In Swift, I am trying to downloaded JSON data from a URL, then use it to create & populate a Swift object.
I do not know the character encoding of the returned JSON.
I have seen some uses of JSONDecoder#decode<T>(_ type: T.Type, from data: Data)
.
How does the above function know what character encoding to use?
The Data
that I've seen used is from an argument to the completionHandler
argument from the following function from URLSession
:
func dataTask(
with request: URLRequest,
completionHandler: @escaping @Sendable (Data?, URLResponse?, (any Error)?) -> Void
) -> URLSessionDataTask
I don't see any character encoding info in the Data
class, but I do see it in URLResponse#textEncodingName
.
Does Data
somehow know what its character encoding is? Or does JSONDecoder#decode<T>(…)
try to autodetect the character encoding? Or?
If Data
doesn't contain the character encoding info itself, how can I use it from URLResponse#textEncodingName
in some function to create & populate a new object?
Also, if the input is is not proper JSON, I want to output the non-JSON text. From what I've seen, I'm supposed to use String(data:encoding:)
to convert from Data
to String?
. I currently do not have the URLResponse
available to me where I need to call this (I just have the Data
) due to existing code from someone else.
If the Data
has the character encoding, is there a function I can call that just takes the Data
without requiring the String.Encoding
argument? Or is there some way to read the character encoding from the Data
so I can use it to provide the String.Encoding
argument?
If I can avoid directly using the URLResponse
, it will save me from needing to refactor everything to make the URLResponse
available in certain places.
Data
does not know what encoding it is encoded with. Data
is just a collection of bytes. It doesn't even necessarily represent text at all.
Though JSONDecoder
does not document what encodings it supports, JSONSerialization.jsonObject(with:options:)
does.
The data must be in one of the 5 supported encodings listed in the JSON specification: UTF-8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE. The data may or may not have a BOM.
As far as I know, JSONDeocder
uses the JSONSerialization
APIs under the hood, in which case it would support the same encodings too. From a quick check, this is at least true for the JSONDecoder
implementation in swift-foundation. Here is how swift-foundation autodetect the encoding.
So if you can assume that the encoding will be one of those listed above, you can just use JSONDecoder
as usual.
Otherwise, you would have to read the textEncodingName
from the URLResponse
(or autodetect it), decode the text, and re-encode it into data using a supported encoding.
You also mentioned that you want to get the response data as plain text if it is not valid JSON. In that case, you must get the encoding via textEncodingName
(or autodetect it), as well.