func callFunctionName(parameters: String) -> returnType
{
var somevalue = parameters
var returnValue = somevalue()
return returnValue
}
Is there a way to take in a input and use it as a function name?
example: let say input is green, I want to call function green. if input is red call function red etc...
Or to have a huge if statement to check each input to call different functions
This is not possible in Swift. You will have to store any functions you want to call in your own dictionary, and then use that to look up functions by name.
A "huge statement" might be feasible for a small number of functions, and it would certainly perform faster, but the ideal approach would be to store them in a dictionary.
However, if you are dealing with objects:
if exampleObject.respondsToSelector("exampleFunction")
{
exampleObject.performSelector("exampleFunction")
}
This approach currently works with all classes, be it Objective-C or Swift.
This is easily possible in Objective-C by using:
[self performSelector:NSSelectorFromString(#"green")];
But Swift is less dynamically typed than Objective-C and has less support for reflection. The Objective-C way I described above is very prone to crashes at runtime if the input (e.g. "purple" if you didn't have a function for purple) doesn't match a function that exists.
Using a big if statement is not an unreasonable way to approach it.
As the other answers said, an if statement is probably the best way to go about this.
override func viewDidLoad() {
if someValue = green {
green() //This will run whatever you have in the green() function below
}
}
func green() {
//put code for output of green here
}
Then all you have to do is make separate functions for all your outputs such as the green() func
This is the closest I could get. (And it's partially based on the answer of Vatsal Manot)
The idea is to use closures.
First of all we define the return type of the closure: let's use Int (of course you can change this later).
typealias colorClosureReturnType = Int
Now lets define the type of a closure that receives no parameters (you can change this too) and returns colorClosureReturnType
typealias colorClosureType = () -> (colorClosureReturnType)
Fine, now lets create a dictionary where the key is a String and the value is a closure of type colorClosureType:
let dict : [String: colorClosureType] = [
"red": { return 0 /* you can write here all the logic you need */ },
"green": { return 1 /* also here */},
"blue": { return 2 /* and here */}
]
Usually I let Swift Type Inference to infer the type of the variable/constant. But this time for sake of clarity I explicitly declared the type of the dictionary.
Now we can build a simple function that receives a String and return an optional colorClosureReturnType.
func callClosure(colorName: String) -> colorClosureReturnType? {
return dict[colorName]?()
}
As you can see the function look in the dictionary a closure associated to the key received as param. If it does found it then runs the closure and returns the results.
If the dictionary does not contain the requested key then the function returns nil. That's why the return type of this function is colorClosureReturnType? and not colorClosureReturnType.
Finally some tests:
callClosure("red") // 0
callClosure("green") // 1
callClosure("blue") // 2
func callFunctionName(parameters: String) -> ()
{
_ = NSTimer.scheduledTimerWithTimeInterval(0.1, target: self, selector: Selector(parameters), userInfo: nil, repeats: false)
}
func green() {
}
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 have in in my app an function. Is there a way to transfer it to other Viewcontroller? if I use UserDefaults.standard.set(function(), forKey: "function")
I don't know how to load it, because
let function() = UserDefaults.standard.object(forKey: "function") as? CGFunction
doesn't work.
Thanks for answers!
Passing and returning functions
The following function is returning another function as its result which can be later assigned to a variable and called.
func jediTrainer () -> ((String, Int) -> String) {
func train(name: String, times: Int) -> (String) {
return "\(name) has been trained in the Force \(times) times"
}
return train
}
let train = jediTrainer()
train("Obi Wan", 3)
Yes, Swift allows you to pass functions or closures to other objects, since functions and closures are themselves first class objects.
However, you cannot save a function or closure to UserDefaults. To the best of my knowledge there is no way to serialize functions or closures, and in any case they certainly are not one of the very small list of types that can be saved to UserDefaults. (Known as "property list objects" since the same small set of types can be store to both property lists and to UserDefaults.)
Why do you want to pass a function to another view controller?
In Swift, functions are Closures. You can simply pass closures in code.
Class A {
var someFunction: (Int) -> String
init(f: (Int) -> String) {
someFunction = f
}
}
func f2(a: Int) -> String {
return "Value is: \(a)"
}
let AInstance = A(f: f2)
print(AInstance.someFunction(5)) // prints "Value is: 5"
or specific someFunction as optional like var someFunction: ((Int) -> String)! and set it later in code.
I'm going to answer your initial question:
I have in in my app an function. Is there a way to transfer it to other Viewcontroller?
Yes, there is a way: use your segue rather than trying to store the function in userDefaults.
1) Make sure that the destination view controller has an instance variable that can hold your function. (Note that in Swift 4, you'll have to make sure you either set a default value for that variable, or create a custom initializer to ensure the variable is given a value on initialization.)
