I'm having a real hard time trying to create an extension that converts a Single array to an Observable. So for example I have a Single<[Address]> and I want to convert it to an Observable<Address>.
The code I have to write each time I want to do this is
mySingleVariable.asObservable().flatMap({ addresses in Observable.from(addresses) })
This operation is very common and the code is quite verbose. My attempt to create an extension looks like the following
extension PrimitiveSequence where Trait == SingleTrait {
func flatObservable<R: Collection>() -> Observable<R.Element> {
return asObservable().flatMap({ element in Observable.from(element) })
}
}
The code above unfortunately does not work. The error I get is "Generic parameter 'R' is not used in function signature". This is because the function returns Observable<R.Element>. If it were to return Observable<R>, the error would go away, but it's not the result I'm trying to achieve.
Try this:
extension PrimitiveSequence where Trait == SingleTrait, Element: Sequence {
func flatObservable<R>() -> Observable<R> where R == Element.Element {
return asObservable().flatMap { Observable.from($0) }
}
}
Related
this is my first question and I hope you guys can help me out.
Lets assume I've written a Framework in Swift that does stuff purely with native Swift Types. This is our interface:
class Foo {
#discardableResult
public func perform(query: String) -> [String: Any]? {
return [:]
}
}
Note that our result is discardable.
I would now like to support other Libraries optionally. I know we have Codable and stuff but lets assume that I want to support SwiftyJSON optionally by creating an extension that only compiles when SwiftyJSON can be imported.
#if canImport(SwiftyJSON)
import SwiftyJSON
extension Foo {
#discardableResult
func perform(query: String) -> JSON? {
// Call internal func => cast JSON => return JSON
return JSON()
}
}
#endif
Without SwiftyJSON (can not import) this compiles:
let bar = Foo().perform(query: "")
With SwiftyJSON it does not because its ambiguous.
Return type has to be explicitly defined like:
let baz: [String: Any]? = Foo().perform(query: "")
// or
let bar: JSON? = Foo().perform(query: "")
For calls where I want the result thats fine.
BUT: Since we have a discardableResult calls like
Foo().perform(query: "")
will always be ambiguous.
The following calls work but are not very nice imho.
Foo().perform(query: "") as JSON?
Foo().perform(query: "") as [String: Any]?
What is the best way to deal with this problem? Should the method just not be overloaded and have a different name instead? Or am I overlooking something that makes the calls non ambiguous in a nice way?
Edit:
A comment suggested to remove discardableResult. I really do not wanna do that since that would lead to a lot of calls that look like this:
_ = Foo().perform(query: "")
The more I think about the problem it occurs to me that there might just not be a good solution for this..
Actually, removing #discardableResult wouldn't work, and _ = Foo().perform(query: "") wouldn't compile, it would still have the same ambiguity: it's not that the compiler is being nitpicky, but it has literally no way of knowing which of your two functions to call!
It might be interesting to think about what behavior you'd expect from this line, and why.
You seem to want to have a function based on an existing function which:
Returns a different type;
Has the same name and parameters;
Has a discardable result.
Unfortunately, it looks like it's a classic "pick any two" type of scenario…
Well, let's do just that! Let's see what happens if we stop insisting on any one of the three conditions.
Let me use a simpler example:
#discardableResult func doSomething() -> String {
return "a"
}
#discardableResult func doSomething() -> Int {
return 1
}
doSomething() // won't compile
let x:String
x = doSomething() // "a"
We have a function overloaded with two return types (String and Int), and we see that we cannot discard its result as it would result in unresolvable ambiguity.
1. Single return type: use an enum with a payload
Let's eliminate the first condition, and try using a single return type. This is possible:
enum ReturnType {
case text(_ payload:String)
case number(_ payload:Int)
}
#discardableResult func doSomething() -> ReturnType {
if ... // some condition
{
return .text("a")
} else {
return .number(1)
}
}
doSomething() // works
let x = doSomething() // ReturnType.text("a") or ReturnType.number(1)
Here we cannot (easily) extend the functionality via extensions; rather, we'd need to touch the enum and function code with all the possible options every time we want to add a new return type. (Of course advanced APIs can also be created that help third parties easily write extensions… If it's worth the effort.)
We would also need a way to determine which return type to choose: it can be a function parameter, or a value stored elsewhere, etc. Here's an example with a parameter:
#discardableResult func doSomething(useText:Bool = false) -> ReturnType {
if useText {
return .text("a")
} else {
return .number(1)
}
}
doSomething() // works
doSomething(useText:true) // works
let x = doSomething() // ReturnType.number(1)
let x2 = doSomething(useText:false) // ReturnType.number(1)
let x3 = doSomething(useText:true) // ReturnType.text("a")
Note: Here, we lose the convenience of having the compiler infer the type from the call site; since the type is now an opaque wrapper, it's no longer the compiler's business to make sure that the wrapped type is correct. This may be a high cost for simply maintaining "result discardability," but then it may also allow us to abstract away some details, only unwrapping the "payload" when needed, which can have its own benefits.
