I have a manager class for my data which is configured by two properties, one to set to a category and another to select items which correspond with that category. Based on that it will expose the relevant pieces of data. I am using a couple of different forms or making those selections, including a pair of IndexSets.
My problem is that I would also like to be able to save the selected items for each category, so that whenever the category is changed the items previously selected for it are restored. This is easy to achieve when accessed programmatically, but using bindings to allow a view in a macOS app to be able to provide that configuration unfortunately does not work properly
Changing the category causes the object bound to its selection to empty or 'preserve' the selected items before the category is actually updated. So the actual selection gets overwritten with, with noway I can see to tell the difference between this behaviour and a user action.
Here are the test code I have used for experimenting, with viewDidLoad generating some random test data to roughly mimic the structure o the real class. This does not attempt to save or restore the selection, but simply shows the overwriting behaviour.
class Thing: NSObject {
#objc dynamic var name: String
required init(name: String) {
self.name = name
}
}
class Stuff: NSObject {
#objc dynamic var name: String
#objc dynamic var things: [Thing]
required init(name: String, things: [Thing]) {
self.name = name
self.things = things
}
}
class StuffManager: NSObject {
#objc dynamic var stuff = [Stuff]()
#objc dynamic var stuffIndex = IndexSet() {
didSet {
print("STUFF: ", Array(stuffIndex))
}
}
#objc dynamic var things = [Thing]()
#objc dynamic var thingsIndex = IndexSet() {
didSet {
print("THING: ", Array(thingsIndex))
}
}
}
class ViewController: NSViewController {
#objc dynamic var stuffManager = StuffManager()
override func viewDidLoad() {
super.viewDidLoad()
(1...10).forEach { stuffManager.things.append(Thing(name: "Thing \($0)")) }
(1...9).forEach {
let randomThings = Array(stuffManager.things.shuffled()[0...Int.random(in: 0..<10)])
stuffManager.stuff.append(Stuff(name: "Collection \($0)", things: randomThings))
}
stuffManager.stuff.append(Stuff(name: "Collection 10", things: []))
}
}
In Interface Builder I have a view containing an NSPopButton to select the Stuff, a multiple selection NSTableView to select the Things, and a pair of NSArrayControllers for each. The bindings are:
Stuff Array Controller
Content Array:
Binding to: ViewController, Model Key Path: stuffManager.stuff
Selection Indexes:
Binding to: ViewController, Model Key Path: stuffManager.stuffIndex
Things Array Controller
Content Array:
Binding to: Stuff Array Controller, Controller Key: Selection, Model Key Path: things
Selection Indexes:
Binding to: ViewController, Model Key Path: stuffManager.thingIndex
The two interface objects are bound to these controllers in the standard way, the Content to the arrangedObjects and the Selection Indexes to the selectionIndexes of their respective array controller.
What this test code shows is that when the value in the popup button is changed the THING debug line appears before the STUFF debug line, that is it changes the selection of Things before it changes the Stuff. So any code in the property observer on stuffManager.things to save the new selection will save this change before being aware that the Stuff has changed.
Obviously this behaviour is to avoid the selection being made incorrect by the change to the content, or worse selecting out of bounds if the new content is shorter. But is there any way to detect when this is happening, rather than a user changing the selection? Or a way to override it to gain manual control over the process rather than having to accept the default behaviour of 'Preserve Selection' or the selection being cancelled if that option is disabled?
And what makes it more awkward is if this behaviour only occurs when the selection would change. If the selected Things exist for the new Stuff, or if nothing is selected, then nothing happens to trigger the property observer. Again this is understandable, but it prevents being able to cache the change and then only save the previous one if the Stuff has not changed.
I did wonder if using a separate IndexSet for each Stuff would avoid this problem, because then there would be no need for the NSTableView to manage the selection. I do not like the idea of keeping an IndexSet in the model but would accept it if it worked. But it does not. Again understandable, because the table view has no idea the Selection Indexes binding will be changed. Unless I am missing something?
