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@JaviSoto
Created January 4, 2022 17:07
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AsyncJSONDecoder Swift Playground
import Foundation
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
let json = """
[
{
"a": 1,
"b": {"c": false, "d": "foo"}
},
{
"a": 2,
"b": {"c": true, "d": "[{bar}]"}
},
]
"""
let jsonData = json.data(using: .utf8)!
struct Model: Codable {
struct Inner: Codable {
var c: Bool
var d: String
}
var a: Int
var b: Inner
}
let value = try! JSONDecoder().decode([Model].self, from: jsonData)
print("Serial parsing:")
print(value)
enum JSONStructureTokenizer {
struct Token: Equatable, CustomStringConvertible {
enum Kind: Character, Equatable {
case openArray = "["
case closeArray = "]"
case openDictionary = "{"
case closeDictionary = "}"
case stringDelimiter = "\""
}
let kind: Kind
let index: String.UnicodeScalarView.Index
var description: String {
return "\"\(kind.rawValue)\" at \(index)"
}
}
static func tokenize(json: String) -> [Token] {
var tokens: [Token] = []
var insideString = false
for scalarIndex in json.unicodeScalars.indices {
let scalar = json.unicodeScalars[scalarIndex]
if let tokenKind = Token.Kind(rawValue: Character(scalar)) {
switch tokenKind {
// Note: this is incredibly naive and doesn't handle escaped quotes inside strings
case .stringDelimiter:
insideString.toggle()
default:
if !insideString {
tokens.append(.init(kind: tokenKind, index: scalarIndex))
}
}
}
}
return tokens
}
}
struct AsyncJSONDecoder<T: Decodable>: AsyncSequence {
typealias Element = T
private let json: String
init(json: String) {
self.json = json
}
func makeAsyncIterator() -> AsyncJSONIterator {
return AsyncJSONIterator(json: json)
}
struct AsyncJSONIterator: AsyncIteratorProtocol {
// TODO: Add better metadata to errors
enum Error: Swift.Error {
case invalidArray
case invalidDictionary
}
private var lastTokenIndex: Int?
private let json: String
private let tokens: [JSONStructureTokenizer.Token]
init(json: String) {
self.json = json
self.tokens = JSONStructureTokenizer.tokenize(json: json)
}
private mutating func consumeNextDictionary() throws -> (openDictionaryTokenIndex: Int, closeDictionaryTokenIndex: Int)? {
var dictionaryNestLevel = 0
var openTokenIndex: Int?
var closeTokenIndex: Int?
let remainingTokens = tokens.enumerated().dropFirst((lastTokenIndex ?? 0) + 1).dropLast()
for (index, token) in remainingTokens {
switch token.kind {
case .openDictionary:
if dictionaryNestLevel == 0 && openTokenIndex == nil {
openTokenIndex = index
} else {
dictionaryNestLevel += 1
}
case .closeDictionary:
if dictionaryNestLevel == 0 {
closeTokenIndex = index
break
} else {
dictionaryNestLevel -= 1
}
default:
// Note: This is also very naive, and will fail in different cases of malformed JSON
break
}
}
guard let openTokenIndex = openTokenIndex else {
// No more dictionaries
return nil
}
guard let closeTokenIndex = closeTokenIndex else {
throw Error.invalidDictionary
}
lastTokenIndex = closeTokenIndex
return (openDictionaryTokenIndex: openTokenIndex, closeDictionaryTokenIndex: closeTokenIndex)
}
private let jsonDecoder = JSONDecoder()
mutating func next() async throws -> T? {
if lastTokenIndex == nil {
guard !tokens.isEmpty else { return nil }
guard tokens.first?.kind == .openArray
&& tokens.last?.kind == .closeArray else {
throw Error.invalidArray
}
}
if let nextDictionary = try consumeNextDictionary() {
let range = tokens[nextDictionary.openDictionaryTokenIndex].index...tokens[nextDictionary.closeDictionaryTokenIndex].index
let subJSON = json[range].data(using: .utf8)!
let nextValue = try jsonDecoder.decode(T.self, from: subJSON)
return nextValue
} else {
return nil
}
}
}
}
let asyncDecoder = AsyncJSONDecoder<Model>(json: json)
async {
do {
for try await value in asyncDecoder {
print(value)
}
print("No more values")
}
catch {
print("Error: \(error)")
}
}
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