I've got couple of classes that inherit from SKNode.
Both of them have a setup() method. They are pretty different from each other.
I've made a protocol like this:
protocol LevelProtocol {
func setup(level: Int)
}
and two of my classes look something like thes:
class Puzzle: SKNode, LevelProtocol {
func setup(level: Int) {do something}
class Action: SKNode, LevelProtocol {
func setup(level: Int) {do something}
And in my main class I need to use one of these classes in one variable.
How do I define this variable properly?
I've tried this:
var lvl: LevelProtocol
if myLevel > 5 {
lvl = Puzzle()
else {
lvl = Action()
}
lvl.setup(myLevel)
self.addChild(lvl)
But the compiler says that LevelProtocol is not convertible to SKNode
I know Im doing something wrong, just cannot understand what exactly.
I'm not really good in Swift but I think you could try self.addChild(lvl as SKNode)
Another solution:
Well, I think, then you need to redesign the inheritance. Try making class Level inheriting from SKNode and declare just one method setup there and leave its body empty. Then you create classes Puzzle and Action deriving from Level and having their own implementation of setup. Then your code is:
var lvl: Level
if myLevel > 5 {
lvl = Puzzle()
}
else {
lvl = Action()
}
lvl.setup(myLevel)
self.addChild(lvl)
Related
I'm faced with the problem of using generic class and inheritance.
Brief description of the problem:
I have a base class called BookPageDataSource and two inherited classes (ReadingBookPageDataSource and StarsBookPageDataSource) with different implementations.
Also, I have a generic class BookPageViewController that contains the generic parameter of this data source and two inherited classes (ReadingBookPageViewController and StarsBookPageViewController) from this class.
I need to write a method the return parameter of which is BookPageViewController<DataSource>.
// Data Sources
class BookPageDataSource { }
class ReadingBookPageDataSource: BookPageDataSource { }
class StarsBookPageDataSource: BookPageDataSource { }
// Controllers
class BookPageViewController<DataSource: BookPageDataSource>: UIViewController {
let dataSource: DataSource
init(dataSource: DataSource) {
self.dataSource = dataSource
super.init(nibName: nil, bundle: nil)
}
required init?(coder aDecoder: NSCoder) {
return nil
}
}
final class ReadingBookPageViewController: BookPageViewController<ReadingBookPageDataSource> { }
final class StarsBookPageViewController: BookPageViewController<StarsBookPageDataSource> { }
// Communication
class Pager {
func currentPageController<DataSource>(at index: Int) -> BookPageViewController<DataSource> {
// for example
if index == 0 {
// How to remove the cast from the line below?
return readingPageController() as! BookPageViewController<DataSource>
}
return starsPageController() as! BookPageViewController<DataSource>
}
private func readingPageController() -> ReadingBookPageViewController {
return ReadingBookPageViewController(dataSource: ReadingBookPageDataSource())
}
private func starsPageController() -> StarsBookPageViewController {
return StarsBookPageViewController(dataSource: StarsBookPageDataSource())
}
}
The method currentPageController always crashes, because the DataSource is always equals to BookPageDataSource, not to ReadingBookPageDataSource or StarsBookPageDataSource.
Conceptual Discussion
Your concept for the architecture is flawed and this is leading to your issue.
Simple Generics Example
Here's a very simple example of a generic function, which just returns the value you give it:
func echo <T> (_ value: T) -> T { return value }
Because this function is generic, there is ambiguity about the type that it uses. What is T? Swift is a type-safe language, which means that ultimately there is not allowed to be any ambiguity about type whatsoever. So why is this echo function allowed? The answer is that when I actually use this function somewhere, the ambiguity about the type will be removed. For example:
let myValue = echo(7) // myValue is now of type Int and has the value 7
In the act of using this generic function I have removed the ambiguity by passing it an Int, and therefore the compiler has no uncertainty about the types involved.
Your Function
func currentPageController <DataSource> (at index: Int) -> BookPageViewController<DataSource>
Your function only uses the generic parameter DataSource in the return type, not in the input - how is the compiler supposed figure out what DataSource is?* I assume this is how you imagined using your function:
let pager = Pager()
let controller = pager.currentPageController(at: 0)
But now, what is the type of controller? What can you expect to be able to do with it? It seems that you're hoping that controller will take on the correct type based on the value that you pass in (0), but this is not how it works. The generic parameter is determined based on the type of the input, not the value of the input. You're hoping that passing in 0 will yield one return type, while 1 will yield a different one - but this is forbidden in Swift. Both 0 and 1 are of type Int, and the type is all that can matter.
As is usually the case with Swift, it is not the language/compiler that is preventing you from doing something. It is that you haven't yet logically formulated what is even is that you want, and the compiler is just informing you of the fact that what you've written so far doesn't make sense.