2) In the first view controller, wherever you handle your segue, instantiate your destination view controller. Then set the variable to your function. (You can do this, for example, in an override of the prepare(for segue: UIStoryboardSegue, sender: Any?) method.)
I am using Firebase to observe event and then setting an image inside completion handler
FirebaseRef.observeSingleEvent(of: .value, with: { (snapshot) in
if let _ = snapshot.value as? NSNull {
self.img = UIImage(named:"Some-image")!
} else {
self.img = UIImage(named: "some-other-image")!
}
})
However I am getting this error
Closure cannot implicitly capture a mutating self parameter
I am not sure what this error is about and searching for solutions hasn't helped
The short version
The type owning your call to FirebaseRef.observeSingleEvent(of:with:) is most likely a value type (a struct?), in which case a mutating context may not explicitly capture self in an #escaping closure.
The simple solution is to update your owning type to a reference once (class).
The longer version
The observeSingleEvent(of:with:) method of Firebase is declared as follows
func observeSingleEvent(of eventType: FIRDataEventType,
with block: #escaping (FIRDataSnapshot) -> Void)
The block closure is marked with the #escaping parameter attribute, which means it may escape the body of its function, and even the lifetime of self (in your context). Using this knowledge, we construct a more minimal example which we may analyze:
struct Foo {
private func bar(with block: #escaping () -> ()) { block() }
mutating func bax() {
bar { print(self) } // this closure may outlive 'self'
/* error: closure cannot implicitly capture a
mutating self parameter */
}
}
Now, the error message becomes more telling, and we turn to the following evolution proposal was implemented in Swift 3:
SE-0035: Limiting inout capture to #noescape contexts
Stating [emphasis mine]:
Capturing an inout parameter, including self in a mutating
method, becomes an error in an escapable closure literal, unless the
capture is made explicit (and thereby immutable).
Now, this is a key point. For a value type (e.g. struct), which I believe is also the case for the type that owns the call to observeSingleEvent(...) in your example, such an explicit capture is not possible, afaik (since we are working with a value type, and not a reference one).
The simplest solution to this issue would be making the type owning the observeSingleEvent(...) a reference type, e.g. a class, rather than a struct:
class Foo {
init() {}
private func bar(with block: #escaping () -> ()) { block() }
func bax() {
bar { print(self) }
}
}
Just beware that this will capture self by a strong reference; depending on your context (I haven't used Firebase myself, so I wouldn't know), you might want to explicitly capture self weakly, e.g.
FirebaseRef.observeSingleEvent(of: .value, with: { [weak self] (snapshot) in ...
Sync Solution
If you need to mutate a value type (struct) in a closure, that may only work synchronously, but not for async calls, if you write it like this:
struct Banana {
var isPeeled = false
mutating func peel() {
var result = self
SomeService.synchronousClosure { foo in
result.isPeeled = foo.peelingSuccess
}
self = result
}
}
You cannot otherwise capture a "mutating self" with value types except by providing a mutable (hence var) copy.
Why not Async?
The reason this does not work in async contexts is: you can still mutate result without compiler error, but you cannot assign the mutated result back to self. Still, there'll be no error, but self will never change because the method (peel()) exits before the closure is even dispatched.
To circumvent this, you may try to change your code to change the async call to synchronous execution by waiting for it to finish. While technically possible, this probably defeats the purpose of the async API you're interacting with, and you'd be better off changing your approach.
Changing struct to class is a technically sound option, but doesn't address the real problem. In our example, now being a class Banana, its property can be changed asynchronously who-knows-when. That will cause trouble because it's hard to understand. You're better off writing an API handler outside the model itself and upon finished execution fetch and change the model object. Without more context, it is hard to give a fitting example. (I assume this is model code because self.img is mutated in the OP's code.)
Adding "async anti-corruption" objects may help
I'm thinking about something among the lines of this:
a BananaNetworkRequestHandler executes requests asynchronously and then reports the resulting BananaPeelingResult back to a BananaStore
The BananaStore then takes the appropriate Banana from its inside by looking for peelingResult.bananaID
Having found an object with banana.bananaID == peelingResult.bananaID, it then sets banana.isPeeled = peelingResult.isPeeled,
finally replacing the original object with the mutated instance.
You see, from the quest to find a simple fix it can become quite involved easily, especially if the necessary changes include changing the architecture of the app.
If someone is stumbling upon this page (from search) and you are defining a protocol / protocol extension, then it might help if you declare your protocol as class bound. Like this:
protocol MyProtocol: class {
...
}
You can try this! I hope to help you.
struct Mutating {
var name = "Sen Wang"
mutating func changeName(com : #escaping () -> Void) {
var muating = self {
didSet {
print("didSet")
self = muating
}
}
execute {
DispatchQueue.global(qos: .background).asyncAfter(deadline: .now() + 15, execute: {
muating.name = "Wang Sen"
com()
})
}
}
func execute(with closure: #escaping () -> ()) { closure() }
}
var m = Mutating()
print(m.name) /// Sen Wang
m.changeName {
print(m.name) /// Wang Sen
}
Another solution is to explicitly capture self (since in my case, I was in a mutating function of a protocol extension so I couldn't easily specify that this was a reference type).
