I have a class called HTTPHelper which is responsible for doing all my back end API requests. All methods in this class are class methods. As you know instance properties cannot be used from within a class method. I have some properties that need initialization with a do{}catch{} block which currently are being initiated from within an init(){} like so:
class HTTPHelper{
init(){
do {
//initiate property
} catch{}
}
}
My question is, is there a mechanism similar to an init(){} that would initiated a static property for a singleton?
You can assign a property to the result of a closure or function (note the = and () at the end to execute the closure—this is not the same as a computed property, where the value is re-created every time). Instead, the first time you access the property the value is lazily computed once using your function/closure and then stored in the property for future access.
class MyClass {
static let myProperty: String = {
do {
return try blah()
} catch {
// error handling
}
}()
}
Of course, this is just a special case of assigning the result of a function to a property:
class MyClass {
static let myProperty: String = MyClass.createMyString()
static func createMyString() -> String {
do {
return try blah()
} catch {
// error handling
}
}
}
Related
I come from Java world. Now I am programming in Swift 4.
I would like to implement abstract class in Swift, I know in Swift there is no such concept of abstract class. But I know we could mimic this concept in Swift by using protocol. For example this is what I tried:
// With protocol, I can define functions that concrete class have to implement
protocol ProductProvider {
func getProductNumber() -> Int
}
// with protocol extension, I can define shared (computed) properties and functions among concrete classes that comply with this protocol
extension ProductProvider {
var maxProductCount: Int {
return 10
}
}
But now, I would like to have a shared variable that could be set & get ("shared" means to be shared with classes that comply with this protocol):
extension ProductProvider {
var maxProductCount: Int {
set(newValue) {
// set to what if I couldn't define private stored variable in extension to hold the most recent set value ?
}
get{
// how to return the most recent value set?
}
}
}
My question is in the comment of above code. How can I do that set and get for a variable in extension of protocol in Swift 4? If it is impossible, what are the workarounds possible?
The simplest way i think is define the variable in the protocol with the getter and setter. Then in your conform object you should declare the variable to conformance.
An Example:
protocol AbstractObject {
var maxProductCount: Int { get set }
}
struct ConformObject: AbstractObject {
var maxProductCount: Int
}
So now you can use your variable in your default implementations
extension AbstractObject {
mutating func addOne() -> Int {
self.maxProductCount += 1
return self.maxProductCount
}
}
Aside from the discussion if its a right way to achieve what you want, you can use object association.
public final class ObjectAssociation<T: AnyObject> {
private let policy: objc_AssociationPolicy
/// - Parameter policy: An association policy that will be used when linking objects.
public init(policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN_NONATOMIC) {
self.policy = policy
}
/// Accesses associated object.
/// - Parameter index: An object whose associated object is to be accessed.
public subscript(index: AnyObject) -> T? {
get { return objc_getAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque()) as! T? }
set { objc_setAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque(), newValue, policy) }
}
}
And in your extension
extension SomeType {
private static let association = ObjectAssociation<NSObject>()
var simulatedProperty: NSObject? {
get { return SomeType.association[self] }
set { SomeType.association[self] = newValue }
}
}
It is not possible to store Swift types via object association directly. You can store e.g. NSNumber instead of Int.
Source: https://stackoverflow.com/a/43056053/1811810
Usecase
I have a superclass (FirebaseObject) with subclasses for most data items in my Firebase (ex: RecipeItem, User). I made a function in the superclass that automatically updates the data that is in the subclass, now I am trying to make a function with closures that get called when the object is updated.
Code
class FirebaseObject {
private var closures: [((FirebaseObject) -> Void)] = []
public func didChange(completion: #escaping (((FirebaseObject) -> Void))) {
// Save closures for future updates to object
closures.append(completion)
// Activate closure with the current object
completion(self)
}
//...
}
This calls the closure with the initial object and saves it for later updates. In my Firebase observer I can now activate all the closures after the data is updated by calling:
self.closures.forEach { $0(self) }
To add these closures that listen for object changes I need to do:
let recipeObject = RecipeItem(data)
recipeObject.didChange { newFirebaseObject in
// Need to set Type even though recipeObject was already RecipeItem
// this will never fail
if let newRecipeObject = newFirebaseObject as? RecipeItem {
// Do something with newRecipeObject
}
}
Question
Is there a way to have the completion handler return the type of the subclass so I don't have to do as? Subclass even though it won't ever fail? I tried to do this with generic type but I can't figure it out and I am not sure if this is the correct solution.
I would like to keep most code in the FirebaseObject class so I don't need to add a lot of code when creating a new subclass.
Edit
Based on this article I tried to add the type when creating a subclass:
class RecipeItem: FirebaseObject<RecipeItem> {
//...
