Hey Eric, let's break down how to fix this multi-BLE device sync control issue and get those LEDs firing consistently!

First, Let's Diagnose the Warning #

That is not a valid characteristic error usually pops up for a few key reasons:

• You're using a stale CBCharacteristic reference (maybe the device reconnected and the old instance is no longer valid)
• You're sending commands before the characteristic is properly cached and verified
• Bombarding CoreBluetooth with concurrent command requests overwhelms its serial processing queue

Best Practices for Stable Multi-Device BLE Control #

1. Fix Characteristic Caching in Your BLEDevice Class

Right now, you're probably fetching the characteristic every time you send a command (or relying on an outdated reference). Update your BLEDevice to cache the target characteristic directly, and add a readiness check:

class BLEDevice {
    var uuid: String!
    var peripheral: CBPeripheral!
    var rssi: NSNumber!
    var name: String!
    var advertData: [String: Any]!
    var type = 0
    // Cache your target LED control characteristic
    var ledControlCharacteristic: CBCharacteristic?
    
    // Quick check if the device is ready to receive commands
    var isCommandReady: Bool {
        return peripheral.state == .connected && ledControlCharacteristic != nil
    }
    
    init() { }
}

When you discover characteristics (in your didDiscoverCharacteristicsFor delegate), save the FFE1 characteristic to the corresponding BLEDevice instance immediately.

2. Use a Serial Queue for BLE Commands

CoreBluetooth operates on a serial queue—sending commands in a raw for loop can cause race conditions. Create a dedicated serial queue to process commands one at a time:

// Create a serial queue for all BLE command operations
private let bleCommandQueue = DispatchQueue(label: "com.yourapp.ble.commandQueue")

func sendSyncLEDCommand(toAllDevices commandData: Data) {
    bleCommandQueue.async { [weak self] in
        guard let self = self else { return }
        // Filter only devices that are ready to receive commands
        let readyDevices = self.bleDeviceDict.values.filter { $0.isCommandReady }
        
        // Use a DispatchGroup to send commands sequentially (avoid overwhelming CB)
        let commandGroup = DispatchGroup()
        for device in readyDevices {
            commandGroup.enter()
            self.sendCommand(to: device, data: commandData) { success in
                if !success {
                    print("Failed to send command to device: \(device.name ?? "Unknown")")
                    // Add retry logic here if needed
                }
                commandGroup.leave()
            }
        }
        
        // Optional: Wait for all commands to finish, or use notify for post-processing
        commandGroup.notify(queue: .main) {
            print("All sync commands initiated")
        }
    }
}

private func sendCommand(to device: BLEDevice, data: Data, completion: @escaping (Bool) -> Void) {
    guard let char = device.ledControlCharacteristic else {
        completion(false)
        return
    }
    
    // Check if the characteristic supports your desired write type
    if char.properties.contains(.writeWithoutResponse) {
        device.peripheral.writeValue(data, for: char, type: .withoutResponse)
        completion(true) // No ACK, so assume success (adjust if you need reliability)
    } else if char.properties.contains(.write) {
        device.peripheral.writeValue(data, for: char, type: .withResponse)
        // For write-with-response, you'll need to trigger completion from the delegate
        // Tip: Store a temporary callback in BLEDevice tied to the characteristic/command
        completion(true)
    } else {
        print("Characteristic doesn't support write operations")
        completion(false)
    }
}

3. Add Command Queueing for Unready Devices

If some devices aren't ready (still connecting, reconnected, etc.), add a command queue to each BLEDevice to hold pending commands until it's ready:

// Update BLEDevice with a command queue
class BLEDevice {
    // ... existing properties ...
    private var pendingCommands: [Data] = []
    
    func enqueueCommand(_ data: Data) {
        pendingCommands.append(data)
        // If ready, process the queue immediately
        if isCommandReady {
            processPendingCommands()
        }
    }
    
    private func processPendingCommands() {
        guard isCommandReady else { return }
        for command in pendingCommands {
            // Use your existing sendCommand logic here
        }
        pendingCommands.removeAll()
    }
}

Then, in your peripheralDidUpdateState delegate, call processPendingCommands() when a device reconnects and becomes ready.

4. Optimize for Near-Sync

For true near-simultaneous control:

• Use Write Without Response (faster, no ACK overhead) when possible—trade a tiny bit of reliability for speed
• If you need precision, first send a "prepare" command to all devices, wait for their ACKs (via notifications), then send the final LED control command all at once
• Avoid large payloads—stick to BLE's default MTU (20 bytes) unless you've negotiated a larger one

Final Notes #

Always validate the device state and characteristic reference before sending any commands. The serial queue ensures CoreBluetooth doesn't get swamped, and command queueing handles devices that aren't ready immediately.

内容的提问来源于stack exchange，提问作者Eric