I've just started learning Combine and am quite confused with behaviour of KVO publishers. They just do not publish any events except for the initial value.
Here is the sample code I used:
#objc class SampleClass: NSObject {
#objc var name: NSString = "1"
}
var a = SampleClass()
let kvoPublisher = a.publisher(for: \.name)
.sink(receiveCompletion: {
print("completion \($0)")
}, receiveValue: { newVal in
print("new val - \(newVal)")
})
a.name = "2"
a.name = "3"
print("Finished; publisher = \(kvoPublisher) | a.name = \(a.name)")
The console output is
new val - 1
Finished; publisher = Combine.AnyCancellable | a.name = 3
Could you please explain what am I missing here and how to fix it?
Thanks.
You also need to mark the property as dynamic in order for it to be KVO compliant. publisher(for:) only works with KVO compliant properties, since it uses KVO under the hood.
#objc class SampleClass: NSObject {
#objc dynamic var name: NSString = "1"
}
Once you do that, the KVO publisher emits the updated values as expected.
For more information on #objc vs #objc dynamic, see this Q&A.
Bear in mind that you should only use KVO publishers when interacting with code that you cannot change. When you want to observe property values of types that you control, use #Published instead.
I've just started learning Combine and am quite confused with behaviour of KVO publishers. They just do not publish any events except for the initial value.
Here is the sample code I used:
#objc class SampleClass: NSObject {
#objc var name: NSString = "1"
}
var a = SampleClass()
let kvoPublisher = a.publisher(for: \.name)
.sink(receiveCompletion: {
print("completion \($0)")
}, receiveValue: { newVal in
print("new val - \(newVal)")
})
a.name = "2"
a.name = "3"
print("Finished; publisher = \(kvoPublisher) | a.name = \(a.name)")
The console output is
new val - 1
Finished; publisher = Combine.AnyCancellable | a.name = 3
Could you please explain what am I missing here and how to fix it?
Thanks.
You also need to mark the property as dynamic in order for it to be KVO compliant. publisher(for:) only works with KVO compliant properties, since it uses KVO under the hood.
#objc class SampleClass: NSObject {
#objc dynamic var name: NSString = "1"
}
Once you do that, the KVO publisher emits the updated values as expected.
For more information on #objc vs #objc dynamic, see this Q&A.
Bear in mind that you should only use KVO publishers when interacting with code that you cannot change. When you want to observe property values of types that you control, use #Published instead.
Hello my problem is simple I have to constantly check whether the value of a bool is true or false, what I have tried so far is to use the:
override func update(_ currentTime: TimeInterval)
function in swift and it is way to fast and once it checks the values it will constantly repeat the action even though I only wish for it to perform the action only once, so basically what I'm saying is that all I want to do is to check whether the bool value is true or false once and then stop checking until it changes again. Please help, thank you.
Property Observers
You can use Property Observers in Swift to accomplish what you need... Here is what docs says about these:
Property observers observe and respond to changes in a property’s
value. Property observers are called every time a property’s value is
set, even if the new value is the same as the property’s current
value.
There are willSet and didSet property observers:
willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.
To solve your problem, you could do something like this:
var myProperty:Int = 0 {
willSet {
print("About to set myProperty, newValue = \(newValue)")
}
didSet{
print("myProperty is now \(myProperty). Previous value was \(oldValue)")
}
}
You can implement either one or both of property observers on your property.
Getters and Setters
As an alternative, You can use getters and setters on a stored property to solve your problem:
private var priv_property:Int = 0
var myProperty:Int{
get {
return priv_property
}
set {
priv_property = newValue
}
}
Computed properties do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.
You should use observers or callbacks. Read comments below and see #Whirlwind's answer. The solution below is not really recommended since it would be inefficient and might complicate your code. But if you wanted or needed to do it in the update(), here's how I'd do it:
// Assume stored property
// It might be an API call and so on
var boolToWatch = false
var lastValueOfWatchedBool: Bool?
var lastCheck: TimeInterval = 0
let checkInterval = 1.0 // check every second
override func update(_ currentTime: TimeInterval) {
// In case boolToWatch is an expensive API call might be good to
// check less frequently
if currentTime - lastCheck > checkInterval {
lastCheck = currentTime
// Check for the initial case
if lastValueOfWatchedBool == nil {
lastValueOfWatchedBool = boolToWatch
}
// Detect change
if boolToWatch != lastValueOfWatchedBool {
lastValueOfWatchedBool = boolToWatch
// Do what you need to do when the value changed here
print("I feel different!")
}
}
}
I was doing some research about the reasons we should use Get and Set for our properties.
I've noticed 3 main reasons for it
When you want to do/check something before you actually set the
property
When you want to have a property that you can only Get from it
(maybe for security purposes I guess? ), or give it different access
levels.
