Why does my Swift app intermittently fail to receive server-emitted Socket.IO events?

Below are some suggestions and debugging techniques you can try:

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1. Confirm Your Event Listener Registration Order

Make sure you register the event listener before any events might be emitted by the server. If you add your listener after you call socket.connect(), there’s a chance that the server has already sent some events that your handler wasn’t listening for. For example, you might consider this order:

SWIFTCopied
guard let url = URL(string: BASE_URL_SOCKET) else { return }
socketManager = SocketManager(
    socketURL: url,
    config: [
        .log(true),
        .compress,
        .connectParams(params),
        .path("..."),
        .reconnects(true),
        .forceNew(true),
        .reconnectAttempts(-1),
        .reconnectWait(5)
    ]
)

socket = socketManager?.defaultSocket

// Register event listeners *before* connecting.
socket?.on("room-listener", callback: { data, ack in
    // Process received data
})

// Then connect
socket?.connect()

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2. Attach Connection and Error Handlers

Sometimes a connection issue may not be obvious if you’re only listening for custom events. Check for common client lifecycle events such as connect, disconnect, error, and reconnect so you can see if the connection state changes unexpectedly. For example:

SWIFTCopied
socket?.on(clientEvent: .connect) { data, ack in
    print("Socket connected")
}

socket?.on(clientEvent: .disconnect) { data, ack in
    print("Socket disconnected: \(data)")
}

socket?.on(clientEvent: .error) { data, ack in
    print("Socket encountered an error: \(data)")
}

These logs can help determine whether you’re missing events because of connection issues.

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3. Confirm Server-Side Emission Logic

Since you mentioned that events work when the app emits first (and the server then acknowledges), compare that flow with the “server-only” events. Make sure that when the server is emitting the event (without a preceding client emit) it’s targeting all connected sockets (or the intended namespace/room) and that it isn’t inadvertently sent before the client has registered its listener.

It might help to ask your Back End team to review timing of their event emissions.

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4. Check for Multiple Socket Instances or Listener Duplication

Sometimes using options such as .forceNew(true) can lead to multiple socket instances if you’re not careful. Double-check that you’re not creating more than one socket and accidentally overwriting or removing event listeners.

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5. Version or Library-Specific Issues

Since you’re on Socket.IO version 16.1.1, it’s worth checking the release notes or issues on the project’s repository. There may be known issues regarding receiving events or subtle differences in the order of connection and listener registration.

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Summary

To summarize, try the following:

• Attach event listeners before calling connect().
• Listen for connection lifecycle events (connect, disconnect, error) to capture any hidden issues.
• Verify server emission logic for events that aren’t responses to client emits.
• Watch out for multiple socket instances that might lead to unexpected behavior.
• Check Socket.IO release notes for any version-related anomalies.

Implementing these steps should give you better insight into why some events are not received and help you narrow down the root cause.