Swift Casting doesn't work as Expected

Question

I have created a custom protocol,which i am planning to use in place of Any

But i doesn't work when i try to cast it from JSONSerialization.jsonObject

Here is my custom protocol

public protocol StringOrNumber {}

extension String:StringOrNumber {}
extension NSNumber:StringOrNumber {}
extension Bool:StringOrNumber {}
extension Float:StringOrNumber {}
extension CGFloat:StringOrNumber {}
extension Int32:StringOrNumber {}
extension Int64:StringOrNumber {}
extension Int:StringOrNumber {}
extension Double:StringOrNumber {}
extension Dictionary:StringOrNumber {}
extension Array:StringOrNumber {}
extension NSDictionary:StringOrNumber {}
extension NSArray:StringOrNumber {}
extension NSString:StringOrNumber {}
extension NSNull:StringOrNumber {}

Here is the code which i expect to work but it doesn't

let json = try JSONSerialization.jsonObject(with: data, options: [])
           if let object = json as? [String: StringOrNumber] {
               // json is a dictionary
               print(object)
           }

However if i try to convert this in 2 steps, It works as below

let json = try JSONSerialization.jsonObject(with: data, options: [])
if let object = json as? [String: Any] {
    // json is a dictionary
    print(object)

    if let newObject:[String:StringOrNumber] = object as? [String:StringOrNumber] {
        // json is a newer dictionary
        print(newObject)
    }


}

Here is the sample JSON i am reading from file.( Doesn't matter you can try your own too )

{
     "firstName": "John",
}

I don't understand why the first piece of code doesn't works and 2nd one does ...

Thanks

Answer 1

it is not Swift (language) specific, it is Apple specific ...

even worst if you change data to

let d = ["first":"john", "last":"doe", "test int": 0, "test null": NSNull()] as [String:Any]

linux version works as expected,

["test null": <NSNull: 0x0000000000825560>, "last": "doe", "first": "john", "test int": 0]
["test null": <NSNull: 0x0000000000825560>, "last": "doe", "first": "john", "test int": 0]
["test int": 0, "last": "doe", "first": "john", "test null": <NSNull: 0x0000000000825560>]

but apple prints

[:]
[:]
{
    first = john;
    last = doe;
    "test int" = 0;
    "test null" = "<null>";
}

it looks very strange. next code snippet explain why

import Foundation

public protocol P {}

extension String:P {}
extension Int:P {}
extension NSNull:P {}

let d = ["first":"john", "last":"doe", "test null": NSNull(), "test int": 10] as [String:Any]
print("A)",d, type(of: d))

let d1 = d as? [String:P] ?? [:]
print("B)",d1, type(of: d1))
print()

if let data = try? JSONSerialization.data(withJSONObject: d, options: []) {

    if let jobject = try? JSONSerialization.jsonObject(with: data, options: []) {

        let o = jobject as? [String:Any] ?? [:]
        print("1)",o, type(of: o))

        var o2 = o as? [String:P] ?? [:]
        print("2)",o2, type(of: o2), "is it empty?: \(o2.isEmpty)")
        print()

        if o2.isEmpty {
            o.forEach({ (t) in
                let v = t.value as? P
                print("-",t.value, type(of: t.value),"as? P", v as Any)
                o2[t.key] = t.value as? P ?? 0
            })
        }
        print()
        print("3)",o2)
    }
}

on apple it prints

A) ["test null": <null>, "test int": 10, "first": "john", "last": "doe"] Dictionary<String, Any>
B) ["test null": <null>, "test int": 10, "first": "john", "last": "doe"] Dictionary<String, P>

1) ["test null": <null>, "test int": 10, "first": john, "last": doe] Dictionary<String, Any>
2) [:] Dictionary<String, P> is it empty?: true

- <null> NSNull as? P Optional(<null>)
- 10 __NSCFNumber as? P nil
- john NSTaggedPointerString as? P nil
- doe NSTaggedPointerString as? P nil

3) ["test null": <null>, "test int": 0, "first": 0, "last": 0]

while on linux it prints

A) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019d8c40>] Dictionary<String, Any>
B) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019d8c40>] Dictionary<String, P>

1) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>] Dictionary<String, Any>
2) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>] Dictionary<String, P> is it empty?: false

3) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>]

finally , I used the slightly modified source code of JSONSerialization from open source distribution (to avoid conflict with apple Foundation I rename the class to _JSONSerialization :-) and change the code such a way it works in my Playground without any warnings and errors and ... it prints the expected results :)

Why it works now? The key is

/* A class for converting JSON to Foundation/Swift objects and converting Foundation/Swift objects to JSON.    An object that may be converted to JSON must have the following properties:
 - Top level object is a `Swift.Array` or `Swift.Dictionary`
 - All objects are `Swift.String`, `Foundation.NSNumber`, `Swift.Array`, `Swift.Dictionary`,  or `Foundation.NSNull`
 - All dictionary keys are `Swift.String`s
 - `NSNumber`s are not NaN or infinity  */

Now the conditional downcasting of all possible values to P works as expected

to be honest, try this snippet on linux :-) and on apple.

let d3 = [1.0, 1.0E+20]
if let data = try? JSONSerialization.data(withJSONObject: d3, options: []) {
    if let jobject = try? JSONSerialization.jsonObject(with: data, options: []) as? [Double] ?? [] {
        print(jobject)
    }
}

apple prints

[1.0, 1e+20]

while linux

[]

and with really big value will crash. this bug goes from (in the open sourced JSONSerialization)

if doubleResult == doubleResult.rounded() {
        return (Int(doubleResult), doubleDistance)
}

replace it with

if doubleResult == doubleResult.rounded() {
         if doubleResult < Double(Int.max) && doubleResult > Double(Int.min) {
                 return (Int(doubleResult), doubleDistance)
         }
 }

and 'deserialization' works as expected (serialization has other bugs ...)

Answer 2

If you want to just check the protocol conformance:

Use is instead of as

if json is [String : StringOrNumber] {

    print("valid")
}
else {
    print("invalid")
}

If you want to use the converted type:

if let object = (json as? [String: Any]) as? [String : StringOrNumber] {

    print("valid object = \(object)")
}
else {
    print("invalid")
}