Making a Variable in View Model Writtable Instead of Get-Only - Swift - ios

What I'm trying to do is add an insert to an empty array variable in my view model through an if else statement. However when I try to add an inset, I get an error saying that my view model array variable is a get-only property.
My question is how can I make my variable in a read and write format so I can add inserts to it? Thanks!
// ViewModel Variable
var selectedTimesOfDay: [String] {
return jobService.selectedTimesOfDay
}
// My Function
#objc func handleContinue() {
guard let jobService = jobService else { return }
let viewModel = JobServiceViewModel(jobService: jobService)
if morningButton.isSelected {
viewModel.selectedTimesOfDay.insert("Morning", at: 0)
}
}

I had to put my view model variable in a setter and getter so it can be writable. Then set an empty array in the initializer so I can add an insert in my controller.
// View Model
var _selectedTimesOfDay: [String]
var selectTimesOfDay: [String] {
set {
_selectedTimesOfDay = newValue
}
get {
return _selectedTimesOfDay
}
}
init(jobService: JobService) {
self.jobService = jobService
self._selectedDate = ""
self._selectedTimesOfDay = []
}

Related

How to call a method once two variables have been set

I am using iOS Swift, and I am trying to understand how to execute a method once the value of two variables have been set up (non-null value) once the requests have finished.
After reading some documentation, I have found out some concepts which are interesting. The first one would be didSet, which works as an observer.
I could call the method using this method by simply using didSet if I would require just one variable
didSet
var myVar: String 0 {
didSet {
print("Hello World.")
}
}
Nevertheless, I also need to wait for the second one myVar2, so it would not work.
I have also found DispatchQueue, which I could use to wait a second before calling the method (the requests that I am using are pretty fast)
DispatchQueue
DispatchQueue.main.asyncAfter(deadline: .now() + 2, execute: {
print("Hello world")
})
but I consider that this solution is not efficient.
Is there anyway to combine these two variables or requests in order to call a method once they have finishing setting the value?
Update
I have tried to replicate David s answer, which I believe is correct but I get the following error on each \.
Type of expression is ambiguous without more context
I copy here my current code
var propertiesSet: [KeyPath<SearchViewController, Car>:Bool] = [\SearchViewController.firstCar:false, \SearchViewController.secondCar:false] {
didSet {
if propertiesSet.allSatisfy({ $0.value }) {
// Conditions passed, execute your custom logic
print("All Set")
} else {
print("Not yet")
}
}
}
var firstCar: Car? {
didSet {
propertiesSet[\SearchViewController.firstCar] = true
}
}
var secondCar: Car? {
didSet {
propertiesSet[\SearchViewController.secondCar] = true
}
}
The variables are set individually, each one on its own request.
You could make your properties optional and check they both have values set before calling your function.
var varA: String? = nil {
didSet {
if varA != nil && varB != nil {
myFunc()
}
}
}
var varB: String? = nil {
didSet {
if varA != nil && varB != nil {
myFunc()
}
}
}
Or you can call your function on each didSet and use a guard condition at the start of your function to check that both of your properties have values, or bail out:
var varA: String? = nil {
didSet {
myFunc()
}
}
var varB: String? = nil {
didSet {
myFunc()
}
}
func myFunc() {
guard varA != nil && varB != nil else { return }
// your code
}
First, you should think very carefully about what your semantics are here. When you say "set," do you mean "assigned a value" or do you mean "assigned a non-nil value?" (I assume you mean the latter in this case.) You should ask yourself, what should happen if your method has already fired, and then another value is set? What if one of the properties has a value is set, then nil is set, then another value set? Should that fire the method 1, 2, or 3 times?
Whenever possible you should work to make these kinds of issues impossible by requiring that the values be set together, in an init rather than mutable properties, for example.
But obviously there are cases where this is necessary (UI is the most common).
If you're targeting iOS 13+, you should explore Combine for these kinds of problems. As one approach:
class Model: ObservableObject {
#Published var first: String?
#Published var second: String?
#Published var ready = false
private var observers: Set<AnyCancellable> = []
init() {
$first.combineLatest($second)
.map { $0 != nil && $1 != nil }
.assign(to: \.ready, on: self)
.store(in: &observers)
}
}
let model = Model()
var observers: Set<AnyCancellable> = []
model.$ready
.sink { if $0 { print("GO!") } }
.store(in: &observers)
model.first = "ready"
model.second = "set"
// prints "GO!"
Another approach is to separate the incidental state that includes optionals, from the actual object you're constructing, which does not.
// Possible parameters for Thing
struct Parameters {
var first: String?
var second: String?
}
// The thing you're actually constructing that requires all the parameters
struct Thing {
let first: String
let second: String
init?(parameters: Parameters) {
guard let first = parameters.first,
let second = parameters.second
else { return nil }
self.first = first
self.second = second
}
}
class TheUIElement {
// Any time the parameters change, try to make a Thing
var parameters: Parameters = Parameters() {
didSet {
thing = Thing(parameters: parameters)
}
}
// If you can make a Thing, then Go!
var thing: Thing? {
didSet {
if thing != nil { print("GO!") }
}
}
}
let element = TheUIElement()
element.parameters.first = "x"
element.parameters.second = "y"
// Prints "GO!"
You need to add a didSet to all variables that need to be set for your condition to pass. Also create a Dictionary containing KeyPaths to your variables that need to be set and a Bool representing whether they have been set already.
Then you can create a didSet on your Dictionary containing the "set-state" of your required variables and when all of their values are true meaning that all of them have been set, execute your code.
This solution scales well to any number of properties due to the use of a Dictionary rather than manually writing conditions like if aSet && bSet && cSet, which can get out of hand very easily.
class AllSet {
var propertiesSet: [KeyPath<AllSet, String>:Bool] = [\.myVar:false, \.myVar2:false] {
didSet {
if propertiesSet.allSatisfy({ $0.value }) {
// Conditions passed, execute your custom logic
print("All Set")
} else {
print("Not yet")
}
}
}
var myVar: String {
didSet {
propertiesSet[\.myVar] = true
}
}
var myVar2: String {
didSet {
propertiesSet[\.myVar2] = true
}
}
init(myVar: String, myVar2: String) {
self.myVar = myVar
self.myVar2 = myVar2
}
}
let all = AllSet(myVar: "1", myVar2: "2")
all.myVar = "2" // prints "Not yet"
all.myVar2 = "1" // prints "All set"

