What is analogue of Objective C static variable in Swift? - swift

It was very convenient to have static variables in Objective C (static variable's value is maintained throughout all function/method calls), however I couldn't find anything like this in Swift.
Is there anything like this?
This is an example of static variable in C:
void func() {
static int x = 0;
/* x is initialized only once across four calls of func() and
the variable will get incremented four
times after these calls. The final value of x will be 4. */
x++;
printf("%d\n", x); // outputs the value of x
}
int main() { //int argc, char *argv[] inside the main is optional in the particular program
func(); // prints 1
func(); // prints 2
func(); // prints 3
func(); // prints 4
return 0;
}

After seeing your updated answer, here is the modification for your Objective-C code:
func staticFunc() {
struct myStruct {
static var x = 0
}
myStruct.x++
println("Static Value of x: \(myStruct.x)");
}
Call is anywhere in your class
staticFunc() //Static Value of x: 1
staticFunc() //Static Value of x: 2
staticFunc() //Static Value of x: 3

Declare the variable at the top level of a file (outside any classes) which is called global variable.
variables at the top level of a file are initialised lazily! So you
can set the default value for your variable to be the result of
reading the file, and the file won't actually be read until your code
first asks for the variable's value.
Reference from HERE.
UPDATE:
From your C example you can achieve same thing in swift this way:
var x = 0 //this is global variable
func staticVar() {
x++
println(x)
}
staticVar()
x //1
staticVar()
x //2
staticVar()
x //3
Tested with playground.
From Apple Document:
In C and Objective-C, you define static constants and variables
associated with a type as global static variables. In Swift, however,
type properties are written as part of the type’s definition, within
the type’s outer curly braces, and each type property is explicitly
scoped to the type it supports.
You define type properties with the static keyword. For computed type
properties for class types, you can use the class keyword instead to
allow subclasses to override the superclass’s implementation. The
example below shows the syntax for stored and computed type
properties:
struct SomeStructure {
static var storedTypeProperty = "Some value."
static var computedTypeProperty: Int {
// return an Int value here
}
}
enum SomeEnumeration {
static var storedTypeProperty = "Some value."
static var computedTypeProperty: Int {
// return an Int value here
}
}
class SomeClass {
static var storedTypeProperty = "Some value."
static var computedTypeProperty: Int {
// return an Int value here
}
class var overrideableComputedTypeProperty: Int {
// return an Int value here
}
}
NOTE
The computed type property examples above are for read-only computed type >properties, but you can also define read-write computed
type properties with the same syntax as for computed instance
properties.

Related

Swift Pass Variable to Reference

In C++ you can do
int x = 5;
int &y = x;
So both x and y point to the same place. Can I do this in Swift?
I'm programming a class with some functionality, and another class which is basically a driver.
class Driver {
var otherClass: OtherClass
var referenceVariable: Int = otherClass.referenceVariable // how to make a reference here
...
}
Or is there a better / more semantically correct way to do this in Swift?
You could use a computed property for this functionality:
class Driver {
var otherClass: OtherClass
var referenceVariable: Int {
get { otherClass.referenceVariable }
set { otherClass.referenceVariable = newValue }
}
...
}
A computed property doesn't store a value itself. It provides closures for reading and writing the property. In this case, we're operating on the value stored in the otherClass.
Int is a struct in Swift. Structs in Swift are first citizen. Basically
structs are value types. Values types usually are copied on assigning.
If you want to use reference type, just use classes.
One of the basic solution for your task is to wrap Int value in a class
final class IntWrapper {
let value: Int
init(value: Int) {
self.value = value
}
}
Now, changing referenceVariable: IntWrapper will change all references to it
class Driver {
var otherClass: OtherClass
var referenceVariable: IntWrapper = otherClass.referenceVariable // how to make a reference here
...
}
If you want to get pointer on your class IntWrapper you can create it this way.
var pointer = UnsafePointer(&i)
But, I suppose, you aren't gonna need it

Cannot create class object or array of class objects

I'm working on a project thatand am simply trying to create a object instance of a simple custom class:
import Foundation
class Set {
private var gam1: Int!
private var gam2: Int!
init (gam1: Int, gam2: Int) {
self.gam1 = gam1
self.gam2 = gam2
}
//returns set info as array
func getStats () -> [Int] {
let arr = [gam1!, gam2!]
return arr
}
}
The class simply stores a few variables for use later and I want an array of such objects to store several values. However, when I try to create a n instance of the class in a different class I get errors:
import Foundation
class AnotherClass {
var mySet = Set(gam1: 6, gam2: 5) //error 1
//array of set objects
var setArray = [Set]() // error 2
//returns array of set objects
func getSets () -> [Set] { //error 3
return setArray
}
}
The errors state:
Cannot find an initializer for type 'Set' that accepts an argument list of type '(gam1: Int, gam2: Int)'
Cannot invoke initializer for type '[Set]' with no arguments
and
Reference to generic type 'Set' requires arguments in <...>
Any ideas of what the issue is here? could the 'Set' name of the class be conflicting with a reserved keyword?
Many thanks,
Kw
The issue that you are having is due to the naming conflict between Set in the Swift standard library and the one you defined.
This is never a good idea. Instead, give it a more descriptive name (and one that doesn't conflict with anything else). For instance, call it gamHolder and initialize it gamHolder(gam1: <an int>, gam2: <another int>).
Also, if you have defined variables inside the init function they do not need to be forced unwrapped optionals.
For example:
class myClass {
var myInt: Int
init(anInt: Int) {
myInt = anInt
}
}
You defined 2 parameters in your init method (since gam1 and gam2 are not optional). So, you have 2 solutions:
1 - You add parameters into your init call (like this):
var mySet = Set(gam1: 1, gam2: 2)
2 - You change gam1 and gam2 to optionals and you add a zero parameters init:
class Set {
private var gam1: Int?
private var gam2: Int?
init() {
}
init(gam1: Int, gam2: Int) {
self.gam1 = gam1
self.gam2 = gam2
}
// returns set info as array
func getStats() -> [Int] {
let arr = [gam1!, gam2!]
return arr
}
}
So, you will be able to call it like this: var mySet = Set()
Also: be careful: Set is a class used in the Swift standard library. It's never a good idea to use same class names than Swift Standard Library. A name like TenisSet would be better.
Edit:
Here is a final working example with a renamed class:
class TenisSet {
private var gam1: Int?
private var gam2: Int?
init() {
}
init(gam1: Int, gam2: Int) {
self.gam1 = gam1
self.gam2 = gam2
}
// returns set info as array
func getStats() -> [Int] {
let arr = [gam1!, gam2!]
return arr
}
}
class AnotherClass {
var mySet = TenisSet()
// array of set objects
var setArray = [TenisSet]()
// returns array of set objects
func getSets() -> [TenisSet] {
return setArray
}
}