So instead of this:
functionWithClosure(completion: { _ in
self.property = newValue
})
I have this:
var closureSelf = self
functionWithClosure(completion: { _ in
closureSelf.property = newValue
})
Which seems to have silenced the warning.
Note this does not work for value types so if self is a value type you need to be using a reference type wrapper in order for this solution to work.
I want to run different functions depending on selected level Integer
so if selected level is 1 then runfunc1(), if 2 then runfunc2()...
I know this is possible using if else
if levelselected == 1 {
runfunc1()
} else if levelseletecd == 2 {
runfunc2()
// ... and so on
}
Is there any better way than this, perhaps something like this
runfunc%i(),levelselected // I know its not correct but something similar
I dont want to write new code for every level, so any better way?
You can use something like:
var levelSelected = 0 //
var selector = Selector("runFunc\(levelSelected)")
if self.respondsToSelector(selector) {
NSThread.detachNewThreadSelector(selector, toTarget: self, withObject: nil)
}
You could have an array or dictionary of functions. A dictionary might be nicer since the logic for checking if the level is valid is a lot simpler:
let funcs = [1: runfunc1, 2: runfunc2]
if let funcToRun = funcs[levelselected] {
funcToRun()
}
However, you won't be able to easily dynamically build a function name from strings and numbers without using #objc functionality.
(except in the sense that you could make the key to the dictionary a string of the function name, but you still have to build the dictionary using actual function names determined at compile time)
That said, you can add to the funcs variable from elsewhere in the code so it does mean to can "hook up" new levels without changing this dispatching logic.
Not the exact solution you are looking for but this can make it easier :
Declare an array of the desired functions:
var levelFunctions: [()->()] = [runfunc1, runfunc2, runfunc3]
This syntax declares an array of functions that have zero argument and return nothing. You initialize this array with the required function names and then execute the desired function using the levelselected variable:
levelFunctions[levelselected]() // Or levelselected-1 if the variable is not zero-based
EDIT:
As Airspeed Velocity mentioned in the comment and his answer you should make sure the level is in the array bounds.
I prefer to create a function, for example runFuncFromLevel::Int -> (() -> Void). runFuncFromLevel return a proper function that you need.
func runFuncFromLevel(level: Int) -> () -> Void
{
switch level
{
case 1: return runfunc1
case 2: return runfunc2
default: return {}
}
}
For convenience, in a little experiment I am doing, I would like to extend Array to provide some app specific functionalities. This specific extension is not necessary best practice, but I am just curious about solving the Swift issues I am having.
Given a custom class Section, my extension (with partially extended closure) is:
extension Array {
func onlyFullSection() -> Array<Section> {
return self.filter {
(a:Section) -> Bool in
return a.isFullSection()
}
}
}
The error I get is: "T" is not a subtype of "Section".
I tried to fix it with all the sauces (changing types, casting, etc...) but still get similar errors.
This other variant:
extension Array {
func onlyFullSection() -> Array<Section> {
return (self as Array<Section>).filter {
(a:Section) -> Bool in
return a.isFullSection()
} as Array<Section>
}
throws: Cannot convert the expression's type 'Array<Section>' to type 'Array<Section>'
Any clue on what I am doing wrong? Thanks!
It is because you are extending T[] and not Section[]. That means that Int[] will also have your additional method. That might not be the best idea (since it will crash badly).
Swift currently does not allow you to extend a specialised generic type like Section[].
But if you really, really want to do it, here is one way to force a cast, use reinterpretCast, which Apple describes as follows
/// A brutal bit-cast of something to anything of the same size
func reinterpretCast<T, U>(x: T) -> U
You can use it like this:
extension Array {
func onlyFullSection() -> Section[] {
let sections : Section[] = reinterpretCast(self)
return sections.filter{ $0.isFullSection() }
}
}
But please don't.
The problem is that since the Array class is actually a generic Array<T>, you are extending Array<T>. And apparently you can't cast between generic types (i.e. <T> to <Section>), so I believe you'll have to make a new array and just push the appropriate objects into it.
17> extension Array {
18. func onlyFullSection() -> Array<Section> {
19. var ary = Array<Section>()
20. for s in self {
21. if (s as Section).isFullSection() {
22. ary.append(s as Section)
23. }
24. }
25. return ary
26. }
27. }
You could also create a helper method to convert between generic types for you, but in this instance that would just create an unnecessary temporary object.
Remember that the language is still heavily in flux so it's possible this will change. I think it's unlikely that we'll get the ability to cast between generic types, but I hope we'll at least be able to extend particular generics.