Sending a variable to be modified in another class?

How can one send a variable from a viewcontroller to be modified by another viewcontroller?
I've tried setting the variable in the performSegue, but it does not get modified.
Sample code:
class VC1: ViewController{
var myVar: MyVar
....
prepare(for segue:...) {
let nextVC = segue.destination as! VC2
nextVC.var = myVar
}
....
}
class VC2: ViewController {
var var: MyVar
....
override func viewDidLoad() {
super.viewDidLoad()
var = MyVar("newValue")
}
}
After this code is executed, the value of myVar in VC1 is not changed to the new value. I believe it only gets a shallow copy of myVar, not a deep copy.
Is what I want achievable in swift?
Classes in Swift are pass by reference, whereas structs are pass by value. Assuming that MyVar is a class, you need to modify the properties of the class, ie:
myVar.property = "xyz"
instead of defining a new instance of the class as you have done in your question.
When you set var = MyVar("newValue") it assign new instance to var.
Examine the results from the following code in a playground. It should give you more insight into what you should expect, without the complication of segues and controllers.
class Variable {
var name:String
init(_ nameString:String) {
name = nameString
}
}
class Sender {
var myVar = Variable("Start name")
func showChanges() {
let receiver = Receiver(myVar)
print(myVar.name)
receiver.modify()
print(myVar.name)
receiver.replace()
print(myVar.name)
}
}
class Receiver {
var received: Variable
init(_ variable:Variable) {
received = variable
}
func modify() {
received.name = "Changed name"
}
func replace() {
received = Variable("New variable")
}
}
let s = Sender()
s.showChanges()

How can I make a Sequence to iterate over all the ancestors (superviews) from a UIView up the view hierarchy to the root?

