iOS蓝牙开发：点击APP按钮无数据发至外设，外设与特征值为nil排查

It sounds like the core issue here is that your TemperatureViewController isn’t properly accessing the Bluetooth objects managed in AppDelegate, or those objects aren’t being passed/updated correctly when the peripheral connects and characteristics are discovered. Let’s break down the exact causes and how to fix them:

Common Root Causes #

• Your ViewController has uninitialized, separate Bluetooth objects: The centralManager, heartRatePeripheral, and characteristic in TemperatureViewController are not linked to the working instances in AppDelegate—they’re never assigned values, so they stay nil.
• No communication between AppDelegate and ViewController: When AppDelegate discovers a peripheral or characteristic, it doesn’t notify your ViewController to update its references.
• You’re calling writeValue too early: If the button is pressed before the peripheral is connected and the target characteristic is fully discovered, those variables will still be nil.

Step-by-Step Fixes #

1. Expose AppDelegate’s Bluetooth Instances #

First, update your AppDelegate to make its Bluetooth properties accessible to other parts of the app, and ensure it properly tracks connected peripherals and characteristics:

import UIKit
import CoreBluetooth

@main
class AppDelegate: UIResponder, UIApplicationDelegate, CBCentralManagerDelegate, CBPeripheralDelegate {
    var window: UIWindow?
    var centralManager: CBCentralManager!
    var connectedPeripheral: CBPeripheral?
    var targetWriteCharacteristic: CBCharacteristic?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        centralManager = CBCentralManager(delegate: self, queue: nil)
        return true
    }

    // MARK: - CBCentralManagerDelegate
    func centralManagerDidUpdateState(_ central: CBCentralManager) {
        if central.state == .poweredOn {
            // Replace with your peripheral's service UUID
            centralManager.scanForPeripherals(withServices: [CBUUID(string: "YOUR_SERVICE_UUID")])
        }
    }

    func centralManager(_ central: CBCentralManager, didDiscover peripheral: CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber) {
        centralManager.connect(peripheral, options: nil)
        connectedPeripheral = peripheral
        peripheral.delegate = self
    }

    func centralManager(_ central: CBCentralManager, didConnect peripheral: CBPeripheral) {
        peripheral.discoverServices([CBUUID(string: "YOUR_SERVICE_UUID")])
    }

    // MARK: - CBPeripheralDelegate
    func peripheral(_ peripheral: CBPeripheral, didDiscoverServices error: Error?) {
        guard let services = peripheral.services else { return }
        for service in services {
            // Replace with your characteristic's UUID
            peripheral.discoverCharacteristics([CBUUID(string: "YOUR_WRITE_CHAR_UUID")], for: service)
        }
    }

    func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
        guard let characteristics = service.characteristics else { return }
        for char in characteristics {
            if char.uuid == CBUUID(string: "YOUR_WRITE_CHAR_UUID") {
                targetWriteCharacteristic = char
                // Notify ViewController the characteristic is ready to use
                NotificationCenter.default.post(name: NSNotification.Name("CharacteristicReady"), object: nil)
            }
        }
    }
}

2. Update Your ViewController to Use AppDelegate’s Objects #

Modify your TemperatureViewController to access the shared Bluetooth instances from AppDelegate, and add safeguards to avoid nil references:

import UIKit
import CoreBluetooth

class TemperatureViewController: UIViewController {
    // Remove local Bluetooth properties—we'll use AppDelegate's instead
    // var centralManager: CBCentralManager!
    // var heartRatePeripheral: CBPeripheral!
    // var characteristic: CBCharacteristic?

    @IBOutlet weak var sendValueButton: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Disable button until characteristic is ready
        sendValueButton.isEnabled = false
        // Listen for notification from AppDelegate when characteristic is discovered
        NotificationCenter.default.addObserver(self, selector: #selector(enableWriteButton), name: NSNotification.Name("CharacteristicReady"), object: nil)
    }

    @objc func enableWriteButton() {
        sendValueButton.isEnabled = true
    }

    // Button action to send value
    @IBAction func sendValueTapped(_ sender: UIButton) {
        writeValue(value: 25) // Example temperature value
    }

    func writeValue(value: Int8) {
        // Get reference to AppDelegate
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
            print("Failed to access AppDelegate")
            return
        }

        // Validate all required objects exist and support writing
        guard let peripheral = appDelegate.connectedPeripheral,
              let characteristic = appDelegate.targetWriteCharacteristic,
              characteristic.properties.contains(.write) || characteristic.properties.contains(.writeWithoutResponse) else {
            print("Peripheral/Characteristic is nil or doesn't support writing")
            return
        }

        // Convert Int8 value to Data
        let data = Data(bytes: &value, count: MemoryLayout<Int8>.size)

        // Write to peripheral (use .withoutResponse if you don't need confirmation)
        peripheral.writeValue(data, for: characteristic, type: .withResponse)
    }
}

3. Key Additional Checks #

• Wait for proper initialization: Only enable the send button after the CharacteristicReady notification fires—this ensures the peripheral is connected and the characteristic is ready.
• Verify UUIDs: Double-check that the service and characteristic UUIDs in your code match exactly what the Bluetooth peripheral is advertising. Mismatched UUIDs will prevent discovery.
• Check characteristic properties: Ensure the peripheral’s characteristic actually supports writing (look for .write or .writeWithoutResponse in its properties).

Debugging Tips #

• Add print statements in AppDelegate’s delegate methods to confirm when the peripheral connects and the characteristic is discovered.
• In your writeValue method, print peripheral and characteristic to confirm they’re not nil when the button is pressed.
• Use Xcode’s debug navigator to inspect the state of AppDelegate’s Bluetooth objects at runtime.

内容的提问来源于stack exchange，提问作者Nazia Islam