I am building a UIPresentationController subclass. UIPresentationController defines an default initializer like this:
init(presentedViewController: UIViewController , presentingViewController: UIViewController?)
Now, in order for this to work, I want my presentedViewController to conform to some protocol, say MyRandomProtocol.
How can I re-write my initializer such that it takes the first argument as both a UIViewController subclass, and one that specifically conforms to MyRandomProtocol?
You can use generics.
init<T: UIViewController>(presentedViewController: T, ...) where T: MyRandomProtocol {
//initialization code
}
Related
I'm trying to write a delegate interface like this:
// This delegate is just a sample. It could be any delegate.
// What's important here is the third function's signature.
protocol MyViewDelegate {
func myView(_ myView: MyView, didDoSomething something: String)
func myView(_ myView: MyView, didDoAnotherThing thing: String at: Date)
func myView(_ myView: MyView, didDoYetSomethingElse)
}
However, the third function is invalid syntax. It's used to notify the delegate that some specific event happened, so the signature is important and I want to keep all function signatures consistent.
Question: What's the recommended signature for the third function?
It may not be very satisfying, but you simply can't do that. There is lots of precedent for writing these kinds of methods like this:
func myViewDidAskUserName(_ myView: MyView)
For example, a common one from Apple:
func applicationDidFinishLaunching(_ application: UIApplication)
You are trying to give the delegate a choice of two kinds of thing that happened: said something, or asked user name. That sort of choice among possibilities, in Swift, is an enum:
enum WhatHappened {
case didSaySomething(String)
case didAskUserName
}
Now write your method signature like this:
protocol MyViewDelegate {
func myView(_ myView: MyView, _ whatHappened: WhatHappened)
}
The method can be called by saying
myView(theView, .didSaySomething("hello"))
or by saying
myView(theView, .didAskUserName)
I have a method:
public func someMethod<Controller>(controller: Controller) {
print(controller)
typealias HandlerType = Controller -> UIViewController
let handler: HandlerType
print(handler.dynamicType)
}
After calling it with the instance of child of UIViewController, which is ViewController in my case, it prints:
<Test.ViewController: 0x7fc43be479d0>
UIViewController -> UIViewController
What is my goal is to have a typealias like this:
Test.ViewController -> UIViewController
I have typealias and the declared valriable:
public typealias ControllerType = UIViewController
public private(set) static var visibleController: ControllerType?
And the method invocation is:
if let controller = visibleController {
someMethod(controller)
}
I do not understand why ViewController becomes UIViewController in typealias, so if anyone faced with such kind of an issue, and/or have a solution, Please let me know.
Thank you in advance.
This is due to the fact that the static and dynamic type of your controller differ.
The static type is taken from your ControllerType typealias, which you've defined to be UIViewController. The dynamic type is the same type as the instance that visibleController points to – in this case Test.ViewController.
Because generics are inferred from the static type that you pass into the function (their specialisation takes place at compile-time), Controller will be inferred to be UIViewController. Therefore your typealias will be UIViewController -> UIViewController, and that's what dynamicType of your handler will return (it appears to fallback to the static type when unassigned).
On the other hand, the dynamic type of your controller will indeed be Test.ViewController, as that's the dynamicType of the instance that you pass in.
As for solutions, without knowing exactly what you're trying to achieve here, it's hard to say.
I tried write a static method for UIView which instantiates view of that class from the nib. Method should be generic and work on every UIView subclasses. Also I want to save the type information – so, for example, in this code
let myView = MyView.loadFromNib()
compiler infers that myView has MyView class. After few trials I decided to use protocol extensions, because otherwise I wouldn't have access to Self inside method body.
Looks like this should work:
protocol NibLoadable {
static func loadFromNib(name: String?) -> Self
}
extension NibLoadable where Self: UIView {
static func loadFromNib(name: String? = nil) -> Self {
let nibName = name ?? "\(self)"
let nib = UINib(nibName: nibName, bundle: nil)
return nib.instantiateWithOwner(nil, options: nil)[0] as! Self
}
}
extension UIView: NibLoadable {}
But it doesn't. I get compilation error
Method 'loadFromNib' in non-final class 'UIView' must return `Self` to conform to protocol 'NibLoadable'
And two strange things happen. First, if I change protocol declaration to
protocol NibLoadable {
static func loadFromNib(name: String?) -> UIView
}
Everything works just great, including type inference. And second, I can go further and remove where clause altogether:
extension NibLoadable {
...
}
And it keeps working!
