Is there a way to use guard statements more concisely? - swift

Question

I'm using Gloss for my JSON instantiation. Here is a sample class:
public class MyObj: Decodable
{
let id_user : String
let contact_addr1 : String
let contact_addr2 : String?
let contact_city : String
let contact_state : String
let contact_zip : String
let points : Int
// Deserialization
required public init?(json: JSON)
{
guard let id_user : String = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
guard let contact_addr1 : String = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
guard let contact_city : String = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
guard let contact_state : String = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
guard let contact_zip : String = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
guard let points : Int = "somekey" <~~ json else {
assertionFailure("MyObj - invalid JSON. Missing key: wouldbenicetonotwritethisforeachmember")
return nil
}
self.id_user = id_user
self.contact_addr1 = contact_addr1
self.contact_addr2 = "somekey" <~~ json
self.contact_city = contact_city
self.contact_state = contact_state
self.contact_zip = contact_zip
self.contact_points = points
}
}
I have a lot of model classes. Hundreds of members between them. Writing a multi-line guard statement for each one really junks up my code. Is there any way I can encapsulate the guard functionality into something more concise? Maybe a function or something like:
shortGuard("memberName", "jsonKey")
Maybe there is a way to guard against an array of string keys?

Answer 1

There are a huge variety of ways to accomplish this. They all boil down to writing a wrapper function to map your keys to values. Here are a couple quick examples I thought of, but as I say there are many ways to do this depending on what you're after:
enum JSONError: Error {
case keyNotFound(String)
}
extension JSON {
func values<T>(for keys: [String]) throws -> [T] {
var values = [T]()
for key in keys {
guard let value: T = key <~~ self else {
throw JSONError.keyNotFound(key)
}
values.append(value)
}
return values
}
func values<T>(for keys: [String], closure: ((_ key: String, _ value: T) -> Void)) throws {
for key in keys {
guard let value: T = key <~~ self else {
throw JSONError.keyNotFound(key)
}
closure(key, value)
}
}
}
The first validates all keys before you can use any of them and will throw if one isn't present. You'd use it like so:
do {
let keys = ["foo", "bar"]
// The type of the values constant is important.
// In this example we're saying look for values of type Int.
let values: [Int] = try json.values(for: keys)
for (index, key) in keys.enumerated() {
print("value for \(key): \(values[index])")
}
} catch JSONError.keyNotFound(let key) {
assertionFailure("key not found \(key)")
}
The second one will pass back key, value pairs to a closure as they appear in your keys array and will throw at the first one it finds that doesn't exist.
do {
let keys = ["foo", "bar"]
// The type of the closure's value argument is important.
// In this example we're saying look for values of type String.
try json.values(for: keys) { (key, value: String) in
print("value for key \(key) is \(value)")
}
} catch JSONError.keyNotFound(let key) {
assertionFailure("key not found \(key)")
}
Using the first version in an init?() function for your class, we have something like this:
public struct MyObj: Decodable {
public let id_user : String
public let contact_addr1 : String
public let contact_addr2 : String?
public let points : Int
public init?(json: S) {
do {
let stringKeys = ["id_user", "contact_addr1"]
let stringValues: [String] = try json.values(for: stringKeys)
id_user = stringValues[0]
contact_addr1 = stringValues[1]
// this isn't required, so just extract with no error if it fails
contact_addr2 = "contact_addr2" <~~ json
let intKeys = ["points"]
let intValues: [Int] = try json.values(for: intKeys)
points = intValues[0]
} catch JSONError.keyNotFound(let key) {
assertionFailure("key \(key) not found in JSON")
return nil
} catch {
return nil
}
}
}

Answer 2

I have not used Gloss, and it mostly seems to be unnecessary considering that it is simple enough to parse JSON safely without needing an extra library, or using unfamiliar syntax.
Option 1:
You can group the optional unwrapping in a single guard statement.
Example:
public struct MyObj {
let id_user : String
let contact_addr1 : String
let contact_addr2 : String?
let points : Int
public init?(json: Any) {
guard
let entities = json as? [String : Any],
let id_user = entities["some key"] as? String,
let contact_addr1 = entities["some key"] as? String,
let points = entities["some key"] as? Int
else {
assertionFailure("...")
return nil
}
self.id_user = id_user
self.contact_addr1 = contact_addr1
self.contact_addr2 = entities["some key"] as? String
self.contact_points = points
}
}
Option 2:
Another approach would be to eliminate the guard statements altogether, and let the parser throw an error during parsing, and use an optional try to convert the result to nil.
Example:
// Helper object for parsing values from a dictionary.
// A similar pattern could be used for arrays. i.e. array.stringAt(10)
struct JSONDictionary {
let values: [String : Any]
init(_ json: Any) throws {
guard let values = json as? [String : Any] else {
throw MyError.expectedDictionary
}
self.values = values
}
func string(_ key: String) throws -> String {
guard let value = values[key] as? String else {
throw MyError.expectedString(key)
}
return value
}
func integer(_ key: String) throws -> Int {
guard let value = values[key] as? Int else {
throw MyError.expectedInteger(key)
}
return value
}
}
Parser:
public struct MyObj {
let id_user : String
let contact_addr1 : String
let contact_addr2 : String?
let points : Int
public init(json: Any) throws {
// Instantiate the helper object.
// Ideally the JSONDictionary would be passed by the caller.
let dictionary = try JSONDictionary(json),
self.id_user = try dictionary.string("some key"),
self.contact_addr1 = try dictionary.string("some key"),
self.points = try dictionary.integer("some key")
// Results in an optional if the string call throws an exception
self.contact_addr2 = try? dictionary.string("some key")
}
}
Usage:
// Instantiate MyObj from myJSON.
// myObject will be nil if parsing fails.
let myObject = try? MyObj(json: myJSON)