So I'm getting started with Combine, and I want to have a component which publishes events like a simple event emitter.
So in other words, I want to have a model like this (pseudocode):
class MyModel {
var onNewEvent: EventPublisher
func foo(bar: Bar) {
onNewEvent.publish(Event(bar))
}
}
let model: MyModel...
model.onNewEvent.sink(
receiveValue: { event in print(event) }
)
I can achieve something like this using a notification center publisher, but I want to avoid this unnecessary step if possible.
Is there such a thing as a simple publisher which just publishes values on command, or else what would be the idiomatic way to handle this in combine?
you can use a private(outside world cannot change/emit values) subject inside your model. and then expose it using a lazy publisher to let the outside world to subscribe.
class MyModel {
private var myPassthroughSubject = PassthroughSubject<Bar, Never>() // make this subject `private`, so only inside methods can send values.
lazy var myPublisher = myPassthroughSubject.eraseToAnyPublisher() // expose this publisher to the outside world.
func foo(bar: Bar) {
myPassthroughSubject.send(bar)
}
}
struct Bar {
}
let model = MyModel()
model.myPublisher.sink(
receiveValue: { event in print(event) }
)
model.foo(bar: .init())
model.foo(bar: .init())
Related
I have an ObservableObject that publishes some values using #Published property wrappers. This object also holds a timer.
The question is, how can I fire an event as soon as the timer is executed and handle that event in a view in SwiftUI (I'd prefer using something like onReceive)?
Using the Combine framework for publishing changing values already, I'd like to implement this event triggering / handling properly. But all that I've read so far about Combine is always about handling value changes. But in my case it's rather a single simple event (without any values).
I know that I could simply use a closure and call that when the timer expires, and I will do that if there's no better, combine-like solution.
This is a conceptual question for a very simple problem so I think it's self explaining without me coming up with a code example?
The way SwiftUI works with Combine is via .onReceive, which expects a publisher. An object can expose a publisher - whether Timer or something else - as a property.
Combine publishers work by emitting values, and if you just need to signal that an event has happened, you can emit () aka Void values.
This object, by the way, need not be an ObservableObject, but it could be.
class Foo: ObservableObject {
let timer = Timer.publish(every: 1, on: .main, in: .default)
.autoconnect()
.map { _ in } // map to Void
.eraseToAnyPublisher() // optional, but a good practice
}
Now, you can use .onReceive to subscribe to the timer event:
struct ContentView: View {
#StateObject var foo = Foo()
#State var int: Int = 0
var body: some View {
Text("\(int)")
.onReceive(timer) {
self.int += 1
}
}
}
Of course, you're not restricted to a TimerPublisher. For example, if some random event happens, you can use a PassthroughSubject to publish a value:
class Foo {
let eventA: AnyPublisher<Void, Never>
private let subject = PassthroughSubject<Void, Never>()
init() {
eventA = subject.eraseToAnyPublisher()
let delay = Double.random(in: 10...100)
DispatchQueue.main.asyncAfter(deadline: .now() + delay) { [weak self] in
// something random happened, notify
self?.subject.send()
}
}
}
Just an idea, but you could dedicate a specific thread to that Object? And then listen and fire things on that Queue specifically. I've never done this before but it seems like the right sort of idea.
https://developer.apple.com/documentation/dispatch/dispatchqueue
https://www.freecodecamp.org/news/ios-concurrency/
I've been working with RxSwift for a few years now, and am starting to explore Combine with SwiftUI and am having some trouble trying to replicate some functionality from RxSwift in Combine.
On the RxSwift GitHub there is an example in a file called ActivityIndicator.swift.
Basic usage is as follows:
class Foo {
let activityIndicator = ActivityIndicator()
lazy var activity = activityIndicator.asDriver()
var disposeBag = DisposeBag()
func doSomething() {
Observable
.just("this is something")
.trackActivity(activityIndicator)
.subscribe()
.disposed(by: disposeBag)
}
}
What this does is allow you to then drive off of the activity driver and it will emit boolean values every time something subscribes or a subscription completes.
You can then directly drive something like a UIActivityIndicatorView's isAnimating property using RxCocoa.
I've been trying to figure out how to create something similar to this in Combine but am not having any luck.
Say I have a viewModel that looks like this:
class ViewModel: ObservableObject {
#Published var isActive = false
func doSomething() -> AnyPublisher<Void, Never> {
Just(())
.delay(for: 2.0, scheduler: RunLoop.main)
.eraseToAnyPublisher()
}
}
What I would like to do is create an operator for a Publisher that will function similarly to how the Rx operator worked where I can forward the events from the subscription through the chain, but change the isActive value every time something subscribes/completes/cancels.