So could anyone please explain me why my first variant fails, why second and third work fine and how it's related to final classes?
Here's my understanding of what you are seeing:
You get the compilation error Method 'loadFromNib' in non-final class 'UIView' must return 'Self' to conform to protocol 'NibLoadable' at the point where you declare extension UIView: NibLoadable {}. Let's look at what this statement means to the compiler. It's saying "UIView (and all of its subclasses since it is a non-final class) are adopting the NibLoadable protocol. It means, for UIView, that there will be method with the signature static func loadFromNib(name: String?) -> UIView, because Self in this context is UIView."
But what does this mean for subclasses of UIView? They inherit their conformance and might inherit the implementation of the method from UIView itself. So any subclass of UIView could have the method with the signature static func loadFromNib(name: String? = nil) -> UIView. However, the NibLoadable protocol that all subclasses also conform to says that the return type of that method must be Self. And in the case of any UIView subclass (for example, let's say "MyView"), the return type of the inherited method will be UIView and not MyView. So any subclass would then violate the contract of the protocol. I realize that your protocol extension uses Self and wouldn't create that issue, but technically, you could still also implement the method directly in a UIView extension, and it seems like the Swift compiler just won't allow it at all for that reason. A better implementation might find the Swift compiler verifying that a protocol extension exists which provides the implementation and there is no conflicting inherited implementation, but this appears to just not exist at this time. So for safety, my guess is that the compiler prevents ANY protocols that have methods with Self return types from being adopted by a non-final class. Thus the error you see.
However, making UIView a final class makes that whole inheritance of a non-conforming method possibility and issue go away, which fixes the error.
The reason why changing the return type in the protocol to UIView fixes everything is because not having 'Self' as the return type now relieves the compiler's concern about inherited versions of the method having a non-conforming return type. E.g., if UIView were to implement the method static func loadFromNib(name: String?) -> UIView, and subclasses inherited that method, the protocol contract would still hold for those subclasses, so there is no problem!
On the other hand, the type inference works, because the subclasses of UIView are getting their method implementation from the protocol extension (since the method is not implemented directly in UIView). That implementation returns the type Self, which tells the compiler that the returned value has the same type as the type the method was called on, and the protocol is satisfied, because any subclass of UIView will have a Self type that is a subclass of the required type of UIView.
Removing the where clause works only in this specific case because you changed the protocol method to return UIView, and the protocol extension defines a matching method implementation that returns Self, and then only UIView is getting that extension in your sample code. So the protocol requirement of the method returning UIView matches the implementation UIView is getting which returns Self (which happens to be UIView in this case). But, should you try to make any type other than UIView get the protocol extension method, e.g.
class SomeClass : NibLoadable {}
or even
class MyView:UIView, NibLoadable {}
the compiler wouldn't allow it, because the Self return type in the protocol extension method would not match the UIView required in the protocol. I feel like in the case of "MyView" or other UIView subclasses though, the compiler error might be a bug, since a method that returns MyView would satisfy the protocol requirement that a method return a UIView, if MyView did inherit from UIView.
To summarize some key points:
It doesn't look like the protocol extension has any role in the compiler error you noted. Just this will also create the error:
protocol NibLoadable {
static func loadFromNib(name: String?) -> Self
}
extension UIView: NibLoadable {}
So it looks like the compiler doesn't allow non-final classes to adopt protocols by using default implementations of methods that have a return type of Self, period.
If you change the protocol method signature to return UIView instead of Self that particular compiler warning goes away because there is no longer the possibility of subclasses inheriting a superclass return type and breaking the protocol. And you can then add conformance to the protocol to UIView with your protocol extension. However, you will get a different error if you try to adopt the protocol for any type other than UIView, because the protocol return type of UIView will not match the protocol extension method's return type of Self except in the single case of UIView. This may be a bug, in my opinion, because Self for any subclass of UIView should meet the required UIView return type contract.
But strangely enough, if you adopt the protocol in UIView only, subclasses of UIView will inherit their conformance to the protocol (avoiding the triggering of any of the two above compiler errors) and get their generic implementations from the protocol extension as long as UIView doesn't explicitly implement the protocol method itself. So the subclasses will get the type inference of appropriate Self, and meet the protocol contract for having that method return a UIView.
I'm pretty sure there are one or more bugs mixed up in all this, but someone on the Swift team would have to confirm that to be sure.
