如何让iOS NFC交互函数同步执行？无需usleep等待结果

你的核心问题是sendI2CCommand依赖异步的NFC请求，但当前实现无法等待异步回调完成就返回结果，用usleep是不可靠的临时方案，信号量崩溃则是因为线程死锁——NFC回调默认在主线程执行，若你在主线程调用semaphore.wait()，主线程会被阻塞，回调永远无法触发signal()，导致死锁。

以下是两种可靠的解决方案：

方案一：修复信号量实现（兼容iOS 14及以下） #

确保信号量的等待操作在后台线程执行，同时所有分支都必须调用signal()，避免永久阻塞。修改后的代码如下：

public static func sendI2CCommand(tag: NFCISO15693Tag, command: UInt8) -> String {
    var res = ""
    let semaphore = DispatchSemaphore(value: 0)
    
    tag.sendRequest(requestFlags: 0x12, commandCode: 0xD4, data: Data([0x04, 0x02, 0x00, command])) { result in
        switch result {
        case .success:
            semaphore.signal()
        case .failure:
            tag.sendRequest(requestFlags: 0x12, commandCode: 0xD5, data: Data([0x04, 0x02, 0x00])) { secondResult in
                switch secondResult {
                case .success:
                    semaphore.signal()
                case .failure:
                    tag.sendRequest(requestFlags: 0x12, commandCode: 0xD2, data: Data([0x04, 0x00, 0x06])) { thirdResult in
                        switch thirdResult {
                        case .success((let responseFlag, let response2)):
                            var foo = response2
                            foo.insert(responseFlag.rawValue, at: 0)
                            let responseHex = foo.hexadecimalCompressed.substring(with: 2..<4)
                            res = responseHex
                            semaphore.signal()
                        case .failure, default:
                            semaphore.signal()
                        }
                    }
                default:
                    semaphore.signal()
                }
            }
        default:
            semaphore.signal()
        }
    }
    
    // 在后台线程等待，避免主线程死锁
    DispatchQueue.global().sync {
        semaphore.wait()
    }
    
    return res
}

方案二：使用Async/Await（iOS 15+，推荐） #

通过withCheckedThrowingContinuation将异步的sendRequest包装为Async函数，彻底消除回调嵌套和线程死锁问题，代码更易维护。

1. 封装NFC请求为Async方法 #

extension NFCISO15693Tag {
    func sendRequestAsync(requestFlags: UInt8, commandCode: UInt8, data: Data) async throws -> (NFCISO15693ResponseFlag, Data) {
        return try await withCheckedThrowingContinuation { continuation in
            self.sendRequest(requestFlags: requestFlags, commandCode: commandCode, data: data) { result in
                switch result {
                case .success(let response):
                    continuation.resume(returning: response)
                case .failure(let error):
                    continuation.resume(throwing: error)
                }
            }
        }
    }
}

2. 修改sendI2CCommand为Async函数 #

public static func sendI2CCommand(tag: NFCISO15693Tag, command: UInt8) async -> String {
    var res = ""
    do {
        // 执行第一条命令
        try await tag.sendRequestAsync(requestFlags: 0x12, commandCode: 0xD4, data: Data([0x04, 0x02, 0x00, command]))
    } catch {
        do {
            // 第一条失败，执行第二条
            try await tag.sendRequestAsync(requestFlags: 0x12, commandCode: 0xD5, data: Data([0x04, 0x02, 0x00]))
        } catch {
            do {
                // 第二条失败，执行第三条并处理结果
                let (responseFlag, response2) = try await tag.sendRequestAsync(requestFlags: 0x12, commandCode: 0xD2, data: Data([0x04, 0x00, 0x06]))
                var foo = response2
                foo.insert(responseFlag.rawValue, at: 0)
                let responseHex = foo.hexadecimalCompressed.substring(with: 2..<4)
                res = responseHex
            } catch {
                // 处理所有错误，返回空字符串或自定义错误标记
                res = ""
            }
        }
    }
    return res
}

3. 调用方式 #

用Task启动异步任务，自动按顺序执行命令：

Task {
    guard let tag = self.tag else { return }
    let fw_id = await NFCUtils.sendI2CCommand(tag: tag, command: NFCConstants.CONFIG_FW_ID)
    print("Read FirmwareID \(fw_id)")
    
    let sw_id = await NFCUtils.sendI2CCommand(tag: tag, command: NFCConstants.CONFIG_SW_ID)
    print("Read SotfwareID \(sw_id)")
    
    let ex_id = await NFCUtils.sendI2CCommand(tag: tag, command: NFCConstants.CONFIG_EX_ID)
    print("Read ExternalID \(ex_id)")
}

内容的提问来源于stack exchange，提问作者tiamat