How to override 'UIScrollView.delegate'? - ios

When I look at UITableView, I find that UITableView 's UITableViewDelegate inherit from UIScrollViewDelegate in order to override UIScrollView's delegate
Here is how i implement my own subclass of 'UIScrollView'.
protocol UIFormViewDelegate:NSObjectProtocol, UIScrollViewDelegate{
}
class UIFormView: UIScrollView {
override var delegate:UIFormViewDelegate?
}
The problem is that the compiler gives a warning:
Property 'delegate' with type 'UIFormViewDelegate?' cannot override a property with type 'UIScrollViewDelegate?'
Can anybody tell me what's wrong here?

You cannot override the type of delegate
What you can do is something like that
class DelegateClass: NSObject, UIFormViewDelegate {
}
class UIFormView: UIScrollView {
func setUpDelegate() {
self.delegate = DelegateClass()
}
}
And then call the setUpDelegate from somewhere where it makes sense

Related

Swift 4.2: Overriding 'restoreUserActivityState' must be as available as declaration it overrides

I'm trying to implement Siri Shortcuts. To handle them I have to override the restoreUserActivityState function, but when I override it into my class which inherits from UIViewController, it results into this error: "Overriding 'restoreUserActivityState' must be as available as declaration it overrides". I tried to make my class and my function public but the error persists. Any idea how can I resolve this issue?
When you override a overridden method from a superclass, you make sure that, you are setting method in the subclass with higher access level than the superclass you inherit from. You can use open keyword.
Example:
class ViewController1: UIViewController {
override func restoreUserActivityState(_ activity:NSUserActivity) {
}
}
class ViewController2: ViewController1 {
open override func restoreUserActivityState(_ activity: NSUserActivity) {
}
}
I Hope, it solves your problem.

Requiring a UIViewController subclass to override a specific method

I've got a superclass UIViewController class that implements a couple of default methods, e.g.:
class Super: UIViewController {
override func viewDidLoad () {
setupView()
}
func setupView () {
initToolbar()
setPageTitle()
}
func initToolbar () {
// some code..
}
func setPageTitle () {
// nothing?
}
}
and subclasses that inherit from Super:
class Sub: Super {
override func setPageTitle () {
self.title = "custom title"
}
}
I'd like to force all subclasses to override the setPageTitle() method (forcing a compile time error if no implementation is present). However, the only way I've managed to achieve this is providing a default implementation in the Super class that contains an assert statement, causing the app to crash if it has not been overriden. This is not really what I was after as the error is only present at runtime and ideally i'd like a compile time warning/error if the method has no implementation. Is there any way to set this method to be overriden as a requirement? Similar to abstract methods in other languages?
I thought about using protocols & extensions but it looks like with extensions I can't override the viewDidLoad method and this is necessary for the superclass.
Any ideas?

Alternative to override extension's method

I want to extend UIView by adding some functions, and override them in any subclass of UIView that I want. I found in apple documentations that I can't override extensions (and the compiler will complain) which make some sense. So
I need someone to suggest an alternative way to the below:
extension UIView {
func hide() { //do almost nothing }
}
class myLabel: UILabel {
override func hide() {
//do work on uilabel that can't be done on imgView
}
}
class myImageView: UIImageView {
override func hide() {
//do work on imgView that can't be done on uilabel
}
}
And the reason I want this is that later in my code I will face the below code and I have to many subclasses and I don't want to write too many if-lets trying to cast the view to myLabel, myTextView, myImageView... etc
let view = cell.viewWithTag(someTag)
// and I want to write this below without casting
view.hide()
I tried with protocols and protocol extensions but I couldn't make it though.
Any thoughts?
Note: func hide() is just an example. My func will have more to do.
**Question updated to be clear.
EDIT: Updating answer to make use of protocols also
Protocols does in various ways enable to you replace subclassing in some cases however you still need your class to conform to the protocol to be able to see and override those methods
You can have a protocol for example:
protocol SomeProtocol {
func hide()
}
To do what you are intending to do it is best to have a parent subclass UIView with all functions that can be overridden for example (in this updated answer you can have your methods to override inside the protocol and have your subclasses conform to it):
class ParentView : UIView, SomeProtocol {
func hide() {
print("PARENT")
}
func anyOtherMethod() {
}
}
and then have all the other UIView's that need to override those methods subclass ParentView:
class ViewOne : ParentView {
override func hide() {
print("VIEW ONE")
}
}
class ViewTwo : ParentView {
override func hide() {
print("VIEW TWO")
}
}
So even if you later place this code:
let view = cell.viewWithTag(someTag)
// and I want to write this below without casting
view.hide()
you won't need to explicitly cast your UIView's, the view will call it's intended overridden method, unless and until you call super in your overridden method also
EDIT: More on making use of protocols
In the case you need other controls to also have a hide() method to override then you can still have to subclass, for example in the case of UILabel you need to override it:
class ParentLabel : UILabel, SomeProtocol {
func hide() {
print("PARENT LABEL")
}
}
then you can write the intended code with casting to your protocol
if let view = cell.viewWithTag(someTag) as? SomeProtocol {
view.hide() // prints PARENT LABEL
}
and either use that subclassed UILabel control or if you need in some cases some label to override that behavior then you can still create a child subclass of ParentLabel:
class LabelOne : ParentLabel {
override func hide() {
print("LABEL ONE")
}
}

Delegate declared in UIView subclass won't work in UIViewController?