In the SwiftUI View I would initiate the doSomething function and sink to it, while also being able to use the published isActive property to show/hide a ProgressView
Something similar to this:
struct SomeView: View {
let viewModel = ViewModel()
var body: some View {
var cancelBag = Set<AnyCancellable>()
VStack {
Text("This is text")
if viewModel.isActive {
ProgressView()
}
}
.onAppear(perform: {
viewModel
.doSomething()
.sink()
.store(in: &cancelBag)
})
}
}
Is there something that works like this already that I am just completely missing?
If not, how can I go about replicating the RxSwift functionality in Combine?
Thank you in advance for the help.
Looks like someone created a Combine version. I don't know if it has the same issue as discussed by #Daniel T. but it looks promising.
https://github.com/duyquang91/ActivityIndicator
Hmm... The key to the ActivityIndicator class is the Observable.using(_:observableFactory:) operator. Unfortunately, I don't believe there is an equivalent operator in Combine.
The using operator creates a resource when the Observable is subscribed to, and then disposes the resource when the Observable sends a stop event (complete or error.) This insures the resource's lifetime. In this particular case, the resource just increments an Int value on creation and decrements it on disposal.
I think you could kind of mimic the behavior with something like this:
extension Publisher {
func trackActivity(_ activityIndicator: CombineActivityIndicator) -> some Publisher {
return activityIndicator.trackActivity(of: self)
}
}
final class CombineActivityIndicator {
var counter = CurrentValueSubject<Int, Never>(0)
var cancelables = Set<AnyCancellable>()
func trackActivity<Source: Publisher>(of source: Source) -> some Publisher {
let sharedSource = source.share()
counter.value += 1
sharedSource
.sink(
receiveCompletion: { [unowned self] _ in
self.counter.value -= 1
},
receiveValue: { _ in }
)
.store(in: &cancelables)
return sharedSource
}
var asPublisher: AnyPublisher<Bool, Never> {
counter
.map { $0 > 0 }
.eraseToAnyPublisher()
}
}
However, the above class will heat up the Publisher and you might miss emitted values because of it. Use at your own risk, I do not recommend the above unless you are desperate.
Maybe someone has written a using operator for Publisher and will be willing to share.
I'm refactoring a project to use MVVM and using protocols to ensure that my view models have a consistent structure. This works fine for defining public properties relating to input and output (which are based on internal structs) but defining actions in the same way is proving problemmatic as, currently, they are defined as closures which have to refer to view model properties. If I use the same approach as I have to input and output properties, I don't think I can access properties of the containing instance.
Example:
protocol ViewModelType {
associatedtype Input
associatedtype Output
associatedtype Action
}
final class MyViewModel: ViewModelType {
struct Input { var test: String }
struct Output { var result: String }
struct Action {
lazy var createMyAction: Action<String, Void> = { ... closure to generate Action which uses a MyViewModel property }
}
var input: Input
var output: Output
var action: Action
}
It's not a deal breaker if I can't do it, but I was curious as I can't see any way of getting access to the parent's properties.
Answer to your question
Let's begin with a note that createMyAction: Action<String, Void> refers to the type (struct) named Action as if it was a generic, but you have not declared it as such and will thus not work.
And to answer your question of the nested struct Action can refer its outer class MyViewModel - yes you can refer static properties, like this:
struct Foo {
struct Bar {
let biz = Foo.buz
}
static let buz = "buz"
}
let foobar = Foo.Bar()
print(foobar.biz)
But you should probably avoid such circular references. And I will omit any ugly hack that might be able to achive such a circular reference on non static properties (would probably involve mutable optional types). It is a code smell.
Suggestion for MVVM
Sounds like you would like to declare Action as a function? I'm using this protocol myself:
protocol ViewModelType {
associatedtype Input
associatedtype Output
func transform(input: Input) -> Output
}
Originally inspired by SergDort's CleanArchitecture.
You can prepare an instance of input (containing Observables) from the UIViewController and call the transform function and then map the Output of transform (being Observabless) to update the GUI.
So this code assumes you have basic Reactive knowledge. As for Observables you can chose between RxSwift or ReactiveSwift - yes their names are similar.
If you are comfortable with Rx, it is an excellent way of achieving a nice MVVM architecture with simple async updates of the GUI. In the example below, you will find the type Driver which is documented here, but the short explanation is that is what you want to use for input from views and input to views, since it updates the views on the GUI thread and it is guaranteed to not error out.