Solutions
Let's move on to giving you a solution though.
UIViewController Functionality
Presumably there is something that you wanted to use controller for. What is it that you actually need? If you just want to push it onto a navigation controller then you don't need it to be a BookPageViewController. You only need it to be a UIViewController to use that functionality, so your function can become this:
func currentPageController (at index: Int) -> UIViewController {
if index == 0 {
return readingPageController()
}
return starsPageController()
}
And you can push the controller that it returns onto a navigation stack.
Custom Functionality (Non-Generic)
If, however, you need to use some functionality which is specific to a BookPageViewController then it depends what it is you want to do. If there is a method on BookPageViewController like this:
func doSomething (input: Int) -> String
which doesn't make use of the generic parameter DataSource then probably you'll want to separate out that function into its own protocol/superclass which isn't generic. For example:
protocol DoesSomething {
func doSomething (input: Int) -> String
}
and then have BookPageViewController conform to it:
extension BookPageViewController: DoesSomething {
func doSomething (input: Int) -> String {
return "put your implementation here"
}
}
Now the return type of your function can be this non-generic protocol:
func currentPageController (at index: Int) -> DoesSomething {
if index == 0 {
return readingPageController()
}
return starsPageController()
}
and you can use it like this:
let pager = Pager()
let controller = pager.currentPageController(at: 0)
let retrievedValue = controller.doSomething(input: 7)
Of course, if the return type is no longer a UIViewController of any sort then you probably want to consider renaming the function and the related variables.
Custom Functionality (Generic)
The other option is that you can't separate out the functionality you need into a non-generic protocol/superclass because this functionality makes use of the generic parameter DataSource. A basic example is:
extension BookPageViewController {
func setDataSource (_ newValue: DataSource) {
self.dataSource = newValue
}
}
So in this case you really do need the return type of your function to be BookPageViewController<DataSource>. What do you do? Well, if what you really want is to use the setDataSource(_:) method defined above then you must have a DataSource object that you plan to pass in as an argument, right? If this is the case then we're making progress. Previously, you only had some Int value which you were passing into your function and the problem was that you couldn't specify your generic return type with that. But if you already have a BookPageDataSource value then it is at least logically possible for you to use this to specialize your
function.
What you say you want, however, is to just use an Int to get the controller at that index, regardless of what the DataSource type is. But if you don't care what the DataSource is of the returned BookPageViewController then how can you expect to set its DataSource to something else using the setDataSource(_:) method?
You see, the problem solves itself. The only reason you would need the return type of your function to be generic is if the subsequent functionality you need to make use of uses that generic type, but if this is the case then the controller you get back can't have just any old DataSource (you just wanted whichever one corresponds to the index you provide) - you need it to have exactly the type of DataSource which you plan to pass in when you use it, otherwise you're giving it the wrong type.
So the ultimate answer to your question is that, in the way that you were conceiving of it, there is no possible use for the function you were trying to construct. What's very cool about the way Swift is architected is that the compiler is actually able to figure out that logical flaw and prevent you from building your code until you've re-conceptualized it.
Footnote:
* It is possible to have a generic function which only uses the generic parameter in the return type and not in the input, but this won't help you here.
I need to implement a class (for convenience name it A) derived from UITableViewController and another(B) from UICollectionViewController. And there are a lot of things common, so I want to put them in class(C) and let A and B inherit C. Now A and B both have two class to inherit, but multiple inheritance is not allowed in swift, so how to implement this? I know there is no multi-inheritance allowed in swift, but I still want to know how to do the things I described above.
As stated in the comments by #Paulw11 is correct. Here is an example that involves A & B inheriting from C. Which I have named DogViewController and CatViewController (which inherits form PetViewController). You can see how a protocol might be useful. This is just an ultra basic example.
protocol Motion {
func move()
}
extension Motion where Self: PetViewController {
func move() {
//Open mouth
}
}
class PetViewController: UIViewController, Motion{
var isLoud: Bool?
func speak(){
//Open Mouth
}
}
class DogViewController:PetViewController {
func bark() {
self.speak()
//Make Bark Sound
}
}
class CatViewController: PetViewController {
func meow() {
self.speak()
//Make Meow Sound
}
}
//....
let cat = CatViewController()
cat.move()
cat.isLoud = false
cat.meow()
You can't have multiple inheritance in Swift, the way to go is to look at Protocols, but it is a rather big topic to be discussed in an answer.
There are also many other questions with the same scope
Multiple inheritance is not allowed in Swift. You can conform any protocol instead. Protocols are like interfaces in Java.