UPDATE
Some clarification from the Swift team in this Twitter thread:
https://twitter.com/_danielhall/status/737782965116141568
As suspected, it's a compiler limitation (though apparently not considered an outright bug) that protocol matching does not consider subtypes, only exact type matches. Which is why extension UIView:NibLoadable {} will work when the protocol method defines a return type of UIView, but extension MyView:NibLoadable {} will not.
Use following code should be ok(in Swift 3):
protocol Nibable {}
extension Nibable {
static func loadFromNib() -> Self? {
return Bundle.main.loadNibNamed(String(describing:
type(of:Self.self)), owner: nil, options: nil)?.first as? Self
}
}
final class ViewFromNib: UIView {}
extension ViewFromNib: Nibable {}
var nibView = ViewFromNib.loadFromNib()
I don't think so. The error I receive states Type 'GameViewController' does not conform to protocol 'MCBrowserViewDelegate' https://developer.apple.com/library/prerelease/ios/documentation/MultipeerConnectivity/Reference/MCBrowserViewController_class/index.html
Assuming that GameViewController is a subclass of UIViewController, you certainly can since MCBrowserViewController is a subclass of UIViewController as well.
The error you are receiving is saying that you are not conforming to the delegate protocols required to use MCBrowserViewController. This means that in order to use a MCBrowserViewController, you first need to add MCBrowserViewDelegate to your class declaration similar to the following.
class GameViewController: UIViewController, MCBrowserViewDelegate {
You will also want to set your GameViewController to be the delegate within viewDidLoad or wherever you create it.
// create the MCBrowserViewController
let browserViewController = MCBrowserViewController(...)
browserViewController.delegate = self
self.presentViewController(browserViewController, animated: true, completion:nil)
My app has a protocol for detail view controllers, stating they must have a viewModel property:
protocol DetailViewController: class {
var viewModel: ViewModel? {get set}
}
I also have a few different classes that implement the protocol:
class FormViewController: UITableViewController, DetailViewController {
// ...
}
class MapViewController: UIViewController, DetailViewController {
// ...
}
My master view controller needs a property that can be set to any UIViewController subclass that implements the DetailViewController protocol.
Unfortunately I can't find any documentation on how to do this. In Objective-C it would be trivial:
#property (strong, nonatomic) UIViewController<DetailViewController>;
It appears that there isn't any syntax available in Swift to do this. The closest I've come is to declare a generic in my class definition:
class MasterViewController<T where T:UIViewController, T:DetailViewController>: UITableViewController {
var detailViewController: T?
// ...
}
But then I get an error saying that "Class 'MasterViewController' does not implement its superclass's required members"
This seems like it should be as easy to do in Swift as it is in Objective-C, but I can't find anything anywhere that suggests how I might go about it.
I think you can get there by adding an (empty) extension to UIViewController and then specifying your detailViewController attribute using a composed protocol of the empty extension and your DetailViewController. Like this:
protocol UIViewControllerInject {}
extension UIViewController : UIViewControllerInject {}
Now all subclasses of UIViewController satisfy protocol UIViewControllerInject. Then with that, simply:
typealias DetailViewControllerComposed = protocol<DetailViewController, UIViewControllerInject>
class MasterViewController : UITableViewController {
var detailViewController : DetailViewControllerComposed?
// ...
}
But, this is not particularly 'natural'.
=== Edit, Addition ===
Actually, you could make it a bit better if you define your DetailViewController using my suggested UIViewControllerInject. Like such:
protocol UIViewControllerInject {}
extension UIViewController : UIViewControllerInject {}
protocol DetailViewController : UIViewControllerInject { /* ... */ }
and now you don't need to explicitly compose something (my DetailViewControllerComposed) and can use DetailViewController? as the type for detailViewController.
As of Swift 4, you can now do this.
Swift 4 implemented SE-0156 (Class and Subtype existentials).
The equivalent of this Objective-C syntax:
#property (strong, nonatomic) UIViewController<DetailViewController> * detailViewController;
Now looks like this in Swift 4:
var detailViewController: UIViewController & DetailViewController
Essentially you get to define one class that the variable conforms to, and N number of protocols it implements. See the linked document for more detailed information.
Another way would be to introduce intermediate base classes for the appropriate UIKit view controllers that implement the protocol:
class MyUIViewControler : UIViewController, DetailViewController ...
class MyUITableViewController : UITableViewController, DetailViewController ...
Then you inherit your view controllers from these view controllers, not the UIKit ones.
This is not natural either, but it doesn't force all your UIViewControllers to satisfy the UIViewControllerInject protocol as GoZoner suggested.