2. Change the name or parameters of the function
This is quite easy:
#discardableResult func doSomething() -> String {
return "a"
}
#discardableResult func doSomethingElse() -> Int {
return 1
}
doSomething() // works
doSomethingElse() // works
let x = doSomething() // "a"
let y = doSomethingElse() // 1
We can also use this in an extension. All we lose is the shared name.
Changing the parameters is also possible but it would be pretty silly in this already-silly example:
#discardableResult func doSomething() -> String {
return "a"
}
#discardableResult func doSomething(thinkOfAnyNumber:Int) -> Int {
return 1
}
doSomething() // "a"
doSomething(thinkOfAnyNumber:42) // 1
Note: I'd obviously only do this if the additional parameters actually make sense.
Note 2: The parameter configuration is different between the two functions, not the parameter values like in the previous case.
3. Do not make the result discardable
Note that simply removing the #discardableResult attribute won't make it impossible to try to discard the result: you'll still be able to try the _ = assignment, or simply ignore the function result (and the compiler warning). Both will ultimately fail to compile, so it will be up to the user of the API to avoid doing either.
Hopefully, your function does some other things (side effects) than provide a return value: in that case, there may be little use in discarding the result of a function that does nothing but provide that result, and one may probably be better off not calling it in the first place.
If the side effects are identical between the two overloaded functions, you can factor them out into a single function:
func doTheActualWork() {
// ...
}
func doSomething() -> String {
doTheActualWork()
return "a"
}
func doSomething() -> Int {
doTheActualWork()
return 1
}
doSomething() // won't compile
doTheActualWork() // use this instead
let z:String = doSomething() // "a"
Note that this can also be done via extensions as long as they re-use existing functions and only overload them with different signatures and/or return types.
Now if the side effects (i.e. doTheActualWork() implementations) are also different in the two cases… Then I give up. But that can be handled as well.
Well, this is what I've been able to gather on this interesting problem. Now I may be dead wrong, and there may be something better out there… We'll find out soon enough.
The following code shows an implementation for a function with a return value.
func isValid() -> Bool {
self.txtComment.text.isEmpty
}
As you may noticed the return keyword not used. even-though, there is no syntax error!
Is this logically correct? or there will be a logical problems?
It's called Functions With an Implicit Return:
If the entire body of the function is a single expression, the function implicitly returns that expression.
Added in swift 5.1. You can check the proposal
Yes, that is logically correct as from swift 5.1. You do not need to explicitly type the return keyword anymore if your function only has one line of code.
Swift lets us skip using the return keyword when we have only one expression in our function. So, these two functions do the same thing:
func doSomething() -> Int {
return 5 + 5
}
func doSomethingMore() -> Int {
5 + 5
}
Well, the ternary operator works here too. For instance:
func sayHello(to name: String) -> String {
if name == "Hasan" {
return "Hey Smart!"
} else {
return "Hello, \(name)"
}
}
we could use the ternary operator like this:
func sayHello(to name: String) -> String {
name == "Hasan" ? "Hey Smart!" : "Hello, \(name)"
}
So, when that function runs it effectively boils down to this:
func greet(name: String) -> String {
"Hey Smart!"
}
reference:
hackingWithSwift
swift-evolution
just trying to implement SwiftUI and Combine in my new project.
But stuck in this:
func task() -> AnyPublisher<Int, Error> {
return AnyPublisher { subscriber in
subscriber.receive(Int(arc4random()))
subscriber.receive(completion: .finished)
}
}
This produces the following compiler error:
Type '(_) -> ()' does not conform to protocol 'Publisher'
Why?
Update
Actually Random here is just as an example. The real code will look like this:
func task() -> AnyPublisher<SomeCodableModel, Error> {
return AnyPublisher { subscriber in
BackendCall.MakeApiCallWithCompletionHandler { response, error in
if let error == error {
subscriber.receive(.failure(error))
} else {
subscriber.receive(.success(response.data.filter))
subscriber.receive(.finished)
}
}
}
}
Unfortunately, I don't have access to BackendCall API since it is private.
It's kind of pseudocode but, it pretty close to the real one.
You cannot initialise an AnyPublisher with a closure accepting a Subscriber. You can only initialise an AnyPublisher from a Publisher. If you want to create a custom Publisher that emits a single random Int as soon as it receives a subscriber and then completes, you can create a custom type conforming to Publisher and in the required method, receive(subscriber:), do exactly what you were doing in your closure.
struct RandomNumberPublisher: Publisher {
typealias Output = Int
typealias Failure = Never
func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
subscriber.receive(Int.random(in: 0...Int.max))
subscriber.receive(completion: .finished)
}
}
Then in your task method, you simply need to create a RandomNumberPublisher and then type erase it.
func task() -> AnyPublisher<Int, Never> {
return RandomNumberPublisher().eraseToAnyPublisher()
}
If all you want is a single random value, use Just
fun task() -> AnyPublisher<Int, Never> {
return Just(Int.random(in: 0...Int.max)).eraseToAnyPublisher()
}
Sidenote: don't use Int(arc4random()) anymore.
