Converting local Realms to synced Realms: 'customSchema' is inaccessible

There is no public customSchema property, you can always refer to the docs (best way to figure out what is intended for public use):

https://realm.io/docs/objc/2.10.1/api/Classes/RLMRealmConfiguration.html

With no better option, I decided to use Objective-C in my Swift project. So, I added a SWIFT_OBJC_BRIDGING_HEADER (Xcode does that automatically when you add an Objective-C file) and created a RealmConverter object:

RealmConverter.h

#import <Foundation/Foundation.h>

@import Realm;

NS_ASSUME_NONNULL_BEGIN

@interface RealmConverter : NSObject

- (void)convertLocalToSyncRealm:(NSURL *)server local:(NSURL *)local username:(NSString *)username password:(NSString *)password completion:(void (^)(NSError * _Nullable))completion;

@end

NS_ASSUME_NONNULL_END

RealmConverter.m

#import "RealmConverter.h"

@import Realm.Dynamic;
@import Realm.Private;

@implementation RealmConverter

- (void)convertLocalToSyncRealm:(NSURL *)server local:(NSURL *)local username:(NSString *)username password:(NSString *)password completion:(void (^)(NSError * _Nullable))completion {
    RLMRealmConfiguration *configuration = [[RLMRealmConfiguration alloc] init];
    configuration.fileURL = local;
    configuration.dynamic = true;
    configuration.readOnly = YES;

    RLMRealm *localRealm = [RLMRealm realmWithConfiguration:configuration error:nil];

    RLMSyncCredentials *credentials = [RLMSyncCredentials credentialsWithUsername:username password:password register:YES];
    [RLMSyncUser logInWithCredentials:credentials authServerURL:server onCompletion:^(RLMSyncUser *syncUser, NSError *error) {
        if (error) {
            completion(error);
            return;
        }

        dispatch_async(dispatch_get_main_queue(), ^{
            RLMRealmConfiguration *syncConfig = [[RLMRealmConfiguration alloc] init];
            syncConfig.syncConfiguration = [[RLMSyncConfiguration alloc] initWithUser:syncUser realmURL:[NSURL URLWithString:[NSString stringWithFormat:@"realm://%@:%@/~/<redacted>", server.host, server.port]]];
            syncConfig.customSchema = [localRealm.schema copy];

            RLMRealm *syncRealm = [RLMRealm realmWithConfiguration:syncConfig error:nil];
            syncRealm.schema = syncConfig.customSchema;

            NSError *error = nil;
            [syncRealm transactionWithBlock:^{
                NSArray *objectSchema = syncConfig.customSchema.objectSchema;
                for (RLMObjectSchema *schema in objectSchema) {
                    RLMResults *allObjects = [localRealm allObjects:schema.className];
                    for (RLMObject *object in allObjects) {
                        RLMCreateObjectInRealmWithValue(syncRealm, schema.className, object, true);
                    }
                }
                completion(nil);
            } error:&error];

            if (error) {
                completion(error);
            }
        });
    }];
}

@end

Then add #import "RealmConverter.h" to your bridging header and then use it in your Swift code like:

RealmConverter().convertLocal(toSyncRealm: URL(string: "http://localhost:9080")!, local: Realm.Configuration.defaultConfiguration.fileURL!, username: "user@example.com", password: "12345678") { error in
    print("Done:", error ?? "nil")
}

Have found a port of the function in Swift here: https://github.com/realm/realm-cocoa/issues/538

import Realm
import Realm.Dynamic
import RealmSwift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    let sourceFilePath = Bundle.main.url(forResource: "fieldFlow", withExtension: "realm")
    let configuration = RLMRealmConfiguration()
    configuration.fileURL = sourceFilePath
    configuration.dynamic = true
    configuration.readOnly = true

    let localRealm = try! RLMRealm(configuration: configuration)

    let creds = SyncCredentials.usernamePassword(username: "admin@realm.io", password: "password")
    SyncUser.logIn(with: creds, server: URL(string: "http://localhost:9080")!) { (syncUser, error) in
        DispatchQueue.main.async {
            if let syncUser = syncUser {
                self.copyToSyncRealmWithRealm(realm: localRealm, user: syncUser)
            }
        }
    }
}

func copyToSyncRealmWithRealm(realm: RLMRealm, user: RLMSyncUser) {
    let syncConfig = RLMRealmConfiguration()
    syncConfig.syncConfiguration = RLMSyncConfiguration(user: user, realmURL: URL(string: "realm://localhost:9080/~/fieldRow")!)
    syncConfig.customSchema = realm.schema

    let syncRealm = try! RLMRealm(configuration: syncConfig)
    syncRealm.schema = syncConfig.customSchema!
    try! syncRealm.transaction {
        let objectSchema = syncConfig.customSchema!.objectSchema
        for schema in objectSchema {
            let allObjects = realm.allObjects(schema.className)
            for i in 0..<allObjects.count {
                let object = allObjects[i]
                RLMCreateObjectInRealmWithValue(syncRealm, schema.className, object, true)
            }
        }
    }
}