Will attempt to recover by breaking constraint

Question

I am connecting my swift app to a bluetooth low energy peripheral. I am very new in Swift and my problem is completely related to swift coding not BLE.

I listen for the peripheral to see if the button is pressed or not. I defined a label and assigned a text to it. when app detected a button press, I want label test to change, but instead my app crashes.

you can see my code here:

   import CoreBluetooth
import UIKit

class ViewController: UIViewController, CBCentralManagerDelegate, CBPeripheralDelegate {

    var manager:CBCentralManager!
    var peripheral:CBPeripheral!
    @IBOutlet weak var statusLabel: UILabel!


    let BEAN_NAME = "Security Tag"
    let BEAN_SCRATCH_UUID = CBUUID(string: "00001C0F-D102-11E1-9B23-000EFB0000A7")
    let BEAN_SERVICE_UUID = CBUUID(string: "00001c00-d102-11e1-9b23-000efb0000a7")

    override func viewDidLoad() {
        super.viewDidLoad()
        manager = CBCentralManager(delegate: self, queue: nil)
    }

    func centralManagerDidUpdateState(_ central:CBCentralManager) {
        if central.state == CBManagerState.poweredOn {
            central.scanForPeripherals(withServices: nil, options: nil)
            debugPrint("Searching ...")
        } else {
            debugPrint("Bluetooth not available.")
        }
    }

    func centralManager(_ central: CBCentralManager, didDiscover peripheral: CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber) {
        let device = (advertisementData as NSDictionary).object(forKey: CBAdvertisementDataLocalNameKey) as? NSString

        if device?.contains(BEAN_NAME) == true {
            debugPrint("Found Bean.")

            self.manager.stopScan()
            self.peripheral = peripheral
            self.peripheral.delegate = self

            manager.connect(peripheral, options: nil)
        }
    }

    func centralManager(_ central: CBCentralManager, didConnect peripheral: CBPeripheral) {
        debugPrint("Getting services ...")
        peripheral.discoverServices(nil)
    }

    func peripheral(_ peripheral: CBPeripheral, didDiscoverServices error: Error?) {
        for service in peripheral.services! {
            let thisService = service as CBService

            debugPrint("Service: ", service.uuid)

            if service.uuid == BEAN_SERVICE_UUID {
                debugPrint("Using scratch.")
                peripheral.discoverCharacteristics(nil, for: thisService)
            }
        }
    }

    func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
        debugPrint("Enabling ...")

        for characteristic in service.characteristics! {
            let thisCharacteristic = characteristic as CBCharacteristic

            debugPrint("Characteristic: ", thisCharacteristic.uuid)

            if thisCharacteristic.uuid == BEAN_SCRATCH_UUID {
                debugPrint("Set to notify: ", thisCharacteristic.uuid)

                // Prepare to show data

                self.peripheral.setNotifyValue(true, for: thisCharacteristic)
           }
        }
    }

    func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
        if characteristic.uuid == BEAN_SCRATCH_UUID {
            let content = String(data: characteristic.value!, encoding: String.Encoding.utf8)

            debugPrint("Notified.")
            self.statusLabel.text = "Your Purchase is Registered."

        }
    }

    func centralManager(_ central: CBCentralManager, didDisconnectPeripheral peripheral: CBPeripheral, error: Error?) {
        debugPrint("Disconnected.")

        central.scanForPeripherals(withServices: nil, options: nil)

        // Hide respective user interface
    }

}

line that says : self.statusLabel.text = "Your Purchase is Registered."

is where I get the exception.

and this is errors I get:

    2017-02-06 13:42:19.825869 kevinbletest[2143:695095] [LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x17008b130 UILabel:0x11dd0cbf0'Waiting for a Purchase Co...'.width == 270   (active)>",
    "<NSLayoutConstraint:0x17008b770 UILabel:0x11dd0cbf0'Waiting for a Purchase Co...'.centerX == UIView:0x11dd0c850.centerX   (active)>",
    "<NSLayoutConstraint:0x17008b7c0 UILabel:0x11dd0cbf0'Waiting for a Purchase Co...'.leading == UIView:0x11dd0c850.leadingMargin + 50   (active)>",
    "<NSLayoutConstraint:0x17408c3f0 'UIView-Encapsulated-Layout-Width' UIView:0x11dd0c850.width == 375   (active)>"
)
    Will attempt to recover by breaking constraint 
    <NSLayoutConstraint:0x17008b130 UILabel:0x11dd0cbf0'Waiting for a Purchase Co...'.width == 270   (active)>    
    Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
    The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
    fatal error: unexpectedly found nil while unwrapping an Optional value
    2017-02-06 13:42:25.299886 kevinbletest[2143:695095] fatal error: unexpectedly found nil while unwrapping an Optional value
    (lldb)

"fatal error: unexpectedly found nil while unwrapping an Optional value" this is your issue. — , Feb 06 '17 at 10:25
I think you're looking at the wrong thing. You have an issue with your constraints. But that should only affect your layout and not crash your app. As @NileshPol said above me here, you have an `Optional` value which you are trying to unwrap who's value is `nil`. That will crash the app. My bet is that your `statusLabel` is not connected in your storyboard. — Mihai Fratu, Feb 06 '17 at 10:38
exactly, as you have stated in question that you are getting exception at line (self.statusLabel.text = "yourText") it means the outlet is not connected in storyboard. This is where your app is crashing — , Feb 06 '17 at 10:55

score 2 · Accepted Answer · answered Feb 06 '17 at 10:38

From what I have read the error lies in here:

<NSLayoutConstraint:0x17008b130 UILabel:0x11dd0cbf0'Waiting for a Purchase Co...'.width == 270   (active)>

If you read it down below it seems that NSLayout is trying to center the label with width of 270 and fails since your text is larger than 270.

Further below it says that your app crashes because you label is actually nil.

Try printf(self.myLabel) to see if it is nil.

Your label could be nil if you added it in storyboard and you press the button before the viewDidAppear finishes.

Also try it without any constrains. Just remove the constrains. + Wait until the ViewController loads up (viewDidAppear) and then click.

You app crashes because it is unwrapping an optinal value which is nil, not because of the constrains.

score 0 · Answer 2 · answered Feb 06 '17 at 10:23

Try to set some content to that label in your storyboard to see if that triggers a visual warning. I suspect that you have set constraints that assume the label has a specific width, which is broken when you assign a text string which is different from what you have in your storyboard. To solve that, try changing the constraint value to accommodate dynamic text - e.g. set the trailing constraint to be greater than or equal to your minimum padding (e.g. 8px).

Will attempt to recover by breaking constraint

2 Answers2