在struct上更新一个可编码可识别的值，并且只有userdefault缓存这个值

Question

我有两个可编码的结构体:Student和地址(链接到Student)

在我的应用程序上，我从Firebase RTDB获取数据，然后我用userdefault存储它假设一个学生更改了他的电子邮件，我只想用更新后的电子邮件更新存储的userdefault。

当我想要存储它时，我需要指定所有其他数据(姓名，地址，工作地点…)或者我可以只更新/存储我的userdefault上的电子邮件而不指定其他数据吗?

struct Adress : Codable, Identifiable {
var id: String { Student_UID }
var Student_UID: String
var Student_Street: String
var Student_Country: String
var Student_City: String

enum CodingKeys: String, CodingKey  {
case Student_UID = "Adress_Student_UID"
case Student_Street = "Student_Street"
case Student_Country = "Student_Country"
case Student_City = "Student_City"

}

}
struct Student_Profile_Data:Codable, Identifiable   {
var id: String { Student_UID }

var Student_UID: String
var Student_firstName: String?
var Student_username : String?
var Student_Email : String?
var Student_lastName: String
var Student_Address: Adress?
var Student_DOB: String?
var Student_Studentpoints : Int?


enum CodingKeys: String, CodingKey {
case Student_UID = "Student_UID"
case Student_firstName = "Student_firstName"
case Student_username = "Student_username"
case Student_Email = "Student_Email"
case Student_lastName = "Student_lastName"
case Student_Address = "Student_Address"
case Student_DOB = "Student_DOB"
case Student_Studentpoints = "Student_Studentpoints"



}


}

The USERDEfault part:


//READ
NSLog("userdefault TEST READ")
let defaults = UserDefaults.standard
if let savedStudent = defaults.object(forKey: "SavedStudent") as? Data {
let decoder = JSONDecoder()
if let loadedStudent = try? decoder.decode(Student_Profile_Data.self, from: savedStudent) {
NSLog("TEST PROFILE- username : (loadedStudent.Student_username)")
}}

//WRITE
let add1 = Adress(Student_UID: "", Student_Street: "", Student_Country: "", Student_City: "")
let stud = Student_Profile_Data(Student_firstName: "", Student_username: "Martin", Student_Email: "", Student_lastName: "", Student_Address: add1, Student_DOB: "", Student_Studentpoints: 0, Student_UID: "")
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(stud) {
let defaults = UserDefaults.standard
defaults.set(encoded, forKey: "SavedStudent")
NSLog("WRITE OK »)
}

//UPDATE ONE (OR TWO) VALUES ?

Answer 1

如果您以这种方式在UserDefault中存储student，那么更新它将涉及读取student，更新其值，然后将其写回的三步过程。

// read
let defaults = UserDefaults.standard
let decoder = JSONDecoder()
var savedStudent = defaults.data(forKey: "SavedStudent").flatMap {
try? decoder.decode(StudentProfileData.self, from: $0)
} ?? StudentProfileData(...) // some "empty" student to use when there is no previously saved student
// update
savedStudent.firstName = "John"
// write
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(savedStudent) {
defaults.set(encoded, forKey: "SavedStudent")
}

为了更方便，您可以将其提取为函数:

func updateSavedStudent(updateBlock: (inout StudentProfileData) -> Void) {
// read
let defaults = UserDefaults.standard
let decoder = JSONDecoder()
var savedStudent = defaults.data(forKey: "SavedStudent").flatMap {
try? decoder.decode(StudentProfileData.self, from: $0)
} ?? StudentProfileData(...) // some "empty" student to use when there is no previously saved student
// update
updateBlock(&savedStudent)
// write
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(savedStudent) {
defaults.set(encoded, forKey: "SavedStudent")
}
}
// usage:
updateSavedStudent {
$0.firstName = "John"
$0.lastName = "Smith"
}

或者，为这个保存的学生创建一个computed属性，并将其放在某个实用程序类中。请注意，对于您更新的每个属性，这将对学生进行一次编码和解码。

static var savedStudent: StudentProfileData {
get {
let defaults = UserDefaults.standard
let decoder = JSONDecoder()
return defaults.data(forKey: "SavedStudent").flatMap {
try? decoder.decode(StudentProfileData.self, from: $0)
} ?? StudentProfileData(...) // some "empty" student to use when there is no previously saved student
}
set {
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(newValue) {
defaults.set(encoded, forKey: "SavedStudent")
}
}
}
// usage
savedStudent.firstName = "John"