Hiding the internal representation of the property while exposing a
property using an alternative representation. (which for me doesn't
make a lot of sense since i can access it on the wrong place using
the Set function anyways)
The code below is a example of how you would implement Get and Set for properties in Swift, taking advantage of those 3 points I mentioned:
class Test
{
private var _testSet:String!
private var _testGetOnly:String
var testSet:String{
get{
return _testSet
}
set{
_testSet = newValue + "you forgot this string"
}
}
var testGetOnly:String!{
get{
return _testGetOnly
}
}
init(testSet:String, testGetOnly:String)
{
_testSet = testSet
_testGetOnly = testGetOnly
}
}
But this other example below also take advantage of those points mentioned but instead of using another computed property to return the private property value I just use the willSet and didSet observers
class Test
{
var testGet:String {
willSet{
fatalError("Operation not allowed")
}
}
var testWillSet:String!{
didSet{
self.testWillSet = self.testWillSet + "you forgot this string"
}
}
init(testGet:String, testWillSet:String)
{
self.testGet = testGet
self.testWillSet = testWillSet
}
}
So I'm curious to know what are the ADVANTAGES and DISADVANTAGES of each implementation.
Thanks in advance
Your question boils down to compile time vs. run time error. To address your 3 questions:
Yes, willCheck is your only option here
Readonly properties fall into 2 types: (a) those whose value derive from other properties, for example, their sum; and (b) those that you want to be able to change by yourself, but not by the users. The first type truly have no setter; the second type has a public getter and a private setter. The compiler can help you check for that and the program will not compile. If you throw a fatalError in didSet you get a runtime error and your application will crash.
There can be state objects that you don't want the user to freely mess with, and yes, you can completely hide those from the users.
Your code first example was too verbose in defining the backing variables - you don't need to do that. To illustrate these points:
class Test
{
// 1. Validate the new value
var mustBeginWithA: String = "A word" {
willSet {
if !newValue.hasPrefix("A") {
fatalError("This property must begin with the letter A")
}
}
}
// 2. A readonly property
var x: Int = 1
var y: Int = 2
var total: Int {
get { return x + y }
}
private(set) var greeting: String = "Hello world"
func changeGreeting() {
self.greeting = "Goodbye world" // Even for private property, you may still
// want to set it, just not allowing the user
// to do so
}
// 3. Hide implementation detail
private var person = ["firstName": "", "lastName": ""]
var firstName: String {
get { return person["firstName"]! }
set { person["firstName"] = newValue }
}
var lastName: String {
get { return person["lastName"]! }
set { person["lastName"] = newValue }
}
var fullName: String {
get { return self.firstName + " " + self.lastName }
set {
let components = newValue.componentsSeparatedByString(" ")
self.firstName = components[0]
self.lastName = components[1]
}
}
}
Usage:
let t = Test()
t.mustBeginWithA = "Bee" // runtime error
t.total = 30 // Won't compile
t.greeting = "Goodbye world" // Won't compile. The compiler does the check for you
// instead of a crash at run time
t.changeGreeting() // OK, greeting now changed to "Goodbye world"
t.firstName = "John" // Users have no idea that they are actually changing
t.lastName = "Smith" // a key in the dictionary and there's no way for them
// to access that dictionary
t.fullName = "Bart Simpsons" // You do not want the user to change the full name
// without making a corresponding change in the
// firstName and lastName. With a custome setter, you
// can update both firstName and lastName to maintain
// consistency
A note about private in Swift 2 vs. Swift 3: if you try this in a Swift 2 playground, you will find t.greeting = "Goodbye world" works just fine. This is because Swift 2 has a strange access level specifier: private means "only accessible within the current file". Separate the class definition and the sample code into different files and Xcode will complain. In Swift 3, that was changed to fileprivate which is both clearer and save the private keyword for something more similar to to Java and .NET
Swift has a property declaration syntax very similar to C#'s:
var foo: Int {
get { return getFoo() }
set { setFoo(newValue) }
}
However, it also has willSet and didSet actions. These are called before and after the setter is called, respectively. What is their purpose, considering that you could just have the same code inside the setter?
The point seems to be that sometimes, you need a property that has automatic storage and some behavior, for instance to notify other objects that the property just changed. When all you have is get/set, you need another field to hold the value. With willSet and didSet, you can take action when the value is modified without needing another field. For instance, in that example:
class Foo {
var myProperty: Int = 0 {
didSet {
print("The value of myProperty changed from \(oldValue) to \(myProperty)")
}
}
}
myProperty prints its old and new value every time it is modified. With just getters and setters, I would need this instead:
class Foo {
var myPropertyValue: Int = 0
var myProperty: Int {
get { return myPropertyValue }
set {
print("The value of myProperty changed from \(myPropertyValue) to \(newValue)")
myPropertyValue = newValue
}
}
}
So willSet and didSet represent an economy of a couple of lines, and less noise in the field list.
