Network request and decoding errors in Swift

This is a listing of raw data returned from different cases of network requests and model decoding in Swift, with emphasis on error cases. Useful for the purpose of mocking error responses.

The Star Wars API is used as reference.

Summary table

Case	Data	Response	Network error	Decoding
1) Successful	Some	200	nil	Ok
2) Non-existing host	nil	nil	`NSURLErrorDomain` (-1003)	N/A
3) Non-existing resource	Some	404	nil	`dataCorrupted`
4) Non-existing query	Some	200(a)	nil	`typeMismatch` (a)
5) Corrupted data	Some	200	nil	`dataCorrupted`
6) Model mismatch	Some	200	nil	`keyNotFound`
7) Offline mode	nil	nil	`NSURLErrorDomain` (-1009)	N/A

(a) Some APIs return 404 when a non-existing query is used, giving keyNotFound as error when decoding. The sample StarWars API returns a valid response, just ignoring the non-existing query.

Sample output of playground follows.

Exercise 1: Successful request

Data: :

Optional("{\"people\":\"https://swapi.co/api/people/\",\"planets\":\"https://swapi.co/api/planets/\",\"films\":\"https://swapi.co/api/films/\",\"species\":\"https://swapi.co/api/species/\",\"vehicles\":\"https://swapi.co/api/vehicles/\",\"starships\":\"https://swapi.co/api/starships/\"}")

Response:

<NSHTTPURLResponse: 0x6000000338c0> { URL: https://swapi.co/api/ } { Status Code: 200, Headers {
    "Content-Encoding" =     (
        gzip
    );
    "Content-Type" =     (
        "application/json"
    );
    Date =     (
        "Wed, 16 May 2018 12:09:45 GMT"
    );
    Etag =     (
        "W/\"c1df070b0509ebefe72e85d721f40bf0\""
    );
    Server =     (
        cloudflare
    );
    Vary =     (
        "Accept, Cookie"
    );
    Via =     (
        "1.1 vegur"
    );
    allow =     (
        "GET, HEAD, OPTIONS"
    );
    "cf-ray" =     (
        "41bdbcbb7f7263df-FRA"
    );
    "expect-ct" =     (
        "max-age=604800, report-uri=\"https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct\""
    );
    "x-frame-options" =     (
        SAMEORIGIN
    );
} }

Network error: none

Decoded model:

API(films: https://swapi.co/api/films/, people: https://swapi.co/api/people/, planets: https://swapi.co/api/planets/, species: https://swapi.co/api/species/, starships: https://swapi.co/api/starships/, vehicles: https://swapi.co/api/vehicles/)

Exercise 2: non-existing host

Data: none

Response: none

Network Error :

Error Domain=NSURLErrorDomain Code=-1003 "A server with the specified hostname could not be found." UserInfo={NSUnderlyingError=0x600001055d80 {Error Domain=kCFErrorDomainCFNetwork Code=-1003 "(null)" UserInfo={_kCFStreamErrorCodeKey=8, _kCFStreamErrorDomainKey=12}}, NSErrorFailingURLStringKey=https://nonexisting.co/api/, NSErrorFailingURLKey=https://nonexisting.co/api/, _kCFStreamErrorDomainKey=12, _kCFStreamErrorCodeKey=8, NSLocalizedDescription=A server with the specified hostname could not be found.}

Exercise 3: non-existing resource

Data:

Optional("<!DOCTYPE html>\n<html lang=\"en\">\n\n<head>\n<meta charset=\"utf-8\">\n<title>\n      SWAPI - The Star Wars API\n    </title>\n<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\">\n<link rel=\"stylesheet\" href=\"//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css\">\n<link href=\"/static/css/bootstrap.css\" rel=\"stylesheet\">\n<link href=\"/static/css/custom.css\" rel=\"stylesheet\">\n<link rel=\"shortcut icon\" href=\"/static/favicon.ico\">\n</style>\n<script src=\"//code.jquery.com/jquery-2.1.0.min.js\" type=\"12b2974bbe4f5b3be4eb1eb3-\"></script>\n<script src=\"//maxcdn.bootstrapcdn.com/bootstrap/3.3.0/js/bootstrap.min.js\" type=\"12b2974bbe4f5b3be4eb1eb3-\"></script>\n</script>\n</head>\n<body>\n<nav class=\"navbar navbar-default\" role=\"navigation\">\n<div class=\"container-fluid\">\n<div class=\"navbar-header\">\n<button

(abridged for brevity) Response:

<NSHTTPURLResponse: 0x60400002eec0> { URL: https://swapi.co/api/chewbacca/ } { Status Code: 404, Headers {
    "Content-Encoding" =     (
        gzip
    );
    "Content-Type" =     (
        "text/html; charset=utf-8"
    );
    Date =     (
        "Wed, 16 May 2018 12:54:30 GMT"
    );
    Server =     (
        cloudflare
    );
    Via =     (
        "1.1 vegur"
    );
    "cf-ray" =     (
        "41bdfe484ca263df-FRA"
    );
    "expect-ct" =     (
        "max-age=604800, report-uri=\"https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct\""
    );
    "x-frame-options" =     (
        SAMEORIGIN
    );
} }

Network error: none

Decoding error:

dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.})))

Exercise 4: non-existing query

Data:

