App Crashing with error: generic parameter 'T' could not be inferred

问题

I'm trying to get custom object which is hashable from UserDefault. My custom model is defined below:

class WorkerProfileResponse: Mappable, Hashable{

    static func == (lhs: WorkerProfileResponse, rhs: WorkerProfileResponse) -> Bool {
        return lhs.id == rhs.id
    } 
    var hashValue: Int{
        return self.id!
    }
    var id, loginStatus, lastLogin, lastActive: Int?
    var username, email, mobileNumber: String?
    var userCategories: [String]?
    var userSubCategories: [String]?
    var biometricToken: String?
    var accessToken: AccessToken?
    var userStatus: UserStatus?
    var userProfile: UserProfile?

    required init(map: Map) {        
    }

    func mapping(map: Map) {
        id <- map["id"]
        loginStatus <- map["is_logged_in"]
        lastLogin <- map["last_login"]
        lastActive <- map["last_active"]
        biometricToken <- map["biometricToken"]
        username <- map["username"]
        email <- map["email"]
        mobileNumber <- map["mobile_number"]
        accessToken <- map["accessToken"]
        userStatus <- map["userStatus"]
        userCategories <- map["userCategories"]
        userSubCategories <- map["userSubCategories"]
        userProfile <- map["userProfile"]
    }
 }

My userdefault method is:

class func getModel<T: Hashable>(key: String) -> T {
        let decoded  = UserDefaults.standard.data(forKey: key)
        let decodedModel = NSKeyedUnarchiver.unarchiveObject(with: decoded!) as! T
        return decodedModel
    }

And I'm calling it like this:

UserDefault.getModel(key: "workerProfile")

App is crashing when I'm calling this method I don't understand the reason, error is:

error: generic parameter 'T' could not be inferred

回答1:

I'm answering my own question, if it helps anyone in the future. It was crashing while decoding because there was no value present in userdefault.

This line had the issue because of force casting:

let decodedModel = NSKeyedUnarchiver.unarchiveObject(with: decoded!) as! T

I've changes this method:

class func getModel<T: Hashable>(key: String) -> T {
        let decoded  = UserDefaults.standard.data(forKey: key)
        let decodedModel = NSKeyedUnarchiver.unarchiveObject(with: decoded!) as! T
        return decodedModel
    }

To this:

class func getModel<T: Hashable>(key: String) -> T? {
        let decoded  = UserDefaults.standard.data(forKey: key)
        if decoded != nil{
            let decodedModel = NSKeyedUnarchiver.unarchiveObject(with: decoded!) as! T
            return decodedModel
        }
        else
        {
            return nil
        }
    }

来源：https://stackoverflow.com/questions/61100434/app-crashing-with-error-generic-parameter-t-could-not-be-inferred

标签

swift

generics

userdefaults

hashable

equatable