SortedJSONCoder is a library for encoding/serialization objects to JSON preserving some sorting rules. Written in Objective-C.

Credits

95% of code is third-party dependencies:

• JSON encoder/decoder https://github.com/TouchCode/TouchJSON
• An ordered dictionary https://github.com/Marxon13/M13OrderedDictionary

Support

• Swift
• Objective-C

Installation

Cocoapods

pod 'SortedJSONCoder', :git => 'https://github.com/mikehouse/SortedJSONCoder.git', :commit => 'cc868564'

No way make support of SPM and Carthage as used third-party libraries do not support these.

Interface

Objective-C

@interface SortedJSONEncoder : NSObject

- (instancetype)init;

- (NSData *)encode:(NSDictionary *)jsonObject
           options:(NSStringCompareOptions)options
             error:(NSError **)error;

- (NSData *)encode:(NSDictionary *)jsonObject
        comparator:(NSComparisonResult (^)(NSString *, NSString *))comparator
             error:(NSError **)error;

@end

Swift

class SortedJSONEncoder {

    init()

    func encode(_ jsonObject: [AnyHashable : Any], 
                     options: NSString.CompareOptions = []) throws -> Data

    func encode(_ jsonObject: [AnyHashable : Any], 
                  comparator: @escaping (String, String) -> ComparisonResult) throws -> Data
}

Usage

Objective-C

#import <SortedJSONCoder/SortedJSONEncoder.h>

- (void)writeJSON:(NSDictionary *)object toPath:(NSURL *)path {
    SortedJSONEncoder *encoder = [SortedJSONEncoder new];
    NSData *data = [encoder encode:object options:NSCaseInsensitiveSearch error:nil];
    [data writeToURL:path atomically:YES];
}

Localized case insensitive order:

#import <SortedJSONCoder/SortedJSONEncoder.h>

- (void)writeJSON2:(NSDictionary *)object toPath:(NSURL *)path {
    SortedJSONEncoder *encoder = [SortedJSONEncoder new];
    NSData *data = [encoder encode:object comparator:^NSComparisonResult(NSString *s1, NSString *s2) {
            return [s1 localizedCaseInsensitiveCompare:s2];
        } error:nil];
    [data writeToURL:path atomically:YES];
}

Swift

import SortedJSONCoder

let encoder = SortedJSONEncoder()
let dict: [String:Any] = ["a":1];
let data = try encoder.encode(dict, options: .literal)
try data.write(to: URL(string: "/path/to/out.json")!)

Custom order logic:

import SortedJSONCoder

let encoder = SortedJSONEncoder()
let dict: [String:Any] = ["a":1];
let data = try encoder.encode(dict, comparator: { (s1: String, s2: String) -> ComparisonResult in
    if s1 == s2 { return .orderedSame }
    else if s1 < s2 { return .orderedAscending }
    else { return .orderedDescending }
})
try data.write(to: URL(string: "/path/to/out.json")!)

Pretty Printed

let encoder = SortedJSONEncoder()
encoder.pretty = true

Why ?

Say we have a json file from server side:

{
    "payload": {
        "prop1_key": "prop1_value",
        "prop2_key": "prop2_value"
    },
    "sha256": "bd575a7f4028fe0b195682193c99eaa734e62def4790280394807068727a4539"
}

where sha256 is SHA256 hash for payload's json data, that is

NSData *jsonData = [[NSData alloc] initWithContentsOfFile:@"/path/to/server.json"];
NSDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:nil];
NSDictionary *payload = jsonObject[@"payload"];
NSData *payloadData = [NSJSONSerialization dataWithJSONObject:payload options:NSJSONWritingSortedKeys error:nil];

NSString *expectedSHA256 = jsonObject[@"sha256"];
NSString *actualSHA256 = [[payloadData sha256] hex];

XCTAssertEqual(expectedSHA256, actualSHA256);

As you can see this hash check may fail when backend and iOS use different algorithms for keys sorting for JSON serialization, moreover on iOS standard SDK no way use different/custom sorting algorithm as only one available is system one. And here this library can help, where you can provide any you'd like sorting algorithm for JSON encoding.

Swift version of this logic:

struct Root: Codable {
    let payload: Payload
    let sha256: String
    struct Payload: Codable {
        let prop1_key: String
        let prop2_key: String
    }
}

let rootJSONData: Data = try Data(contentsOf: URL(string: "/path/to/server.json")!)
let rootObject: Root = try JSONDecoder().decode(Root.self, from: rootJSONData)
let payloadJSONData = try JSONEncoder().encode(rootObject.payload)
let payloadJSONObject = try JSONSerialization.jsonObject(with: payloadJSONData, options: []) as! [AnyHashable : Any]

let sortedPayloadData = try SortedJSONEncoder().encode(payloadJSONObject, options: .caseInsensitive)
let sha256: String = sortedPayloadData.sha256.hex()

assert(sha256 == rootObject.sha256)

Tests

Base cases covered.

TODO

• Pretty json formatting
• Support custom indents for json elements

License

The code of this library itself - MIT
https://github.com/TouchCode/TouchJSON - FreeBSD License
https://github.com/Marxon13/M13OrderedDictionary - MIT