How to upload a file (to REDCap) from an iOS device using swift

Question

I am trying to upload a file saved to a directory within an app (built using ResearchKit) to a server (via REDCap API). REDCap API playground suggests code in several languages, but not Obj-C or Swift. For this case, I need to use Swift, which is a first for me. Below is my attempt at a method, which is unfortunately not working:

func postFileToRedcap (_ fileUrl: URL) {
    guard let url = URL (string: "https://our_redcap_url.edu/api/") else {return}
    let token = "ABCDEFGHIJK123456789" // unique to REDCap project
    let record = "1" // can be set elsewhere
    let event = ""
    let field = "file_upload_field" // name of field in REDCap
    
    if !fileUrl.pathComponents.isEmpty {
        let fileName = URL(fileURLWithPath: fileUrl.absoluteString).lastPathComponent
        var request = URLRequest.init(url: url)
        request.httpMethod = "POST"
        // parameters required by REDCap
        request.addValue(token, forHTTPHeaderField: "token")
        request.addValue(fileName, forHTTPHeaderField: "content")
        request.addValue("import", forHTTPHeaderField: "action")
        request.addValue(record, forHTTPHeaderField: "record")
        request.addValue(field, forHTTPHeaderField: "field")
        request.addValue(event, forHTTPHeaderField: "event")
        request.addValue("json", forHTTPHeaderField: "returnFormat")
        
        let task = URLSession.shared.uploadTask(
            with: request,
            fromFile: fileUrl
        )
        task.resume()
    }
}

The primary problem appears to be the parameter settings, but I can't seem to find a solution that works for REDCap.

All help graciously received. Thanks in advance!

Answer 1

After far too much time, I realised the key was to use

request.addValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")

but it was code snippets from Postman that enabled me to successfully build the parameter array and the payload. Here's a working version that can be simplified, I'm sure.

   import Foundation
     
    guard let url = URL (string: "https://our_redcap_url.edu/api/") else {return}
    let token = "ABCDEFGHIJK123456789" // unique to REDCap project
    let record : = "1" // set elsewhere
    let event = ""
    let field = "file_upload_field" // name of field in REDCap
    let path = "path_of_file_to_be_uploaded"
    let repeat_instance : Int = 1
     
    let parameters = [
        [
        "key": "token",
        "value": token,
        "type": "text"
        ],
        [
        "key": "content",
        "value": "file",
        "type": "text"
        ],
        [
        "key": "action",
        "value": "import",
        "type": "text"
        ],
        [
        "key": "record",
        "value": record,
        "type": "text"
        ],
        [
        "key": "field",
        "value": field_name,
        "type": "text"
        ],
        [
        "key": "event",
        "value": event,
        "type": "text"
        ],
        [
        "key": "repeat_instance",
        "value": repeat_instance,
        "type": "text"
        ],
        [
        "key": "returnFormat",
        "value": "json",
        "type": "text"
        ],
        [
        "key": "file",
        "src": path,
        "type": "file"
        ]] as [[String : Any]]
     
    // build payload
    let boundary = "Boundary-\(UUID().uuidString)"
    var body = ""
    for param in parameters {
    if param["disabled"] == nil {
        let paramName = param["key"]!
        body += "--\(boundary)\r\n"
        body += "Content-Disposition:form-data; name=\"\(paramName)\""
        if param["contentType"] != nil {
            body += "\r\nContent-Type: \(param["contentType"] as! String)"
        }
        let paramType = param["type"] as! String
        if paramType == "text" {
            let paramValue = param["value"] as! String
            body += "\r\n\r\n\(paramValue)\r\n"
        } else {
            let paramSrc = param["src"] as! String
            let fileData = try? NSData(contentsOfFile:paramSrc, options:[]) as Data
            let fileContent = String(data: fileData!, encoding: .utf8)!
            body += "; filename=\"\(paramSrc)\"\r\n"
            + "Content-Type: \"content-type header\"\r\n\r\n\(fileContent)\r\n"
            }
        }
    }
    body += "--\(boundary)--\r\n";
    let postData = body.data(using: .utf8)
     
    // create URL request and session
    var request = URLRequest(url: url,timeoutInterval: Double.infinity)
    request.addValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
     
    request.httpMethod = "POST"
    request.httpBody = postData
     
    let task = URLSession.shared.dataTask(with: request) { data, response, error in
    guard let data = data else {
        print(String(describing: error))
        return
        }
        print(String(data: data, encoding: .utf8)!)
    }
    task.resume()