My understanding is that set and get are for computed properties (no backing from stored properties)
if you are coming from an Objective-C bare in mind that the naming conventions have changed. In Swift an iVar or instance variable is named stored property
Example 1 (read only property) - with warning:
var test : Int {
get {
return test
}
}
This will result in a warning because this results in a recursive function call (the getter calls itself).The warning in this case is "Attempting to modify 'test' within its own getter".
Example 2. Conditional read/write - with warning
var test : Int {
get {
return test
}
set (aNewValue) {
//I've contrived some condition on which this property can be set
//(prevents same value being set)
if (aNewValue != test) {
test = aNewValue
}
}
}
Similar problem - you cannot do this as it's recursively calling the setter.
Also, note this code will not complain about no initialisers as there is no stored property to initialise.
Example 3. read/write computed property - with backing store
Here is a pattern that allows conditional setting of an actual stored property
//True model data
var _test : Int = 0
var test : Int {
get {
return _test
}
set (aNewValue) {
//I've contrived some condition on which this property can be set
if (aNewValue != test) {
_test = aNewValue
}
}
}
Note The actual data is called _test (although it could be any data or combination of data)
Note also the need to provide an initial value (alternatively you need to use an init method) because _test is actually an instance variable
Example 4. Using will and did set
//True model data
var _test : Int = 0 {
//First this
willSet {
println("Old value is \(_test), new value is \(newValue)")
}
//value is set
//Finaly this
didSet {
println("Old value is \(oldValue), new value is \(_test)")
}
}
var test : Int {
get {
return _test
}
set (aNewValue) {
//I've contrived some condition on which this property can be set
if (aNewValue != test) {
_test = aNewValue
}
}
}
Here we see willSet and didSet intercepting a change in an actual stored property.
This is useful for sending notifications, synchronisation etc... (see example below)
Example 5. Concrete Example - ViewController Container
//Underlying instance variable (would ideally be private)
var _childVC : UIViewController? {
willSet {
//REMOVE OLD VC
println("Property will set")
if (_childVC != nil) {
_childVC!.willMoveToParentViewController(nil)
self.setOverrideTraitCollection(nil, forChildViewController: _childVC)
_childVC!.view.removeFromSuperview()
_childVC!.removeFromParentViewController()
}
if (newValue) {
self.addChildViewController(newValue)
}
}
//I can't see a way to 'stop' the value being set to the same controller - hence the computed property
didSet {
//ADD NEW VC
println("Property did set")
if (_childVC) {
// var views = NSDictionaryOfVariableBindings(self.view) .. NOT YET SUPPORTED (NSDictionary bridging not yet available)
//Add subviews + constraints
_childVC!.view.setTranslatesAutoresizingMaskIntoConstraints(false) //For now - until I add my own constraints
self.view.addSubview(_childVC!.view)
let views = ["view" : _childVC!.view] as NSMutableDictionary
let layoutOpts = NSLayoutFormatOptions(0)
let lc1 : AnyObject[] = NSLayoutConstraint.constraintsWithVisualFormat("|[view]|", options: layoutOpts, metrics: NSDictionary(), views: views)
let lc2 : AnyObject[] = NSLayoutConstraint.constraintsWithVisualFormat("V:|[view]|", options: layoutOpts, metrics: NSDictionary(), views: views)
self.view.addConstraints(lc1)
self.view.addConstraints(lc2)
//Forward messages to child
_childVC!.didMoveToParentViewController(self)
}
}
}
//Computed property - this is the property that must be used to prevent setting the same value twice
//unless there is another way of doing this?
var childVC : UIViewController? {
get {
return _childVC
}
set(suggestedVC) {
if (suggestedVC != _childVC) {
_childVC = suggestedVC
}
}
}
Note the use of BOTH computed and stored properties. I've used a computed property to prevent setting the same value twice (to avoid bad things happening!); I've used willSet and didSet to forward notifications to viewControllers (see UIViewController documentation and info on viewController containers)
If I've made a mistake anywhere, please edit to fix it!
You can also use the didSet to set the variable to a different value. This does not cause the observer to be called again as stated in Properties guide. For example, it is useful when you want to limit the value as below:
let minValue = 1
var value = 1 {
didSet {
if value < minValue {
value = minValue
}
}
}
value = -10 // value is minValue now.
These are called Property Observers:
Property observers observe and respond to changes in a property’s
value. Property observers are called every time a property’s value is
set, even if the new value is the same as the property’s current
value.
Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/ca/jEUH0.l
I suspect it's to allow for things we would traditionally do with KVO such as data binding with UI elements, or triggering side effects of changing a property, triggering a sync process, background processing, etc, etc.
