Swift: Accessing JSON received by post request - ios

Question

let task = URLSession.shared.dataTask(with: postRequest) { data, response, error in
guard let data = data, error == nil else {
print(error?.localizedDescription ?? "NoData")
return
}
let jsonString = String(data: data, encoding: String.Encoding.utf8)!
self.submitView.text = jsonString
}
task.resume()
I'm trying to access the return value of this post request. I set a breakpoint and saw that jsonString does get the correct value. However, which I try to set self.submitView.text to anything, I get this error:
2017-06-21 14:20:16.062815-0400 Button[958:197756] This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.
Stack:(
0 CoreFoundation 0x0000000191c82ff8 <redacted> + 148
1 libobjc.A.dylib 0x00000001906e4538 objc_exception_throw + 56
2 CoreFoundation 0x0000000191c82f28 <redacted> + 0
3 Foundation 0x0000000192874338 <redacted> + 128
4 Foundation 0x00000001926bc9f8 <redacted> + 36
5 UIKit 0x0000000197daf76c <redacted> + 816
6 UIKit 0x0000000197dbb668 <redacted> + 1740
7 UIKit 0x0000000197dbaf84 <redacted> + 828
8 UIKit 0x0000000197df90f4 <redacted> + 256
9 UIKit 0x0000000197df1818 <redacted> + 116
10 UIKit 0x00000001987ec308 <redacted> + 48
11 UIKit 0x0000000197e5d880 <redacted> + 280
12 UIFoundation 0x0000000197cdb238 <redacted> + 7012
13 UIFoundation 0x0000000197cdc0c8 <redacted> + 52
14 UIFoundation 0x0000000197d0eb70 <redacted> + 48
15 UIKit 0x0000000197e5d85c <redacted> + 244
16 UIFoundation 0x0000000197ce0214 <redacted> + 2172
17 UIFoundation 0x0000000197d0b058 <redacted> + 240
18 UIFoundation 0x0000000197d2f474 <redacted> + 160
19 UIFoundation 0x0000000197d2ebb4 <redacted> + 92
20 UIKit 0x0000000197e60500 <redacted> + 252
21 UIKit 0x0000000197e603e4 <redacted> + 196
22 Button 0x0000000100061e24 _TFFC6Button14ViewController12submitTappedFT_T_U_FTGSqV10Foundation4Data_GSqCSo11URLResponse_GSqPs5Error___T_ + 784
23 Button 0x0000000100062468 _TTRXFo_oGSqV10Foundation4Data_oGSqCSo11URLResponse_oGSqPs5Error____XFdCb_dGSqCSo6NSData_dGSqS1__dGSqCSo7NSError___ + 224
24 CFNetwork 0x000000019228c34c <redacted> + 32
25 CFNetwork 0x00000001922a4048 <redacted> + 148
26 Foundation 0x0000000192751814 <redacted> + 16
27 Foundation 0x0000000192696770 <redacted> + 96
28 Foundation 0x0000000192686b28 <redacted> + 612
29 Foundation 0x0000000192753bb0 <redacted> + 228
30 libdispatch.dylib 0x0000000100a6da10 _dispatch_client_callout + 16
31 libdispatch.dylib 0x0000000100a7b2e8 _dispatch_queue_serial_drain + 1140
32 libdispatch.dylib 0x0000000100a71634 _dispatch_queue_invoke + 852
33 libdispatch.dylib 0x0000000100a7d630 _dispatch_root_queue_drain + 552
34 libdispatch.dylib 0x0000000100a7d39c _dispatch_worker_thread3 + 140
35 libsystem_pthread.dylib 0x0000000190d43100 _pthread_wqthread + 1096
36 libsystem_pthread.dylib 0x0000000190d42cac start_wqthread + 4
)
2017-06-21 14:20:16.085280-0400 Button[958:197756] This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread. This can lead to engine corruption and weird crashes.
Stack:(
0 CoreFoundation 0x0000000191c82ff8 <redacted> + 148
1 libobjc.A.dylib 0x00000001906e4538 objc_exception_throw + 56
2 CoreFoundation 0x0000000191c82f28 <redacted> + 0
3 Foundation 0x0000000192874338 <redacted> + 128
4 Foundation 0x00000001926bc9f8 <redacted> + 36
5 UIKit 0x0000000197eb18e0 <redacted> + 564
6 UIKit 0x0000000197db0138 <redacted> + 224
7 UIKit 0x0000000197daf8d0 <redacted> + 120
8 Foundation 0x00000001926bca7c <redacted> + 168
9 UIKit 0x0000000197daf76c <redacted> + 816
10 UIKit 0x0000000197dbb668 <redacted> + 1740
11 UIKit 0x0000000197dbaf84 <redacted> + 828
12 UIKit 0x0000000197df90f4 <redacted> + 256
13 UIKit 0x0000000197df1818 <redacted> + 116
14 UIKit 0x00000001987ec308 <redacted> + 48
15 UIKit 0x0000000197e5d880 <redacted> + 280
16 UIFoundation 0x0000000197cdb238 <redacted> + 7012
17 UIFoundation 0x0000000197cdc0c8 <redacted> + 52
18 UIFoundation 0x0000000197d0eb70 <redacted> + 48
19 UIKit 0x0000000197e5d85c <redacted> + 244
20 UIFoundation 0x0000000197ce0214 <redacted> + 2172
21 UIFoundation 0x0000000197d0b058 <redacted> + 240
22 UIFoundation 0x0000000197d2f474 <redacted> + 160
23 UIFoundation 0x0000000197d2ebb4 <redacted> + 92
24 UIKit 0x0000000197e60500 <redacted> + 252
25 UIKit 0x0000000197e603e4 <redacted> + 196
26 Button 0x0000000100061e24 _TFFC6Button14ViewController12submitTappedFT_T_U_FTGSqV10Foundation4Data_GSqCSo11URLResponse_GSqPs5Error___T_ + 784
27 Button 0x0000000100062468 _TTRXFo_oGSqV10Foundation4Data_oGSqCSo11URLResponse_oGSqPs5Error____XFdCb_dGSqCSo6NSData_dGSqS1__dGSqCSo7NSError___ + 224
28 CFNetwork 0x000000019228c34c <redacted> + 32
29 CFNetwork 0x00000001922a4048 <redacted> + 148
30 Foundation 0x0000000192751814 <redacted> + 16
31 Foundation 0x0000000192696770 <redacted> + 96
32 Foundation 0x0000000192686b28 <redacted> + 612
33 Foundation 0x0000000192753bb0 <redacted> + 228
34 libdispatch.dylib 0x0000000100a6da10 _dispatch_client_callout + 16
35 libdispatch.dylib 0x0000000100a7b2e8 _dispatch_queue_serial_drain + 1140
36 libdispatch.dylib 0x0000000100a71634 _dispatch_queue_invoke + 852
37 libdispatch.dylib 0x0000000100a7d630 _dispatch_root_queue_drain + 552
38 libdispatch.dylib 0x0000000100a7d39c _dispatch_worker_thread3 + 140
39 libsystem_pthread.dylib 0x0000000190d43100 _pthread_wqthread + 1096
40 libsystem_pthread.dylib 0x0000000190d42cac start_wqthread + 4
)
2017-06-21 14:20:16.088842-0400 Button[958:197756] *** Assertion failure in void _UIPerformResizeOfTextViewForTextContainer(NSLayoutManager *, UIView<NSTextContainerView> *, NSTextContainer *, NSUInteger)(), /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIFoundation/UIFoundation-491.7/UIFoundation/TextSystem/NSLayoutManager_Private.m:1577
2017-06-21 14:20:16.089727-0400 Button[958:197756] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Only run on the main thread!'
*** First throw call stack:
(0x191c82fe0 0x1906e4538 0x191c82eb4 0x19271a78c 0x197cd93f8 0x197cd90d8 0x197d07e88 0x197d0b188 0x197d2f474 0x197d2ebb4 0x197e60500 0x197e603e4 0x100061e24 0x100062468 0x19228c34c 0x1922a4048 0x192751814 0x192696770 0x192686b28 0x192753bb0 0x100a6da10 0x100a7b2e8 0x100a71634 0x100a7d630 0x100a7d39c 0x190d43100 0x190d42cac)
libc++abi.dylib: terminating with uncaught exception of type NSException
It seems like I can't assign self.submitView.text to anything inside the scope of "task". Any ideas why?
Edit:
Here is what the server returns upon sending a request.
"{\"raw\":\"schedule for John in New York on Monday\",\"location\":\"New York\",\"date\":\"Mon, June 26\",\"name\":\"john\",\"errorResponse\":null,\"stripTest\":\"for John in new york on Monday\"}"
This is what my code looks like:
do {
let info = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String : Any]
DispatchQueue.main.async {
self.submitView.text = info?["name"] as! String
}
} catch {
DispatchQueue.main.async {
self.submitView.text = "ERROR"
}
}
"ERROR" is no longer printing, but "info" seems to have 0 key/value pairs, thus nothing prints.
UPDATE:
print(try? JSONSerialization.jsonObject(with: data, options: [])) prints "nil"
The latter prints:
Optional("\"{\\\"raw\\\":\\\"Schedule for John in New York on Monday\\\",\\\"location\\\":\\\"new york\\\",\\\"date\\\":\\\"6/26/2017 12:00:00 AM\\\",\\\"name\\\":\\\"john\\\",\\\"errorResponse\\\":null,\\\"stripTest\\\":\\\" for john in new york on monday \\\"}\"")
This isn't what prints when I assign it to a text view in the UI.. is it converting the escape sequences to literal characters?