What is the correct way to create preset Structs?

In Swift classes we can use a class function to create preset instances. Like the calendar example below:
let calender = NSCalendar.currentCalendar()
Which will have a similar pattern as this :
class SomeClass {
var attribute : String
init(value:String) {
attribute = value
}
class func testClass() -> SomeClass {
return SomeClass(value: "test")
}
}
let test = SomeClass.testClass()
But there are no class functions in structs obviously. Xcode recommends using static instead. This is very close to the singleton pattern.
struct SomeStruct {
var attribute : String
init(value:String) {
attribute = value
}
static var testStruct = SomeStruct(value: "test")
}
Singleton pattern
class Singleton {
static let shared = Singleton()
private init() {
}
}
So is this an ok way to init a struct with preset values since structs are value types. If it is not ok, what is the correct way?
The equivalent of class func for struct types is static func:
static func testStruct() -> SomeStruct {
return SomeStruct(value: "foo")
}
and a static property (the "singleton-pattern") works identically
with both class and struct types:
static let singleStruct = SomeStruct(value: "foo")
testStruct() creates a value on each call, whereas singleStruct
creates the value once (on the first call).
In most cases that would make no difference because structures are
value types. The static property has advantages if creating the
value is "expensive". Also, as #Lance noticed in a comment,
this pattern is used by Apple frequently, such as CGRect.zero.
However, it makes a difference if the struct has properties which
are reference types (or pointers to unmanaged memory). Here is an example:
class MyClass {
var attribute : String
init(value : String) {
attribute = value
}
}
struct SomeStruct {
var ptr : MyClass
init(value : String) {
ptr = MyClass(value: value)
}
static func testStruct() -> SomeStruct {
return SomeStruct(value: "foo")
}
static let singleStruct = SomeStruct(value: "foo")
}
Using the static function:
let test1 = SomeStruct.testStruct()
print(test1.ptr.attribute) // foo
let test2 = SomeStruct.testStruct()
test2.ptr.attribute = "bar"
print(test1.ptr.attribute) // foo
Here test1 and test2 are separate values and we get the expected
output.
Using the static property:
let test1 = SomeStruct.singleStruct
print(test1.ptr.attribute) // foo
let test2 = SomeStruct.singleStruct
test2.ptr.attribute = "bar"
print(test1.ptr.attribute) // bar <--- What?
Here, test1 and test2 are set to the same value returned from
the static property. Changing test2.ptr does not mutate test2,
resulting in the somewhat unexpected output for test1.ptr.attribute
See Friday Q&A 2015-04-17: Let's Build Swift.Array for an interesting article on how this can be solved.
Btw, static can be used with class types as well, here static
is a shortcut for class final: a type method that cannot be overridden
in a subclass. Since there is no inheritance for struct types it makes
sense that type methods for struct types are written as static.

Instance member cannot be used on type

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
}
}

cannot convert expression type void to type integer in Swift using XCTAssertEqual

I am very new to the Swift language and XCode. Have a error message from this code:
Class Deck
class Deck {
var decks : Integer = 0
init () {
decks = 1
}
init (amountOfDecks : Integer){
decks = amountOfDecks
}
func getAmountOfCards() -> Integer {
return 0
}
}
Test Class
import XCTest
import helloWorldv2
class helloWorldv2Tests: XCTestCase {
override func setUp() {
super.setUp()
}
override func tearDown() {
super.tearDown()
}
func testDeckConstructor() {
var deck = Deck(amountOfDecks: 1)
var amount : Integer = deck.getAmountOfCards()
let expected : Integer = 52
// ERROR: Cannot convert the expression type 'void' to type 'integer'
XCTAssertEqual(expected, amount)
}
}
I set the two variables to type Integer so don't understand why I cant compare the two values...
the Type you should be using is Int (Integer is a protocol, not a Type, and there is not an implementation in Swift for == that accepts arguments conforming to the Integer protocol)
specifying the Type in the way that you are doing it is unnecessary, thanks to Swift's "type inference" - when you declare and assign a value to a variable, it will automatically give that variable the same Type as the value that is being assigned to it (and literal integers in Swift are typed as Int by default)... so let expected = 52 will automatically give your expected constant the Type Int without you having to declare it as such
Integer is a protocol, you should use Int instead as that is an actual struct.