But I tested this by updating the Stuff class to include the following:
#objc dynamic var selected = IndexSet() {
didSet {
print("THING: ", Array(selected))
}
}
Then changing the Selection Indexes binding of the Things Array Controller to:
Binding to: Stuff Array Controller, Controller Key: selection, Model Key Path: selected
Is what I am trying to achieve impossible? I would not have thought it that strange a thing to want to do, to save and restore a selection, but it seems impossible with bindings.
The only solution I can see is to forgo the master-detail style pattern and instead just maintain a separate [Thing] property in my data manager class, bind the Things Array Controller to this (or even just bind the table directly to the property), then whenever the popup button changes update the new property to match the stuff object.
Something like this in the StuffManager, with the table content bound to availableThings:
#objc dynamic var stuffIndex = IndexSet() {
didSet {
print("STUFF: ", Array(stuffIndex))
availableThings = stuff[stuffIndex.first!].things
}
}
#objc dynamic var availableThings = [Thing]()
It appears there is no way to prevent the NSTableView behaviour of automatically resetting its selection when the content changes. Nor any way to detect when this is happening, as it updates this before updating the selection on the NSPopupButton having changed. So here is how I have written the StuffManager class, adding a property for binding to the tableview so I can control the content changing:
class StuffManager: NSObject {
let defaults: UserDefaults = .standard
var canSaveThingsIndex = true
#objc dynamic var stuff = [Stuff]()
#objc dynamic var stuffIndex = IndexSet() {
didSet {
canSaveThingsIndex = false
if stuffIndex.count > 0 {
availableThings = stuff[stuffIndex.first!].things
let thing = stuff[stuffIndex.first!].name
if let items = defaults.object(forKey: thing) as? [Int] {
thingsIndex = IndexSet(items)
} else if availableThings.count > 0 {
thingsIndex = IndexSet(0..<availableThings.count)
} else {
thingsIndex.removeAll()
}
} else {
availableThings.removeAll()
thingsIndex.removeAll()
}
canSaveThingsIndex = true
}
}
#objc dynamic var things = [Thing]()
#objc dynamic var availableThings = [Thing]()
#objc dynamic var thingsIndex = IndexSet() {
didSet {
if canSaveThingsIndex && stuffIndex.count > 0 {
let thing = stuff[stuffIndex.first!].name
defaults.set(Array(thingsIndex), forKey: thing)
}
}
}
}
The Things Array Controller is now bound as:
Content Array:
Binding to: ViewController, Model Key Path: stuffManager.availableThings
Selection Indexes:
Binding to: ViewController, Model Key Path: stuffManager.thingsIndex
Though without being able to use the master-detail benefits of an NSArrayController they are not needed. Both the NSPopupButton and NSTableView can be bound directly to the StuffManager. And this allows the NSPopupButton's Selected Index can be bound to an Int int he Stuff Manager rather than needing to use an IndexSet despite multiple selections being impossible.
The main feature of the workaround is that because I am manually changing the content I can use the canSaveThingsIndex flag before changing the NSTableView content. So whenever its natural behaviour triggers the thingsIndex property observer, this can be ignored to prevent it overwriting the user's selection. It also avoids the unnecessary saving of a selection immediately after being restored.
I have a custom view based on TheNeil's answer, that has two levels: MainContent and SubviewContent, both are Views.
The user can navigate from a single MainContent to many SubviewContent. This means I make the destination view's content dynamically.
The hierarchy looks like this:
struct StackNavigationView<MainContent, SubviewContent>: View where MainContent: View, SubviewContent: View {
subView: () -> SubviewContent
//...
}
struct SomeView: View {
var body: some View {
StackNavigationView(/*init parameters*/) {
//MainView
}
}
private func subView(forIndex index: Int) -> AnyView {
//some stuff...
}
}
SubviewContent is built from a method passed from the MainContent and stored as a variable like so:
let subView: () -> SubviewContent
During initialization, I pass an #escaping method and assign it to the variable subView.
init(#ViewBuilder subView: #escaping () -> SubviewContent, /*more stuff*/) {
self.subView = subView
//more stuff...
}
That method returns a view based on an index, like so:
private func subView(forIndex index: Int) -> AnyView {
switch index {
case 0: return AnyView(/*some content*/)
case 1: return AnyView(/*some content*/)
//etc...
default: return AnyView(ViewNotFound())
}
}
The problem is that if I pass a view that requires parameters, the parameters must be hardcoded. Otherwise, the parameters are nil and the app crashes.