}
class FirebaseObject<ItemType> {
private var handlers: [((ItemType) -> Void)] = []
public func didChange(completion: #escaping (((ItemType) -> Void))) {
//...
This compiles but it crashes as soon as RecipeItem is initialised. I also tried
class RecipeItem: FirebaseObject<RecipeItem.Type> {
//...
}
But this gives an interesting compiler error when I try to access RecipeItem data in didChange closure:
Instance member 'title' cannot be used on type 'RecipeItem'
Ok, so I've been working on this for a day and I have found a way to do it using the method in this answer for the didChange and initObserver functions and taking inspiration from this way of saving data in extensions.
First off, all the functions that need to use the type of the subclass are moved to a protocol.
protocol FirebaseObjectType {}
extension FirebaseObjectType where Self: FirebaseObject {
private func initObserver(at ref: DatabaseReference) {
//...
}
mutating func didChange(completion: #escaping (((Self) -> Void))) {
if observer == nil {
// init Firebase observer here so there will be no Firebase
// observer running when you don't check for changes of the
// object, and so the Firebase call uses the type of whatever
// FirebaseObject this function is called on eg:
// RecipeItem.didChange returns RecipeItem
// and NOT:
// RecipeItem.didChange returns FirebaseObject
initObserver(at: ref)
}
if closureWrapper == nil {
// init closureWrapper here instead of in init() so it uses
// the class this function is called on instead of FirebaseObject
closureWrapper = ClosureWrapper<Self>()
}
// Save closure for future updates to object
closures.append(completion)
// Activate closure with current object
completion(self)
}
}
To save the closures I now use a wrapper class so I can do type checking on that. In FirebaseObject:
class ClosureWrapper<T> {
var array: [((T) -> Void)]
init() {
array = []
}
}
fileprivate var closureWrapper: AnyObject?
Now I can get the closures with the right type in FirebaseObjectType protocol:
private var closures: [((Self) -> Void)] {
get {
let closureWrapper = self.closureWrapper as? ClosureWrapper<Self>
return closureWrapper?.array ?? []
}
set {
if let closureWrapper = closureWrapper as? ClosureWrapper<Self> {
closureWrapper.array = newValue
}
}
}
I can now use didChange on a FirebaseObject subclass without checking its type every time.
var recipe = RecipeItem(data)
recipe.didChange { newRecipe in
// Do something with newRecipe
}
The code below compiles just fine if I make the tester() method an instance method. How can I make it work while keeping it a class method?
protocol Numbers {
}
extension Numbers {
func amountFromText(text: String) -> Int {
return 0
}
}
class CommonDB: Numbers {
class func tester() {
let text = ""
let amount = amountFromText(text)
}
}
The way you have your function defined in your protocol means that it will be an instance function, that is, you need an instance of the class or structure in order to call that function. When you are in a class function, you don't have an instance of that class.
If you want to call amountFromText() from a class function, then declare it to be static. That way, it won't need an instance of the class or structure to be called:
extension Numbers {
static func amountFromText(text: String) -> Int {
return 0
}
}
Is it possible that the variable which initializes my class is computed by a function inside that class?
I first initialize MyClass:
let MyClass = MyClass()
Second, this would be the code in the class:
class MyClass {
var myString = computeRandomString()
func computeRandomString() -> String {
piece of code
return(randomString)
}
}
Whenever I create an instance of MyClass I want the myString to be a randomString. In order for that I made a function within the same class.
There are two options.
First, if computeRandomString does not rely on knowing about anything about specific instances of this class, it could be converted to a class function, or simply moved outside of the class entirely.
For example:
class MyClass {
var myString = MyClass.computeRandomString()
class func computeRandomString() -> String {
return "random string"
}
}
or
func computeRandomString() -> String {
return "random string"
}
class MyClass {
var myString = computeRandomString()
}
The second option is to set it in the initializers (rather than giving it a default value), but you'll only be able to do this after all values (including this one) have been assigned a value.
For example:
class MyClass {
var myString: String
init() {
myString = ""
myString = computeRandomString()
}
func computeRandomString() -> String {
return "random string"
}
}
The reason we can't use an instance method to assign a default value for an instance variable is better explained by the warning generated when we try to use the instance method within an initializer before first giving it a value:
class MyClass {
var myString: String
init() {
myString = computeRandomString()
}
func computeRandomString() -> String {
return "random string"
}
}
On the myString =... line in init, we see the following error:
Use of 'self' in method call 'computeRandomString' before all stored properties are initialized.
This error, unfortunately, does not show up when we use it as the property's default value, as you're trying to do, but it does accurately describe the actual problem.
We cannot use self before our class is fully initialized, and that includes calling methods on self. And until all of our stored properties have valid values, our instance is not fully initialized, so we can never use an instance method to give a non-optional stored property its first value.
