HomeKit crashes reason: '-[__NSDate length]: but Bool sent

Question

tl;dr - The code in the question is correct. Reason for crash at another party.

The challenge is to control an Elgato Eve Energy HomeKit enabled outlet. Up til where the code starts, everything is working OK, so the correct room, accessory and so on is selected. The code does turn off the outlet, but right afterwards the app crashes.

guard let services = accessory?.services else {
    print("No Service")
    return
}
for service in services {
    if service.serviceType == HMServiceTypeOutlet {
        for characteristic in service.characteristics {
            print(characteristic.characteristicType)
            if characteristic.characteristicType == HMCharacteristicTypePowerState {
                print(characteristic.metadata!)
                dump(characteristic)
                // Turning off the outlet
                characteristic.writeValue(false, completionHandler: { (error: Error?) -> Void in
                    if error == nil {
                        print("Yep")
                    } else {
                        print("Nop")
                    }
                })
            }
        }
    }
}

Log

00000023-0000-1000-8000-0026BB765291
00000025-0000-1000-8000-0026BB765291
[%@ Format: bool, Manufacturer Description: Power State ]
- <HMCharacteristic: 0x1702c23e0> #0
  - super: NSObject
00000026-0000-1000-8000-0026BB765291
E863F10A-079E-48FF-8F27-9C2605A29F52
E863F126-079E-48FF-8F27-9C2605A29F52
E863F10D-079E-48FF-8F27-9C2605A29F52
E863F10C-079E-48FF-8F27-9C2605A29F52
2017-02-07 23:19:59.725340 POS[499:100178] -[__NSDate length]: unrecognized selector sent to instance 0x170007710
2017-02-07 23:19:59.725767 POS[499:100178] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDate length]: unrecognized selector sent to instance 0x170007710'
*** First throw call stack:
(0x188a291b8 0x18746055c 0x188a30268 0x188a2d270 0x18892680c 0x1895be458 0x1895be350 0x1895be698 0x1006328f8 0x1a249ac8c 0x199674f34 0x19966c588 0x1a249d194 0x101951258 0x101951218 0x10195eaec 0x101954ce0 0x10195f088 0x101960e2c 0x101960b78 0x187abb2a0 0x187abad8c)
libc++abi.dylib: terminating with uncaught exception of type NSException

Sym

2017-02-08 10:17:47.804825 POS[602:171994] -[__NSDate length]: unrecognized selector sent to instance 0x1740169c0

iZettlePayments`-[NSDictionary(Fractionized) dateForKey:]:
0x100973d88 <+0>: stp x22, x21, [sp, #-48]!
0x100973d8c <+4>: stp x20, x19, [sp, #16]
0x100973d90 <+8>: stp x29, x30, [sp, #32]
0x100973d94 <+12>: add x29, sp, #32 ; =32
0x100973d98 <+16>: mov x19, x0
0x100973d9c <+20>: nop
0x100973da0 <+24>: ldr x20, #861056 ; (void *)0x00000001ad8966b8: NSDateFormatter
0x100973da4 <+28>: nop
0x100973da8 <+32>: ldr x21, #851248 ; "izDateFormatterUsingISO8601"
0x100973dac <+36>: mov x0, x2
0x100973db0 <+40>: bl 0x1009e9b54 ; symbol stub for: objc_retain
0x100973db4 <+44>: mov x22, x0
0x100973db8 <+48>: mov x0, x20
0x100973dbc <+52>: mov x1, x21
0x100973dc0 <+56>: bl 0x1009e9b30 ; symbol stub for: objc_msgSend
0x100973dc4 <+60>: mov x29, x29
0x100973dc8 <+64>: bl 0x1009e9b78 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x100973dcc <+68>: mov x20, x0
0x100973dd0 <+72>: nop
0x100973dd4 <+76>: ldr x1, #849044 ; "valueForKey:"
0x100973dd8 <+80>: mov x0, x19
0x100973ddc <+84>: mov x2, x22
0x100973de0 <+88>: bl 0x1009e9b30 ; symbol stub for: objc_msgSend
0x100973de4 <+92>: mov x19, x0
0x100973de8 <+96>: mov x0, x22
0x100973dec <+100>: bl 0x1009e9b48 ; symbol stub for: objc_release
0x100973df0 <+104>: mov x0, x19
0x100973df4 <+108>: bl 0x1009e9b78 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x100973df8 <+112>: mov x19, x0
0x100973dfc <+116>: nop
0x100973e00 <+120>: ldr x1, #851168 ; "dateFromString:"
0x100973e04 <+124>: mov x0, x20
0x100973e08 <+128>: mov x2, x19
0x100973e0c <+132>: bl 0x1009e9b30 ; symbol stub for: objc_msgSend
0x100973e10 <+136>: mov x29, x29
0x100973e14 <+140>: bl 0x1009e9b78 ; symbol stub for: objc_retainAutoreleasedReturnValue
0x100973e18 <+144>: mov x21, x0
0x100973e1c <+148>: mov x0, x19
0x100973e20 <+152>: bl 0x1009e9b48 ; symbol stub for: objc_release
0x100973e24 <+156>: mov x0, x20
0x100973e28 <+160>: bl 0x1009e9b48 ; symbol stub for: objc_release
0x100973e2c <+164>: mov x0, x21
0x100973e30 <+168>: ldp x29, x30, [sp, #32]
0x100973e34 <+172>: ldp x20, x19, [sp, #16]
0x100973e38 <+176>: ldp x22, x21, [sp], #48
0x100973e3c <+180>: b 0x1009e9ac4 ; symbol stub for: objc_autoreleaseReturnValue

Browsing the Elgato Eve services and characteristics, shows the first characteristic in the array is the one holding the name, and the second one is the one I need to manipulate. The printed metadata shows its a bool, and confirms its the Power State, and its both readable and writable, as described in the table linked to before. Since I write a false to the correct characteristic, the outlet turns off, but a couple of seconds later, the app crashes. I have removed any observers and delegate methods that could interfere. This really baffles me. Further, the code does not crash on iOS 9, just iOS 10.2, as well as the 10.3 betas

Solution

Sorry guys, after a whole lot of swearing, a bruised toe, and a lot less hair, and 24 hours later, I found the culprit. Im using iZettle, a payment device, and as soon as the libraries was added to the project, it crashed. The guys over at iZettle admitted they had an extension on NSDictionary

@interface NSDictionary (Fractionized)
    - (NSDate *)dateForKey:(id)aKey;
@end

This one collides with an extension in HomeKit, so thats the reason for the problem. The result is they have to add a prefix to their extension and compile the libraries again.

Answer 1

While a symbolicated stack trace would help, there is one symbol in your logs that may provide a useful hint: something is trying to ask an NSDate for length. In my experience, this happens when a library expects something to be an NSString and it's about to parse it or print it or what have you.

Perhaps you know where NSDates are being used. It will also help to know if the crash is on the same thread that is executing your above code. Given that you are experiencing a slight delay, it's hard to say if it is related to that code at all! Perhaps elsewhere in your app, you are responding to the device itself giving you a status update, and maybe printing a value that you think is a string but is really a date?