Example case with parameter:
case 1: return AnyView( DestinationView(parameter: some_parameter) )
This crashes because some_parameter is nil, when the user tries to navigate.
The same issue appears with optional parameters. When unwrapped, they are nil.
I have tried removing the #escaping bit to solve this issue, but then the app crashes instantly because there are no parameters to pass (again, it passes nil but for a different reason, obviously).
How can I pass a fully constructed View only after the users select the navigation destination?
UPDATE 1
Upon review and discussion of the question, I believe a better way to clarify the problem would be this:
How is it possible to maintain in-method parameters while passing the method as a variable?
If you are just trying to pass imagepaths/text, maybe just make the subviews read it directly from somewhere else?
Class ParamsToPass {
static var data = [
"viewID" : [
["param1key": "param1value"]
["param2key": "param2value"]
]
]
}
On your update:
How is it possible to maintain in-method parameters while passing the method as a variable?
Maybe make the method return said parameters you need?
so - I have a text field on a screen - the data loads asynchronously - so I've created and ObservableObject with a published field, and successfully bind it to the view:
class Blah : ObservableObject
{
#Published var value : Double? = nil
init()
{
load_variable_async().then {result in self.value = result }
}
}
which works perfectly - the view reflects the value of the variable and everything.
But - I want it to work both ways. Published seems to be a bidirectional wrapper, so I want to add something like this:
init()
{
load_variable_async().then {result in self.value = result }
value.when_changed { new_value in asynchronously_save( variable) }
}
and I can't find any way of doing it. Everything I google for puts a sink or some call to a save in the view - which seems completely wrong to me... if I'm reading it in one place, I want to be writing it in the same place - and if I'm already binding the variable to a textfield for instance, and bindings go both ways, then enough connections have already been made
So what am I doing wrong? How do react to value being set, inside my "model" object without explicitly putting some sort of save or other action into the view?
So I found the solution I wanted - the binding from model -> textfield was always working, but the value from textfield -> model I couldn't get working. The magic seems to be this keyword "willSet"
#Published var value : Double? = nil {
willSet( new_value ) {
print("going to save asynchronously now")
}}
works magically.
I have a keyDown function in my application that is used to capture input from a NSTextView named textInput. Some conversions are done with the input which is appended as a NSAttributedString back into the NSTextView.
This works fine currently, but the problem I have is that the value entered into the textbox on keyDown doesn't get added to the textInput.textStorage?.string, until another key is pressed.
For example if I enter the text abcde and nothing more into textInput, and then inside func keyDown() I try to access textInput.textStorage?.string, it will return abcd.
Here is the function without unnecessary parts:
override func keyDown(with event: NSEvent) {
let bottomBox = textInput.textStorage?.string // This returns one character short of what is actually in the text box
if let bottomBox = bottomBox {
var attribute = NSMutableAttributedString(string: bottomBox)
// Do some stuff here with bottomBox and attribute
// Clear and set attributed string
textInput.textStorage?.mutableString.setString("")
textInput.textStorage?.append(attribute)
}
}
If I were to use keyUp, this isn't a problem, although the problem with keyUp is that if the user holds down the key, the attributes on the NSAttributedString don't get set until the user releases the key.
I though maybe there was a way to programatically release the keyDown event during the keyDown function, or generate a keyUp event, but can't seem to find anything.
Is there a way to fix this?
What I like to do is to use Cocoa Bindings with a property observer. Set up your properties like so:
class MyViewController: NSViewController {
#objc dynamic var textInput: String {
didSet { /* put your handler here */ }
}
// needed because NSTextView only has an "Attributed String" binding
#objc private static let keyPathsForValuesAffectingAttributedTextInput: Set<String> = [
#keyPath(textInput)
]
#objc private var attributedTextInput: NSAttributedString {
get { return NSAttributedString(string: self.textInput) }
set { self.textInput = newValue.string }
}
}
Now bind your text view to attributedTextInput with the "Continuously Updates Value" check box checked:
Et voilà, your property will be immediately updated every time you type a character, and your property's didSet will immediately be called.
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.