I am trying a bit of type (aka class) method but am confused on the real world application of such methods. e.g. In the following code from tutorialspoint.com -
class Math
{
class func abs(number: Int) -> Int
{
if number < 0
{
return (-number)
}
else
{
return number
}
}
}
let no = Math.abs(-35)
println(no)
So my question is that what is happening here when I am writing a type method. At what point of my programming may I need this. Can any one explain with a bit clear and simple example.
these kinds of functions are useful when you dont actually need an instance of the type to be made to be able to call it, eg helper methods. take the example you posted, if you call the abs function, you dont really need to make a Math object instantiated to do that (you could be seems unnecessary).
if your abs function wasnt a type method, you would have to go like this
var mathObject = Math()
mathObject.abs(-35)
as apposed to the way you have it in you example
Math.abs(-35)
both statements achieve the same goal, but the 2nd is more elegant (and memory efficient).
there are other reasons as well for using type methods, but this is just the simplest example of one (look up what a singleton is, for another example)
class C {
class func foo(){}
// Type method is always static !!!
static func boo() {}
}
class D: C {
override class func foo() {}
// this is not possible for 'Type method'
override static func boo() {} // error !!!!
}
A WWDC 2015 session video describes the idea of Protocol-Oriented Programming, and I want to adopt this technique in my future apps. I've been playing around with Swift 2.0 for the last couple of days in order to understand this new approach, and am stuck at trying to make it work with the Delegate Pattern.
I have two protocols that define the basic structure of the interesting part of my project (the example code is nonsense but describes the problem):
1) A delegation protocol that makes accessible some information, similar to UITableViewController's dataSource protocol:
protocol ValueProvider {
var value: Int { get }
}
2) An interface protocol of the entity that does something with the information from above (here's where the idea of a "Protocol-First" approach comes into play):
protocol DataProcessor {
var provider: ValueProvider { get }
func process() -> Int
}
Regarding the actual implementation of the data processor, I can now choose between enums, structs, and classes. There are several different abstraction levels of how I want to process the information, therefore classes appear to fit best (however I don't want to make this an ultimate decision, as it might change in future use cases). I can define a base processor class, on top of which I can build several case-specific processors (not possible with structs and enums):
class BaseDataProcessor: DataProcessor {
let provider: ValueProvider
init(provider: ValueProvider) {
self.provider = provider
}
func process() -> Int {
return provider.value + 100
}
}
class SpecificDataProcessor: BaseDataProcessor {
override func process() -> Int {
return super.process() + 200
}
}
Up to here everything works like a charm. However, in reality the specific data processors are tightly bound to the values that are processed (as opposed to the base processor, for which this is not true), such that I want to integrate the ValueProvider directly into the subclass (for comparison: often, UITableViewControllers are their own dataSource and delegate).
First I thought of adding a protocol extension with a default implementation:
extension DataProcessor where Self: ValueProvider {
var provider: ValueProvider { return self }
}
This would probably work if I did not have the BaseDataProcessor class that I don't want to make value-bound. However, subclasses that inherit from BaseDataProcessor and adopt ValueProvider seem to override that implementation internally, so this is not an option.
I continued experimenting and ended up with this:
class BaseDataProcessor: DataProcessor {
// Yes, that's ugly, but I need this 'var' construct so I can override it later
private var _provider: ValueProvider!
var provider: ValueProvider { return _provider }
func process() -> Int {
return provider.value + 10
}
}
class SpecificDataProcessor: BaseDataProcessor, ValueProvider {
let value = 1234
override var provider: ValueProvider { return self }
override func process() -> Int {
return super.process() + 100
}
}
Which compiles and at first glance appears to do what I want. However, this is not a solution as it produces a reference cycle, which can be seen in a Swift playground:
weak var p: SpecificDataProcessor!
autoreleasepool {
p = SpecificDataProcessor()
p.process()
}
p // <-- not nil, hence reference cycle!
Another option might be to add class constraints to the protocol definitions. However, this would break the POP approach as I understand it.
Concluding, I think my question boils down to the following: How do you make Protocol Oriented Programming and the Delegate Pattern work together without restricting yourself to class constraints during protocol design?
It turns out that using autoreleasepool in Playgrounds is not suited to proof reference cycles. In fact, there is no reference cycle in the code, as can be seen when the code is run as a CommandLine app. The question still stands whether this is the best approach. It works but looks slightly hacky.
Also, I'm not too happy with the initialization of BaseDataProcessors and SpecificDataProcessors. BaseDataProcessors should not know any implementation detail of the sub classes w.r.t. valueProvider, and subclasses should be discreet about themselves being the valueProvider.
