Straightforward multipart file add for Swift

I consider that you have already heard in regards to the well-known multipart-data add method that everybody likes to add information and submit kind knowledge, but when not, hopefully this text will aid you somewhat 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 kind specification. To rapidly summarize it first I might wish to inform you a couple of phrases about how the HTTP layer works. In a nutshell, you ship some knowledge with some headers (give it some thought as a key-value consumer data object) to a given URL utilizing a way and as a response you may get again a standing code, some headers and possibly some form of response knowledge too. 🥜

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

The request methodology & URL is fairly easy, the attention-grabbing half is whenever 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 elements and we’ll use the “xxx” boundary string as a separator between the elements. Oh, by the best way every half can have it is personal kind 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 truly ship the precise content material worth.

That is greater than sufficient concept for now, let me snow you ways we are able to implement all of this utilizing Swift 5. To start with, we wish to have the ability to append string values to a Information object, so we’ll prolong Information kind with an ‘append string utilizing encoding’ methodology:

import Basis

public extension Information {

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

Subsequent, we want one thing that may assemble the HTTP multipart physique knowledge, for this function we’ll construct a MultipartRequest object. We will set the boundary once we init this object and we’ll append the elements wanted to assemble the HTTP physique knowledge.

The personal strategies will assist to assemble all the pieces, we merely append string values to the personal knowledge object that holds our knowledge construction. The general public API solely consists of two add capabilities that you need to use to append a key-value primarily based kind subject or a whole file utilizing its knowledge. 👍

public struct MultipartRequest {
    
    public let boundary: String
    
    personal let separator: String = "rn"
    personal var knowledge: Information

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

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

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

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

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

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

The final remaining two public variables are helpers to simply get again the HTTP associated content material kind header worth utilizing the correct boundary and the whole knowledge object that you need to to ship to the server. Here is how one can assemble the HTTP URLRequest utilizing the multipart struct.

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

multipart.add(
    key: "file",
    fileName: "pic.jpg",
    fileMimeType: "picture/png",
    fileData: "fake-image-data".knowledge(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 (knowledge, response) = attempt await URLSession.shared.knowledge(for: request)

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

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