how to pass expression as argument in a function in Swift?

Question

I am a junior iOS developer. I want to refactor my code. currently I want to refactor my realm methods. to save or delete data in Realm, I need to use a lot of do try catch block all over the place, so I want to make a service to handle this. I have tried but I don't know if this is the correct way or not

import RealmSwift

class RealmService {

    private init() {}
    static let shared = RealmService()

    var realm = try! Realm()

    func save(expression: ()) {

        do {
            try realm.write {

            // I want to pass the expression in this block here

                expression
            }
        } catch {
            post(error)
        }

    }


}

usage:

func saveToRealmDatabase(wishList: WishList, product: Product) {


    RealmService.shared.save(expression: {
        // I want to pass expression in the block in here as argument
        wishList.products.append(product)
    }())



}

I want to pass the expression wishList.products.append(product) as argument of the function in RealmService class. is there a better way to achieve that ?

Answer 1

Your "expression" can be represented by the type () -> Void - a closure that accepts no parameters and returns nothing.

This is how your method should be declared:

func save(expression: @escaping () -> Void) {

    do {
        try realm.write {
            expression()
        }
    } catch {
        post(error)
    }

}

This is how you should use it:

func saveToRealmDatabase(wishList: WishList, product: Product) {


    RealmService.shared.save(expression: {
        wishList.products.append(product)
    })
}

Answer 2

From the approach you are trying i can guess you want to do something like,

RealmService.shared.save(expression: {
        wishList.products.append(product)
        realm.add(wishList.products)
    })

So based on the understanding that you want to perform the addition operation in that callback, i would rather suggest the below implementation where you don't need to implement the callback for every operation. Rather when you are interested to know the status of the operation then only you would implement the callback. Check the below snippet,

class RealmService {

    private init() {}
    static let shared = RealmService()

    var realm = try! Realm()

    /// Saving a list of object i.e, List<Object>
    func save<O, L: List<O>>(_ list: L, callback: ((Error?) -> Void)? = nil) where O: Object {
        do {
            try realm.write {
                realm.add(list)
                callback?(nil)
            }
        } catch {
            callback?(error)
        }
    }

    /// Saving a single object i.e, `Object`
    func save(_ object: Object, callback: ((Error?) -> Void)? = nil) {
        do {
            try realm.write {
                realm.add(object)
                callback?(nil)
            }
        } catch {
            callback?(error)
        }
    }
}

Usage

class Dog: Object {
   @objc dynamic var name = ""
   @objc dynamic var age = 0
}

let tomy = Dog()
tomy.name = "Tomy"

let johny = Dog()
johny.name = "Johny"

let dogs = List<Dog>()
dogs.append(tomy)
dogs.append(johny)

//When you are not interested in save operation status
RealmService.shared.save(dogs)

//When you want to handle error while saving
RealmService.shared.save(dogs) { [weak self] in self?.showAlert($0) } 

func showAlert(_ error: Error?) {
    guard let e = error else { return }
    showAlert(alertTitle: "Error", alertMessage: e.localizedDescription, actionTitle: "Back")
}