Variable listener in iOS Swift - swift

Question

I am trying to make an independent class/library that runs on its own and handles errors internally, but also gives feedback (status) to the frontend for possible user interaction. In Android I'd solve this with a Listener:
class Status(errCode:Int=0, msg:String=""){
var errCode:Int=0
var text:String=""
private var listener: ChangeListener? = null
init {
this.errCode=errCode
this.text=msg
}
fun set(errCode:Int, msg:String) {
this.errCode=errCode
this.text=msg
if (listener != null) listener!!.onChange()
}
fun getListener(): ChangeListener? {
return listener
}
fun setListener(listener: ChangeListener?) {
this.listener = listener
}
interface ChangeListener {
fun onChange()
}
}
Whenever I want to update it, e.g. on Exception, I call:
catch (e: IOException) {
this.status.set(109,"someException: $e")
}
And in the MainActivity I just have to handle these changes:
myObj.status.setListener(object : Status.ChangeListener {
override fun onChange() {
when(myObj!!.status.errCode) {
//errors
109 -> log(myObj!!.status.text) //some Exception
111 -> myObj!!.restart() //another exc
112 -> myObj!!.checkAll() //some other error
...
In Swift I found didSet that seems similar. I read you can attach it to a struct so whenever something in the struct changes, it is called. So I added my "struct Status" to my "struct myObj" like this:
struct myObj {
var status:Status=Status(errCode: 0, msg: "Init")
struct Status {
var errCode:Int
var msg:String
mutating func set(err:Int, txt:String){
errCode=err
msg=txt
}
}
and in my code I initialize a new instance of myObj
var mObj:myObj=myObj.init() {
didSet{
switch myObj.status.errCode{
case 1:
promptOkay()
case 113:
obj.errorHandling()
default:
log(txt: mObj.status.msg)
}
}
}
However, it never trigger didSet, even though internal functions should change the Status. I read that didSet is not triggered on init, but I do not run anything right now after initializing the class object that should run quite independently. I want to verify that the approach is okay before I go further and have to unravel everything again.

Answer 1

didSet must be declared on the property, not during initialization:
class MyObj {
var status: Status = Status(errCode: 0, msg: "Init") {
didSet {
// status did change
print("new error code: \(status.errCode)")
}
}
struct Status {
var errCode:Int
var msg:String
mutating func set(err:Int, txt:String){
errCode = err
msg = txt
}
}
}
let obj = MyObj()
obj.status.set(err: 10, txt: "error 10") // prints "new error code: 10"
At this point you can react to every changes made to obj.status in the didSetClosure.
Edit — React to didSet from outside the class
If you want to respond to changes from outside the MyObj, I would recommend using a closure:
class MyObj {
var status: Status = Status(errCode: 0, msg: "Init") {
didSet {
statusDidChange?(status)
}
}
// closure to call when status changed
var statusDidChange: ((Status) -> Void)? = nil
struct Status {
var errCode:Int
var msg:String
mutating func set(err:Int, txt:String){
errCode = err
msg = txt
}
}
}
This way, you can assign a closure from outside to perform custom actions:
let obj = MyObj()
obj.statusDidChange = { status in
// status did change
print("new error code: \(status.errCode)")
}
obj.status.set(err: 10, txt: "error 10") // prints "new error code: 10"
Edit 2 — Call didSet closure directly from initialization
You also can manually call the statusDidChange closure during the init.
class MyObj {
var status: Status = Status(errCode: 0, msg: "Init") {
didSet {
statusDidChange(status)
}
}
// closure to call when status changed
var statusDidChange: (Status) -> Void
init(status: Status, statusDidChange: #escaping (Status) -> Void) {
self.status = status
self.statusDidChange = statusDidChange
self.statusDidChange(status)
}
}
let obj = MyObj(status: MyObj.Status(errCode: 9, msg: "error 09")) { status in
// status did change
print("new error code: \(status.errCode)")
}
obj.status.set(err: 10, txt: "error 10")
This will print
new error code: 9
new error code: 10