Answer 1

You are setting the text property of the UITextField on a background thread.
You need to ensure that all UI work is done on the main thread, You can do this using DispatchQueue...
DispatchQueue.main.async {
self.submitView.text = jsonString
}
Update:
The data is literally the data that was sent over the network, you will need to convert and deserialise it.
You can do this using the JSON Serialisation class like so:
// I create some dummy JSON string for example purposes
let data = "{ \"name\": \"John Smith\"}".data(using: String.Encoding.utf8)!
if let jsonObject = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String:AnyObject] {
print(jsonObject["name"] as! String)
}
This tries to convert the data into a JSON object, which in Swift is usually cast to a dictionary object.
There are loads of resources on networking with Swift available. One I highly recommend would be iOS Networking with Swift this would take a fair bit of time to go through and learn so here is something that is alot quicker to get through Working with JSON in Swift
UPDATE
This code works for me:
let data = "{\"raw\":\"schedule for John in New York on Monday\",\"location\":\"New York\",\"date\":\"Mon, June 26\",\"name\":\"john\",\"errorResponse\":null,\"stripTest\":\"for John in new york on Monday\"}".data(using: String.Encoding.utf8)!
if let jsonObject = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String:AnyObject] {
print(jsonObject)
print(jsonObject["name"] as! String)
}