You're likely better off wrapping this in a Future publisher, possibly also wrapped with Deferred if you want it to response when subscriptions come in. Future is an excellent way to wrap external async API calls, especially ones that you can't fully control or otherwise easily adapt.
There's an example in Using Combine for "wrapping an async call with a Future to create a one-shot publisher" that looks like it might map quite closely to what you're trying to do.
If you want it to return more than a single value, then you may want to compose something out of PassthoughSubject or CurrentValueSubject that gives you an interface of -> AnyPublisher<YourType, Error> (or whatever you're looking for).
I have a Completable being returned from a simple function.
This is not an async call, so I just need to return a succcessful completion or error depending on a conditional (using Rx here so I can tie into other Rx usages):
func exampleFunc() -> Completable {
if successful {
return Completable.just() // What to do here???
} else {
return Completable.error(SomeErrorType.someError)
}
}
The error case works pretty easily, but am having a block on how to just return a successful completable (without needing to .create() it).
I was thinking I just need to use Completable's .just() or .never(), but just is requiring a parameter, and never doesn't seem to trigger the completion event.
.empty() is the operator I was looking for!
Turns out, I had mixed up the implementations of .never() and .empty() in my head!
.never() emits no items and does NOT terminate
.empty() emits no items but does terminates normally
So, the example code above works like this:
func exampleFunc() -> Completable {
if successful {
return Completable.empty()
} else {
return Completable.error(SomeErrorType.someError)
}
}
Here is the documentation on empty/throw/never operators.
I would be more inclined to do the following:
func example() throws {
// do something
if !successful {
throw SomeErrorType.someError
}
}
Then in order to tie it into other Rx code, I would just use map as in:
myObservable.map { try example() }
But then, mapping over a Completable doesn't work because map's closure only gets called on next events. :-(
I tend to avoid Completable for this very reason, it doesn't seem to play well with other observables. I prefer to use Observable<Void> and send an empty event before the completed...
Something like this:
let chain = Observable<Void>.just()
let foo = chain.map { try example() }
foo.subscribe { event in print(event) }
I have had to use type erasure in Swift a few times however it always involved a generic protocol. In this case, it involves both a generic enum and and generic protocol and I'm stumped.
Here is my generic enum and generic protocol with the necessary extension:
enum UIState<T> {
case Loading
case Success([T])
case Failure(ErrorType)
}
protocol ModelsDelegate: class {
associatedtype Model
var state: UIState<[Model]> { get set }
}
extension ModelsDelegate {
func getNewState(state: UIState<[Model]>) -> UIState<[Model]> {
return state
}
func setNewState(models: UIState<[Model]>) {
state = models
}
}
And here is my type erased generic class:
class AnyModelsDelegate<T>: ModelsDelegate {
var state: UIState<[T]> {
get { return _getNewState(UIState<[T]>) } // Error #1
set { _setNewState(newValue) }
}
private let _getNewState: ((UIState<[T]>) -> UIState<[T]>)
private let _setNewState: (UIState<[T]> -> Void)
required init<U: ModelsDelegate where U.Model == T>(_ models: U) {
_getNewState = models.getNewState
_setNewState = models.setNewState
}
}
I'm getting the following errors (they are marked in the code sample):
Error #1:
Cannot convert value of type '(UIState<[T]>).Type' (aka 'UIState<Array<T>>.Type') to expected argument type 'UIState<[_]>' (aka 'UIState<Array<_>>')
I have been working on this for awhile and there have been quite a few variations on this code that "almost worked". The error always has to do with the getter.
The problem that causes this error, as #dan has pointed out, is that on this line you're trying to pass a type as an argument, instead of an instance of that type:
get { return _getNewState(UIState<[T]>) }
However, I would question your use of an argument to this function in the first place, surely a getting function should have no argument at all? In this case you'll simply want your _getNewState function to have the signature () -> UIState<[T]>, and call it like so:
get { return _getNewState() }
Also, if your getNewState and setNewState(_:) functions in your protocol extension only exist in order to forward the getting and setting of your state property to the type-erasure – you can simplify your code by getting rid of them entirely and use closure expressions in the type-erasure's init instead:
_getNewState = { models.state }
_setNewState = { models.state = $0 }
(These work by capturing a reference to the models argument, for more info see Closures: Capturing Values)
Finally, I suspect that you mean to refer to UIState<T> rather than UIState<[T]> throughout your code, as T in this case refers to an element in the array that your .Success case has as an associated value (unless you want a 2D array here).
All in all, with the above proposed changes, you'll want your code to look something like this:
enum UIState<T> {
case Loading
case Success([T])
case Failure(ErrorType)
}
protocol ModelsDelegate: class {
associatedtype Model
var state: UIState<Model> { get set }
}
class AnyModelsDelegate<T>: ModelsDelegate {
var state: UIState<T> {
get { return _getNewState() }
set { _setNewState(newValue) }
}
private let _getNewState: () -> UIState<T>
private let _setNewState: (UIState<T>) -> Void
required init<U: ModelsDelegate where U.Model == T>(_ models: U) {
_getNewState = { models.state }
_setNewState = { models.state = $0 }
}
}