncHTTPClient through a package trait (Swift 6.1+):

dependencies: [
    .package(
        url: "https://github.com/mattt/EventSource.git",
        from: "1.4.0",
        traits: ["AsyncHTTPClient"]
    )
]

Build and test with the trait enabled:

swift build --traits AsyncHTTPClient
swift test --traits AsyncHTTPClient

[!NOTE]

AsyncHTTPClient uses SwiftNIO instead of Foundation URL Loading System.

If traffic is routed through AsyncHTTPClient, URLProtocol-based interception

does not apply.

On Linux, EventSource starts with URLSession transport and switches to

AsyncHTTPClient only after a retryable URLSession failure. Once switched,

that EventSource instance continues using AsyncHTTPClient for reconnect attempts.

Usage

Connecting to an EventSource

Create an EventSource with a URL to establish a persistent connection to an SSE endpoint. The API mirrors the JavaScript [EventSource interface][mdn] with event handlers for connection lifecycle management.

import EventSource
import Foundation
// Initialize with SSE endpoint URL
let sse = EventSource(url: URL(string: "https://example.com/events")!)
// Create an EventSource with the URL
let sse = EventSource(url: url)
// Set up event handlers
sse.onOpen = {
    print("Connection established")
}
sse.onMessage = { e in
    print("Received event: \(e.event): \(e.data)")
}
sse.onError = { error in
    if let error = error {
        print("Error: \(error)")
    } else {
        print("Connection closed")
    }
}
// Later, when done
Task {
    await sse.close()
}

Processing an AsyncSequence of Server-Sent Events

Alternatively, you can process server-sent event data with Swift's modern AsyncSequence API for greater flexibility and control. Use this approach when you need custom request configuration or direct integration with existing async / URL Loading System code.

import EventSource
import Foundation
Task {
    // Create a request to the SSE endpoint
    let url = URL(string: "https://example.com/events")!
    let request = URLRequest(url: url)
do {
        let (stream, _) = try await URLSession.shared.bytes(for: request)
// Iterate through events as they arrive
        for try await event in stream.events {
            switch event.event {
            case "update":
                handleUpdate(event.data)
            case "error":
                handleError(event.data)
            default:
                print("Received: \(event.data)")
            }
        }
    } catch {
        print("Stream error: \(error.localizedDescription)")
    }
}

Parsing Server-Sent Events Directly

Use the low-level parser directly to process raw SSE data. This approach is ideal for custom networking stacks, testing, or scenarios where you need precise control over state management.

import EventSource
import Foundation
// Create a parser instance
let parser = EventSource.Parser()
// Process raw SSE data byte-by-byte
let rawData = """
    id: 123
    event: update
    data: {"key": "value"}
data: Another message
""".utf8
// Parse the data
Task {
    // Feed bytes to the parser
    for byte in rawData { await parser.consume(byte) }
    await parser.finish()
// Extract all parsed events
    while let event = await parser.getNextEvent() {
        handleEvent(event)
    }
// Access state for reconnection logic
    print("Last Event ID: \(await parser.getLastEventId())")
    print("Reconnection time: \(await parser.getReconnectionTime())ms")
}

Examples

Working with Streaming APIs

Inference providers like Anthropic and OpenAI offer APIs with endpoints that stream tokens as they're generated. Here's a simplified example of how you might consume server-sent events for such an API:

import EventSource
import Foundation
// Simple model for LLM token streaming
struct TokenChunk: Codable {
    let text: String
    let isComplete: Bool
}
// Create a request to the LLM streaming endpoint
let url = URL(string: "https://api.example.com/completions")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.setValue("Bearer YOUR_API_KEY", forHTTPHeaderField: "Authorization")
// Track the full response
var completedText = ""
// Process the stream asynchronously
Task {
    do {
        // Get a byte stream from URLSession
        let (byteStream, response) = try await URLSession.shared.bytes(for: request)
// Ensure response is valid
        guard let httpResponse = response as? HTTPURLResponse,
            httpResponse.statusCode == 200,
            let contentType = httpResponse.value(forHTTPHeaderField: "Content-Type"),
            contentType.contains("text/event-stream")
        else {
            throw NSError(domain: NSURLErrorDomain,
                        code: NSURLErrorBadServerResponse,
                        userInfo: nil)
        }
let decoder = JSONDecoder()
// Stream events asynchronously
        for try await event in byteStream.events {
            // Decode each chunk as it arrives
            let chunk = try decoder.decode(TokenChunk.self,
                                           from: Data(event.data.utf8))
// Add the new token to our result
            completedText += chunk.text
            print("Text so far: \(completedText)")
// Check if the response is complete
            if chunk.isComplete {
                print("Final response: \(completedText)")
                break
            }
        }
    } catch {
        print("Stream error: \(error.localizedDescription)")
    }
}

License

This project is available under the MIT license. See the LICENSE file for more info.

[mdn]: https://developer.mozilla.org/en-US/docs/Web/API/EventSource [spec]: https://html.spec.whatwg.org/multipage/server-sent-events.html#the-eventsource-interface