I'm trying to create a reusable UIView that I can place in multiple UIViewControllers. I gave it delegate methods that I want the parent UIViewControllers to access, but it throws me an error (commented in the code below). What's a better way I can solve this?
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var cameraView: CameraView!
override func viewDidLoad() {
super.viewDidLoad()
self.cameraView.delegate = self
//ERROR: Cannot assign a value of type 'viewController' to a value of type 'CameraViewDelegate?'
}
}
protocol CameraViewDelegate {
func cameraViewShutterButtonTapped()
func cameraViewimagePickerTapped(imageData: NSData)
}
class CameraView: UIView {
var delegate:CameraViewDelegate?
//Ect...
}
You have not specified that ViewController conforms to the CameraViewDelegate protocol. You should amend your code to this:
class ViewController: UIViewController, CameraViewDelegate {
…at which point Xcode will complain that you have not implemented cameraViewShutterButtonTapped() and cameraViewimagePickerTapped(), which at least tells you that you're on the right track!
Side note: do you really want the camera view to have a strong reference to its delegate? You might want that to be weak.
You need to have your ViewController class implement the CameraViewDelegate protocol, like so:
class ViewController : UIViewController, CameraViewDelegate { ... }

Overriding delegate property of UIScrollView in Swift (like UICollectionView does)

UIScrollView has a delegate property which conforms to UIScrollViewDelegate
protocol UIScrollViewDelegate : NSObjectProtocol {
//...
}
class UIScrollView : UIView, NSCoding {
unowned(unsafe) var delegate: UIScrollViewDelegate?
//...
}
UICollectionView overrides this property with a different type UICollectionViewDelegate
protocol UICollectionViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
//...
}
class UICollectionView : UIScrollView {
unowned(unsafe) var delegate: UICollectionViewDelegate?
//...
}
When I try to override UIScrollViews delegate with my protocol like so:
protocol MyScrollViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
//...
}
class MyScrollView: UIScrollView {
unowned(unsafe) var delegate: MyScrollViewDelegate?
}
the compiler gives me two warnings:
Property 'delegate' with type 'MyScrollViewDelegate?' cannot override a property with type 'UIScrollViewDelegate?'
'unowned' cannot be applied to non-class type 'MyScrollViewDelegate?'
How can I subclass UIScrollView and override type of delegate property (i.e. use a custom delegate protocol) ?
I think overriding an inherited property is something that's possible in Objective-C but not (at least currently) in Swift. The way I've handled this is to declare a separate delegate as a computed property of the correct type that gets and sets the actual delegate:
#objc protocol MyScrollViewDelegate : UIScrollViewDelegate, NSObjectProtocol {
func myHeight() -> CGFloat
// ...
}
class MyScrollView: UIScrollView {
var myDelegate: MyScrollViewDelegate? {
get { return self.delegate as? MyScrollViewDelegate }
set { self.delegate = newValue }
}
}
This way anything that calls the scroll view delegate normally still works, and you can call your particular delegate methods on self.myDelegate, like this:
if let height = self.myDelegate?.myHeight() {
// ...
}
You can do like this:
protocol ExtendedUIScrollViewDelegate: UIScrollViewDelegate {
func someNewFunction()
}
class CustomScrollView: UIScrollView {
weak var myDelegate: ExtendedScrollViewDelegate?
override weak var delegate: UIScrollViewDelegate? {
didSet {
myDelegate = delegate as? ExtendedScrollViewDelegate
}
}
}
Hope this helps
My favoured method personally is not to subclass scrollviews directly but to make a UIView subclass containing and acting as delegate for a separate scrollview, then forward that scrollview's delegate messages on to the UIView subclass's own delegate where necessary. This also allows for the adding of custom controls outside of the area defined by the scroll view. It may seem a little inelegant compared to a direct subclass, but it does at least avoid unpleasant hacks.
Here is a solution for changing the type of the overriding properties in Swift. It is especially useful when you need to extend protocols of delegates.
#objc protocol ExtendedUIScrollViewDelegate: UIScrollViewDelegate {
func someNewFunction()
}
class CustomScrollView: UIScrollView {
weak var delegateInterceptor: ExtendedScrollViewDelegate?
override var delegate: UIScrollViewDelegate! {
didSet {
if let newValue = delegate {
let castedDelegate = unsafeBitCast(delegate, ExtendedScrollViewDelegate.self)
delegateInterceptor = castedDelegate
}
else {
delegateInterceptor = nil
}
}
}
}
This works as tested with Swift version 1.2. I hope it helps.
You can override get and set method by declare function like:
func setDelegate(delegate:UITableViewDelegate?){
self.delegateInterceptor = delegate;
}
swift compiler the property to method as Objective-c does.
Consider the following situation:
class BaseProp {}
class Base {
var prop: BaseProp
}
Then if you do this:
class DerivedProp: BaseProp {}
class Derived: Base {
override var prop: DerivedProp
}
Then if would break the subclassing principles (namely, the Liskov Substitution Principle). Basically what you are doing is limiting the scope of "var prop" from wider "BaseProp" type to a more narrow "DerivedProp" type. Then this kind of code would be possible, which does not make sense:
class UnrelatedProp: BaseProp {}
let derived = Derived()
let base = derived as Base
base.prop = UnrelatedProp()
Note that we are assigning an instance of UnrelatedProp to the property, which does not make sense for the Derived instance which we actually operate with. ObjectiveC allows such kind of ambiguity, but Swift doesn't.

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