For now, I have solved the initialization problem as follows:
class BaseDataProcessor: DataProcessor {
private var provider_: ValueProvider! // Not great but necessary for the 'var' construct
var provider: ValueProvider { return provider_ }
init(provider: ValueProvider!) {
provider_ = provider
}
func process() -> Int {
return provider.value + 10
}
}
class SpecificDataProcessor: BaseDataProcessor, ValueProvider {
override var provider: ValueProvider { return self } // provider_ is not needed any longer
// Hide the init method that takes a ValueProvider
private init(_: ValueProvider!) {
super.init(provider: nil)
}
// Provide a clean init method
init() {
super.init(provider: nil)
// I cannot set provider_ = self, because provider_ is strong. Can't make it weak either
// because in BaseDataProcessor it's not clear whether it is of reference or value type
}
let value = 1234
}
If you have a better idea, please let me know :)
This is my inheritance structure
Protocols
protocol BaseProtocol {
}
protocol ChildProtocol: BaseProtocol {
}
Classes
class BaseClass: NSObject {
var myVar: BaseProtocol!
}
class ChildClass: BaseClass {
override var myVar: ChildProtocol!
}
I'm receiving a compiler error:
Property 'myVar' with type 'ChildProtocol!' cannot override a property with type 'BaseProtocol!'
What is the best approach to achieve this?
UPDATE
I updated the question trying to implement the solution with generics but it does not work :( This is my code (now the real one, without examples)
Protocols
protocol TPLPileInteractorOutput {
}
protocol TPLAddInteractorOutput: TPLPileInteractorOutput {
func errorReceived(error: String)
}
Classes
class TPLPileInteractor<T: TPLPileInteractorOutput>: NSObject, TPLPileInteractorInput {
var output: T!
}
And my children
class TPLAddInteractor<T: TPLAddInteractorOutput>: TPLPileInteractor<TPLPileInteractorOutput>, TPLAddInteractorInput {
}
Well, inside my TPLAddInteractor I can't access self.output, it throws a compiler error, for example
'TPLPileInteractorOutput' does not have a member named 'errorReceived'
Besides that, when I create the instance of TPLAddInteractor
let addInteractor: TPLAddInteractor<TPLAddInteractorOutput> = TPLAddInteractor()
I receive this other error
Generic parameter 'T' cannot be bound to non-#objc protocol type 'TPLAddInteractorOutput'
Any thoughts?
#tskulbru is correct: it can't be done, and this has nothing to do with your protocols. Consider the example below, which also fails…this time with Cannot override with a stored property 'myVar':
class Foo {
}
class Goo: Foo {
}
class BaseClass: NSObject {
var myVar: Foo!
}
class ChildClass: BaseClass {
override var myVar: Foo!
}
To understand why, let's reexamine the docs:
Overriding Properties
You can override an inherited instance or class property to provide
your own custom getter and setter for that property, or to add
property observers to enable the overriding property to observe when
the underlying property value changes.
The implication is that if you are going to override a property, you must write your own getter/setter, or else you must add property observers. Simply replacing one variable type with another is not allowed.
Now for some rampant speculation: why is this the case? Well, consider on the one hand that Swift is intended to be optimized for speed. Having to do runtime type checks in order to determine whether your var is in fact a Foo or a Bar slows things down. Then consider that the language designers likely have a preference for composition over inheritance. If both of these are true, it's not surprising that you cannot override a property's type.
All that said, if you needed to get an equivalent behavior, #tskulbru's solution looks quite elegant, assuming you can get it to compile. :)
I don't think you can do that with protocols
The way i would solve the problem you are having is with the use of generics. This means that you essentially have the classes like this (Updated to a working example).
Protocols
protocol BaseProtocol {
func didSomething()
}
protocol ChildProtocol: BaseProtocol {
func didSomethingElse()
}
Classes
class BaseClass<T: BaseProtocol> {
var myProtocol: T?
func doCallBack() {
myProtocol?.didSomething()
}
}
class ChildClass<T: ChildProtocol> : BaseClass<T> {
override func doCallBack() {
super.doCallBack()
myProtocol?.didSomethingElse()
}
}
Implementation/Example use
class DoesSomethingClass : ChildProtocol {
func doSomething() {
var s = ChildClass<DoesSomethingClass>()
s.myProtocol = self
s.doCallBack()
}
func didSomething() {
println("doSomething()")
}
func didSomethingElse() {
println("doSomethingElse()")
}
}
let foo = DoesSomethingClass()
foo.doSomething()
Remember, you need a class which actually implements the protocol, and its THAT class you actually define as the generic type to the BaseClass/ChildClass. Since the code expects the type to be a type which conforms to the protocol.
There are two ways you can go with your code, depending what you want to achieve with your code (you didn't tell us).
The simple case: you just want to be able to assign an object that confirms to ChildProtocol to myVar.
Solution: don't override myVar. Just use it in ChildClass. You can do this by design of the language Swift. It is one of the basics of object oriented languages.
Second case: you not only want to enable assigning instances of ChildProtocol, you also want to disable to be able to assign instances of BaseProtocol.
If you want to do this, use the Generics solution, provided here in the answers section.
If you are unsure, the simple case is correct for you.
Gerd