Straightforward multipart file add for Swift

I consider that you’ve got already heard in regards to the well-known multipart-data add method that everybody likes to add recordsdata and submit kind information, but when not, hopefully this text will allow you to a bit bit to grasp this stuff higher.

Let’s begin with some concept. Don’t fret, it is only one hyperlink, in regards to the multipart/form-data content material sort specification. To rapidly summarize it first I might wish to let you know a number of phrases about how the HTTP layer works. In a nutshell, you ship some information with some headers (give it some thought as a key-value consumer information object) to a given URL utilizing a technique and as a response you will get again a standing code, some headers and possibly some kind of response information too. 🥜

• HTTP request = Methodology + URL + Headers + Physique (request information)
• HTTP response = Standing code + Headers + Physique (response information)

The request technique & URL is fairly simple, the fascinating half is while you specify the Content material-Kind HTTP header, in our case the multipart/form-data;boundary="xxx" worth means, that we’ll ship a request physique utilizing a number of components and we’ll use the “xxx” boundary string as a separator between the components. Oh, by the way in which every half can have it is personal sort and title, we’ll use the Content material-Disposition: form-data; title="field1" line to let the server find out about these fields, earlier than we really ship the precise content material worth.

That is greater than sufficient concept for now, let me snow you the way we are able to implement all of this utilizing Swift 5. Initially, we want to have the ability to append string values to a Information object, so we’ll lengthen Information sort with an ‘append string utilizing encoding’ technique:

import Basis

public extension Information {

    mutating func append(
        _ string: String,
        encoding: String.Encoding = .utf8
    ) {
        guard let information = string.information(utilizing: encoding) else {
            return
        }
        append(information)
    }
}

Subsequent, we want one thing that may assemble the HTTP multipart physique information, for this goal we’ll construct a MultipartRequest object. We are able to set the boundary once we init this object and we’ll append the components wanted to assemble the HTTP physique information.

The non-public strategies will assist to assemble all the things, we merely append string values to the non-public information object that holds our information construction. The general public API solely consists of two add features that you need to use to append a key-value based mostly kind discipline or a complete file utilizing its information. 👍

public struct MultipartRequest {
    
    public let boundary: String
    
    non-public let separator: String = "rn"
    non-public var information: Information

    public init(boundary: String = UUID().uuidString) {
        self.boundary = boundary
        self.information = .init()
    }
    
    non-public mutating func appendBoundarySeparator() {
        information.append("--(boundary)(separator)")
    }
    
    non-public mutating func appendSeparator() {
        information.append(separator)
    }

    non-public func disposition(_ key: String) -> String {
        "Content material-Disposition: form-data; title="(key)""
    }

    public mutating func add(
        key: String,
        worth: String
    ) {
        appendBoundarySeparator()
        information.append(disposition(key) + separator)
        appendSeparator()
        information.append(worth + separator)
    }

    public mutating func add(
        key: String,
        fileName: String,
        fileMimeType: String,
        fileData: Information
    ) {
        appendBoundarySeparator()
        information.append(disposition(key) + "; filename="(fileName)"" + separator)
        information.append("Content material-Kind: (fileMimeType)" + separator + separator)
        information.append(fileData)
        appendSeparator()
    }

    public var httpContentTypeHeadeValue: String {
        "multipart/form-data; boundary=(boundary)"
    }

    public var httpBody: Information {
        var bodyData = information
        bodyData.append("--(boundary)--")
        return bodyData
    }
}

The final remaining two public variables are helpers to simply get again the HTTP associated content material sort header worth utilizing the right boundary and the entire information object that it’s best to to ship to the server. Here is how one can assemble the HTTP URLRequest utilizing the multipart struct.

var multipart = MultipartRequest()
for discipline in [
    "firstName": "John",
    "lastName": "Doe"
] {
    multipart.add(key: discipline.key, worth: discipline.worth)
}

multipart.add(
    key: "file",
    fileName: "pic.jpg",
    fileMimeType: "picture/png",
    fileData: "fake-image-data".information(utilizing: .utf8)!
)


let url = URL(string: "https://httpbin.org/publish")!
var request = URLRequest(url: url)
request.httpMethod = "POST"
request.setValue(multipart.httpContentTypeHeadeValue, forHTTPHeaderField: "Content material-Kind")
request.httpBody = multipart.httpBody


let (information, response) = strive await URLSession.shared.information(for: request)

print((response as! HTTPURLResponse).statusCode)
print(String(information: information, encoding: .utf8)!)

As you’ll be able to see it is comparatively simple, you simply add the shape fields and the recordsdata that you just wish to add, and get again the HTTP associated values utilizing the helper API. I hope this text will allow you to to simulate kind submissions utilizing multipart requests with out trouble. 😊