NOTE
willSet and didSet observers are not called when a property is set in an initializer before delegation takes place
The many well-written existing answers cover the question well, but I'll mention, in some detail, an addition that I believe is worth covering.
The willSet and didSet property observers can be used to call delegates, e.g., for class properties that are only ever updated by user interaction, but where you want to avoid calling the delegate at object initialization.
I'll cite Klaas up-voted comment to the accepted answer:
willSet and didSet observers are not called when a property is first
initialized. They are only called when the property’s value is set
outside of an initialization context.
This is a quite neat as it means e.g. the didSet property is a good choice of launch point for delegate callbacks & functions, for your own custom classes.
As an example, consider some custom user control object, with some key property value (e.g. position in rating control), implemented as a subclass of UIView:
// CustomUserControl.swift
protocol CustomUserControlDelegate {
func didChangeValue(value: Int)
// func didChangeValue(newValue: Int, oldValue: Int)
// func didChangeValue(customUserControl: CustomUserControl)
// ... other more sophisticated delegate functions
}
class CustomUserControl: UIView {
// Properties
// ...
private var value = 0 {
didSet {
// Possibly do something ...
// Call delegate.
delegate?.didChangeValue(value)
// delegate?.didChangeValue(value, oldValue: oldValue)
// delegate?.didChangeValue(self)
}
}
var delegate: CustomUserControlDelegate?
// Initialization
required init?(...) {
// Initialise something ...
// E.g. 'value = 1' would not call didSet at this point
}
// ... some methods/actions associated with your user control.
}
After which your delegate functions can be used in, say, some view controller to observe key changes in the model for CustomViewController, much like you'd use the inherent delegate functions of the UITextFieldDelegate for UITextField objects (e.g. textFieldDidEndEditing(...)).
For this simple example, use a delegate callback from the didSet of the class property value to tell a view controller that one of it's outlets have had associated model update:
// ViewController.swift
Import UIKit
// ...
class ViewController: UIViewController, CustomUserControlDelegate {
// Properties
// ...
#IBOutlet weak var customUserControl: CustomUserControl!
override func viewDidLoad() {
super.viewDidLoad()
// ...
// Custom user control, handle through delegate callbacks.
customUserControl = self
}
// ...
// CustomUserControlDelegate
func didChangeValue(value: Int) {
// do some stuff with 'value' ...
}
// func didChangeValue(newValue: Int, oldValue: Int) {
// do some stuff with new as well as old 'value' ...
// custom transitions? :)
//}
//func didChangeValue(customUserControl: CustomUserControl) {
// // Do more advanced stuff ...
//}
}
Here, the value property has been encapsulated, but generally: in situations like these, be careful not to update the value property of the customUserControl object in the scope of the associated delegate function (here: didChangeValue()) in the view controller, or you'll end up with infinite recursion.
The willSet and didSet observers for the properties whenever the property is assigned a new value. This is true even if the new value is the same as the current value.
And note that willSet needs a parameter name to work around, on the other hand, didSet does not.
The didSet observer is called after the value of property is updated. It compares against the old value. If the total number of steps has increased, a message is printed to indicate how many new steps have been taken. The didSet observer does not provide a custom parameter name for the old value, and the default name of oldValue is used instead.
Getter and setter are sometimes too heavy to implement just to observe proper value changes. Usually this needs extra temporary variable handling and extra checks, and you will want to avoid even those tiny labour if you write hundreds of getters and setters. These stuffs are for the situation.
In your own (base) class, willSet and didSet are quite reduntant , as you could instead define a calculated property (i.e get- and set- methods) that access a _propertyVariable and does the desired pre- and post- prosessing.
If, however, you override a class where the property is already defined, then the willSet and didSet are useful and not redundant!
One thing where didSet is really handy is when you use outlets to add additional configuration.
#IBOutlet weak var loginOrSignupButton: UIButton! {
didSet {
let title = NSLocalizedString("signup_required_button")
loginOrSignupButton.setTitle(title, for: .normal)
loginOrSignupButton.setTitle(title, for: .highlighted)
}
I do not know C#, but with a little guesswork I think I understand what
foo : int {
get { return getFoo(); }
set { setFoo(newValue); }
}
does. It looks very similar to what you have in Swift, but it's not the same: in Swift you do not have the getFoo and setFoo. That is not a little difference: it means you do not have any underlying storage for your value.
Swift has stored and computed properties.
A computed property has get and may have set (if it's writable). But the code in the getter and setter, if they need to actually store some data, must do it in other properties. There is no backing storage.
A stored property, on the other hand, does have backing storage. But it does not have get and set. Instead it has willSet and didSet which you can use to observe variable changes and, eventually, trigger side effects and/or modify the stored value. You do not have willSet and didSet for computed properties, and you do not need them because for computed properties you can use the code in set to control changes.