I'd like for UIView to have a property that returns a sequence of all the ancestors of the view up the hierarchy. That would be useful for purposes like finding the nearest one that matches a particular type:
let tableView = cell.ancestors.first(where: { $0 is UITableView })
What's a nice way of implementing that ancestors property?
Using the sequence(first:next:) function, from the Swift Standard Library, an even shorter solution is possible as well:
extension UIView {
var ancestors: AnySequence<UIView> {
return AnySequence<UIView>(
sequence(first: self, next: { $0.superview }).dropFirst())
}
}
You can implement a type that conforms to Sequence and add a property returning it in an extension. A Sequence normally needs a makeIterator() method that returns a type that conforms to IteratorProtocol, but in this case we can make the sequence act as its own iterator and use one type for both, which makes things very simple:
Swift 3:
struct AncestorSequenceIterator: Sequence, IteratorProtocol {
var current: UIView
mutating func next() -> UIView? {
guard let next = current.superview else { return nil }
current = next
return next
}
}
extension UIView {
var ancestors: AncestorSequenceIterator {
return AncestorSequenceIterator(current: self)
}
}
You could create extension and return IteratorProtocol to be able to do first(where:) comparison like so,
extension UIView {
var ancestors: AnyIterator<UIView> {
var current: UIView = self
return AnyIterator<UIView> {
guard let parent = current.superview else {
return nil
}
current = parent
return parent
}
}
}
Since AnyIterator itself conforms to Sequence, the statement that you showed above should work fine.
let tableView = cell.ancestors.first(where: { $0 is UITableView })
Paulo Mattos's implementation is good, but for your specific use, you probably want something like this:
extension UIView {
func nearestAncestor<T: UIView>(ofType type: T.Type) -> T? {
if let me = self as? T { return me }
return superview?.nearestAncestor(ofType: type)
}
}
Then you can use it like this:
guard let tableView = cell.nearestAncestor(ofType: UITableView.self) else { return }
// tableView at this point is type UITableView

Observe values added to Set - Swift

Is it possible to observe values being added to a Set data-structure?
What i'm trying to achieve:
var storedStrings = Set<String>() {
didSet (value) {
// where value is a string that has been added to the Set
}
}
Example:
storedStrings.insert("hello")
didSet called, as a new value has been added.
storedString.insert("world")
didSet called again.
storedString.insert("hello")
didSet not called, as the set already contains the string "hello"
This can be a bit expensive, but you still can do something like:
var storedStrings = Set<String>() {
didSet {
if storedStrings != oldValue {
print("storedStrings has changed")
let added = storedStrings.subtracting(oldValue)
print("added values: \(added)")
let removed = oldValue.subtracting(storedStrings)
print("removed values: \(removed)")
}
}
}
The insert function returns a tuple with the definition: (inserted: Bool, memberAfterInsert: Element).
Therefore the check for a new unique element can be made on insertion instead of using didSet.
var storedStrings = Set<String>()
var insertionResult = storedStrings.insert("Hello")
if insertionResult.inserted {
print("Value inserted") // this is called
}
insertionResult = storedStrings.insert("Hello")
if insertionResult.inserted {
print("Value inserted") // this isn't called
}
You could implement your own inserter for your set, which could emulate the use of a property observer, making use of the fact that the insert method of Set returns a tuple whose first member is false in case the element to be inserted is already present.
func insert(Element)
Inserts the given element in the set if it is not already present.
From the language reference.
E.g.:
struct Foo {
private var storedStrings = Set<String>()
mutating func insertNewStoredString(_ newString: String) {
if storedStrings.insert(newString).0 {
print("Inserted '\(newString)' into storedStrings")
}
}
}
var foo = Foo()
foo.insertNewStoredString("hello") // Inserted 'hello' into storedStrings
foo.insertNewStoredString("hello")
foo.insertNewStoredString("world") // Inserted 'world' into storedStrings

Check if array is nil / or has been loaded in Swift / iOS?

What is the best practice to check if an array of objects has been loaded in Swift?
Say, if I declare an array in a class, and it is lazily loaded. Apple's docs say the array declaration / initialization is something like
var events = [Event]()
I suppose the above means the array is already initialized (ie. not nil).
Then, I need a function like:
func getEvents() -> [Event] {
// check if array is nil, and if so, load the events (not: which could be 0 events)
return events
}
In Java, I would declare something like
ArrayList<Event> events;
public ArrayList<Event> getEvents() {
if(!events) { // null means never been loaded
events = new ArrayList<Event>();
events = loadEvents(); // load the events, which could be zero
}
}
What is the best practice to code the equivalent in Swift?
Lazy Stored Property
In Swift you can declare a lazy stored property: it does exactly what you need.
struct Event { }
class Foo {
lazy var events: [Event] = { self.loadEvents() }()
private func loadEvents() -> [Event] {
print("Loading events")
return [Event(), Event(), Event()]
}
}
We associated a closure to the events property. The closure defines how the events property should be populated and will be executed only when the property is used.
let foo = Foo()
print(foo.events) // now the events property is populated
You may experience an Array in these ways:
var vect = [String]()
if vect.isEmpty {
print ("true")
}else{
print("false")
}
Or
func ceck()->Void{
guard !vect.isEmpty else{
print ("true")
return // return, break, or another logic this is example
}
}

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