A possible solution is a lazy computed property.
The string is created when the property is accessed the first time
class MyClass {
lazy var computeRandomString : String = {
let alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789"
let alphaLength = UInt32(alphabet.characters.count)
var randomString : String = ""
for _ in 0..<20 {
let random = Int(arc4random_uniform(alphaLength))
let index = alphabet.startIndex.advancedBy(random)
randomString += String(alphabet[index])
}
return randomString
}()
}
for _ in 0..<10 {
print(MyClass().computeRandomString)
}
You have to do it like this:
class MyClass {
var myString: String
init() {
self.myString = MyClass.computeRandomString()
}
static func computeRandomString() -> String {
piece of code
return(randomString)
}
}
This will set myString to the results of computeRandomString() on initialisation (creating) of a MyClass-object.
I changed computeRandomString() to static because otherwise it could not be used before initialisation has finished, thanks to #nhgrif.
I have the following class:
class ReportView: NSView {
var categoriesPerPage = [[Int]]()
var numPages: Int = { return categoriesPerPage.count }
}
Compilation fails with the message:
Instance member 'categoriesPerPage' cannot be used on type
'ReportView'
What does this mean?
Sometimes Xcode when overrides methods adds class func instead of just func. Then in static method you can't see instance properties. It is very easy to overlook it. That was my case.
You just have syntax error when saying = {return self.someValue}. The = isn't needed.
Use :
var numPages: Int {
get{
return categoriesPerPage.count
}
}
if you want get only you can write
var numPages: Int {
return categoriesPerPage.count
}
with the first way you can also add observers as set willSet & didSet
var numPages: Int {
get{
return categoriesPerPage.count
}
set(v){
self.categoriesPerPage = v
}
}
allowing to use = operator as a setter
myObject.numPages = 5
For anyone else who stumbles on this make sure you're not attempting to modify the class rather than the instance! (unless you've declared the variable as static)
eg.
MyClass.variable = 'Foo' // WRONG! - Instance member 'variable' cannot be used on type 'MyClass'
instanceOfMyClass.variable = 'Foo' // Right!
It is saying you have an instance variable (the var is only visible/accessible when you have an instance of that class) and you are trying to use it in the context of a static scope (class method).
You can make your instance variable a class variable by adding static/class attribute.
You instantiate an instance of your class and call the instance method on that variable.
Another example is, you have class like :
#obc class Album: NSObject {
let name:String
let singer:Singer
let artwork:URL
let playingSong:Song
// ...
class func getCurrentlyPlayingSongLyric(duration: Int = 0) -> String {
// ...
return playingSong.lyric
}
}
you will also get the same type of error like :
instance member x cannot be used on type x.
It's because you assign your method with "class" keyword (which makes your method a type method) and using like :
Album.getCurrentlyPlayingSongLyric(duration: 5)
but who set the playingSong variable before? Ok. You shouldn't use class keyword for that case :
// ...
func getCurrentlyPlayingSongLyric(duration: Int = 0) -> String {
// ...
return playingSong.lyric
}
// ...
Now you're free to go.
Your initial problem was:
class ReportView: NSView {
var categoriesPerPage = [[Int]]()
var numPages: Int = { return categoriesPerPage.count }
}
Instance member 'categoriesPerPage' cannot be used on type 'ReportView'
previous posts correctly point out, if you want a computed property, the = sign is errant.
Additional possibility for error:
If your intent was to "Setting a Default Property Value with a Closure or Function", you need only slightly change it as well. (Note: this example was obviously not intended to do that)
class ReportView: NSView {
var categoriesPerPage = [[Int]]()
var numPages: Int = { return categoriesPerPage.count }()
}
Instead of removing the =, we add () to denote a default initialization closure. (This can be useful when initializing UI code, to keep it all in one place.)
However, the exact same error occurs:
Instance member 'categoriesPerPage' cannot be used on type 'ReportView'
The problem is trying to initialize one property with the value of another. One solution is to make the initializer lazy. It will not be executed until the value is accessed.
class ReportView: NSView {
var categoriesPerPage = [[Int]]()
lazy var numPages: Int = { return categoriesPerPage.count }()
}
now the compiler is happy!
I kept getting the same error inspite of making the variable static.
Solution: Clean Build, Clean Derived Data, Restart Xcode. Or shortcut
Cmd + Shift+Alt+K
UserNotificationCenterWrapper.delegate = self
public static var delegate: UNUserNotificationCenterDelegate? {
get {
return UNUserNotificationCenter.current().delegate
}
set {
UNUserNotificationCenter.current().delegate = newValue
}
}
Just in case someone really needs a closure like that, it can be done in the following way:
var categoriesPerPage = [[Int]]()
var numPagesClosure: ()->Int {
return {
return self.categoriesPerPage.count
}
}