Optional("{\"count\":7,\"next\":null,\"previous\":null,\"results\":[{\"title\":\"A New Hope\",\"episode_id\":4,\"opening_crawl\":\"It is a period of civil war.\\r\\nRebel spaceships, striking\\r\\nfrom a hidden base, have won\\r\\ntheir first victory against\\r\\nthe evil Galactic Empire.\\r\\n\\r\\nDuring the battle, Rebel\\r\\nspies managed to steal secret\\r\\nplans to the Empire\'s\\r\\nultimate weapon, the DEATH\\r\\nSTAR, an armored space\\r\\nstation with enough power\\r\\nto destroy an entire planet.\\r\\n\\r\\nPursued by the Empire\'s\\r\\nsinister agents, Princess\\r\\nLeia races home aboard her\\r\\nstarship, custodian of the\\r\

(abridged for brevity)

Response:

<NSHTTPURLResponse: 0x6000002237a0> { URL: https://swapi.co/api/films/?nonexisting=query } { Status Code: 200, Headers {
    "Content-Encoding" =     (
        gzip
    );
    "Content-Type" =     (
        "application/json"
    );
    Date =     (
        "Thu, 17 May 2018 09:37:31 GMT"
    );
    Etag =     (
        "W/\"a6f57b83572279727b3a9208d0653a82\""
    );
    Server =     (
        cloudflare
    );
    Vary =     (
        "Accept, Cookie"
    );
    Via =     (
        "1.1 vegur"
    );
    allow =     (
        "GET, HEAD, OPTIONS"
    );
    "cf-ray" =     (
        "41c51b159f046403-FRA"
    );
    "expect-ct" =     (
        "max-age=604800, report-uri=\"https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct\""
    );
    "x-frame-options" =     (
        SAMEORIGIN
    );
} }

Network error: none

Decoding error:

typeMismatch(Swift.Double, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "results", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys(stringValue: "releaseDate", intValue: nil)], debugDescription: "Expected to decode Double but found a string/data instead.", underlyingError: nil))

Exercise 5: corrupted data (non-conforming JSON)

Data: (overriden by playground for sample of corrupted data)

Response:

<NSHTTPURLResponse: 0x600000026fc0> { URL: https://swapi.co/api/ } { Status Code: 200, Headers {
    "Content-Encoding" =     (
        gzip
    );
    "Content-Type" =     (
        "application/json"
    );
    Date =     (
        "Wed, 16 May 2018 12:54:34 GMT"
    );
    Etag =     (
        "W/\"c1df070b0509ebefe72e85d721f40bf0\""
    );
    Server =     (
        cloudflare
    );
    Vary =     (
        "Accept, Cookie"
    );
    Via =     (
        "1.1 vegur"
    );
    allow =     (
        "GET, HEAD, OPTIONS"
    );
    "cf-ray" =     (
        "41bdfe5c684a6415-FRA"
    );
    "expect-ct" =     (
        "max-age=604800, report-uri=\"https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct\""
    );
    "x-frame-options" =     (
        SAMEORIGIN
    );
} }

Network error: none

Decoding error:

dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.})))

Exercise 6: model mismatch / wrong decoding key

Data:

Optional("{\"people\":\"https://swapi.co/api/people/\",\"planets\":\"https://swapi.co/api/planets/\",\"films\":\"https://swapi.co/api/films/\",\"species\":\"https://swapi.co/api/species/\",\"vehicles\":\"https://swapi.co/api/vehicles/\",\"starships\":\"https://swapi.co/api/starships/\"}")

Response:

<NSHTTPURLResponse: 0x60400002e3e0> { URL: https://swapi.co/api/ } { Status Code: 200, Headers {
    "Content-Encoding" =     (
        gzip
    );
    "Content-Type" =     (
        "application/json"
    );
    Date =     (
        "Wed, 16 May 2018 12:54:35 GMT"
    );
    Etag =     (
        "W/\"c1df070b0509ebefe72e85d721f40bf0\""
    );
    Server =     (
        cloudflare
    );
    Vary =     (
        "Accept, Cookie"
    );
    Via =     (
        "1.1 vegur"
    );
    allow =     (
        "GET, HEAD, OPTIONS"
    );
    "cf-ray" =     (
        "41bdfe63fc5863af-FRA"
    );
    "expect-ct" =     (
        "max-age=604800, report-uri=\"https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct\""
    );
    "x-frame-options" =     (
        SAMEORIGIN
    );
} }

Network error: none

Decoding error:

keyNotFound(CodingKeys(stringValue: "count", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"count\", intValue: nil) (\"count\").", underlyingError: nil))

Exercise 7: request in offline mode (disable host connection in playground to get it)

Data: none

Response: none

Network Error :

Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." UserInfo={NSUnderlyingError=0x60400005fbf0 {Error Domain=kCFErrorDomainCFNetwork Code=-1009 "(null)" UserInfo={_kCFStreamErrorCodeKey=50, _kCFStreamErrorDomainKey=1}}, NSErrorFailingURLStringKey=https://swapi.co/api/, NSErrorFailingURLKey=https://swapi.co/api/, _kCFStreamErrorDomainKey=1, _kCFStreamErrorCodeKey=50, NSLocalizedDescription=The Internet connection appears to be offline.}

atineoSE/Network-request-and-decoding-errors-in-Swift

Network request and decoding errors in Swift

Summary table

Exercise 1: Successful request

Exercise 2: non-existing host

Exercise 3: non-existing resource

Exercise 4: non-existing query

Exercise 5: corrupted data (non-conforming JSON)

Exercise 6: model mismatch / wrong decoding key

Exercise 7: request in offline mode (disable host connection in playground to get it)