CleanArchitecture contains e.g. PostsViewModel :
final class PostsViewModel: ViewModelType {
struct Input {
let trigger: Driver<Void>
let createPostTrigger: Driver<Void>
let selection: Driver<IndexPath>
}
struct Output {
let fetching: Driver<Bool>
let posts: Driver<[PostItemViewModel]>
let createPost: Driver<Void>
let selectedPost: Driver<Post>
let error: Driver<Error>
}
private let useCase: PostsUseCase
private let navigator: PostsNavigator
init(useCase: PostsUseCase, navigator: PostsNavigator) {
self.useCase = useCase
self.navigator = navigator
}
func transform(input: Input) -> Output {
let activityIndicator = ActivityIndicator()
let errorTracker = ErrorTracker()
let posts = input.trigger.flatMapLatest {
return self.useCase.posts()
.trackActivity(activityIndicator)
.trackError(errorTracker)
.asDriverOnErrorJustComplete()
.map { $0.map { PostItemViewModel(with: $0) } }
}
let fetching = activityIndicator.asDriver()
let errors = errorTracker.asDriver()
let selectedPost = input.selection
.withLatestFrom(posts) { (indexPath, posts) -> Post in
return posts[indexPath.row].post
}
.do(onNext: navigator.toPost)
let createPost = input.createPostTrigger
.do(onNext: navigator.toCreatePost)
return Output(fetching: fetching,
posts: posts,
createPost: createPost,
selectedPost: selectedPost,
error: errors)
}
}
Recently I have used ReSwift API, And I want to add ReSwiftRecorder too!
The sample of ReSwiftRecorder in Github is very simple app
I need to to something more complicated. I have an object which get data from server and I need to It reloads its data when app is not connected to net. Here is my code:
AppState:
struct AppState: StateType {
var menus: Result<[Menu]>?
}
MenuReducer:
func menusReducer(state: Result<[Menu]>?, action: Action) -> Result<[Menu]>? {
switch action {
case let action as SetMenusAction:
return action.menus
default:
return state
}
}
AppReducer:
struct AppReducer: Reducer {
func handleAction(action: Action, state: AppState?) -> AppState {
return AppState(
menus: menusReducer(state: state?.menus, action: action),
)
}
}
MenuActions:
struct SetMenus: Action {
let menus: Result<[Menu]>
}
I know I need to change MenuAction to Something like this:
let SetMenusActionTypeMap: TypeMap = [SetMenusAction.type: SetMenusAction.self]
struct SetMenusAction: StandardActionConvertible {
static let type = "SET_MENU_ACTION"
let menus: Result<[Menu]>
init() {}
init(_ standardAction: StandardAction) {}
func toStandardAction() -> StandardAction {
return StandardAction(type: SetMenusAction.type, payload: [:], isTypedAction: true)
}
}
but I got error on init functions
Return from initializer without initializing all stored properties
when I set a initializer code the error disappear but app does not restore saved data! How can I fix it?
You will want to add serialization/deserialization code. The menus property needs to be set. Also, you will want to serialize that property as payload:
let SetMenusActionTypeMap: TypeMap = [SetMenusAction.type: SetMenusAction.self]
struct SetMenusAction: StandardActionConvertible {
static let type = "SET_MENU_ACTION"
let menus: Result<[Menu]>
init() {
self.menus = // however you initialize that
}
init(_ standardAction: StandardAction) {
let maybeMenus = standardAction.payload["menus"] as? [Menu]?
self.menus = // create Result from Optional<[Menu]>
}
func toStandardAction() -> StandardAction {
let maybeMenus = self.menus.asOptional // Cannot serialize Result itself
return StandardAction(type: SetMenusAction.type, payload: ["menus" : maybeMenus], isTypedAction: true)
}
}
So problems I see here: JSON serialization depends on Dictionary representation of your payload data, i.e. the properties of your object. Can Result be serialized directly? I guess not, so you need to convert it, probably easiest to nil.
All in all, the payload is the key you missed and now you have to figure out how to use it with the data you have at hand. Also, it makes me a bit suspicious that the Result type itself is part of the AppState. I expected it to be reduced away or handled before dispatching an action, like SettingMenusFailedAction instead of ChangeMenusAction(result:) or similar. Just as a sidenote: actions should be more than typed property setters.
I'm trying to come up with a simple observable object in Swift and thought to use RxSwift. I couldn't find a simple example to do something like this:
protocol PropertyObservable {
typealias PropertyType
var propertyChanged: Event<(PropertyType, Any)> { get }
}
class Car: PropertyObservable {
typealias PropertyType = CarProperty
let propertyChanged = Event<(CarProperty, Any)>()
dynamic var miles: Int = 0 {
didSet {
propertyChanged.raise(.Miles, oldValue as Any)
}
}
dynamic var name: String = "Turbo" {
didSet {
propertyChanged.raise(.Name, oldValue as Any)
}
}
}
The above is pure Swift solution for observables from this blog post; I really like how it's a protocol-based solution and not invasive. In my case, I have an object in my project where each property is set asynchronously under the hood (bluetooth device). So I need to observe/subscribe to the changes instead of getting/setting the properties in real-time.
I keep hearing RxSwift will do just that and more. However, I can't find a simple example to match above and beginning to think RxSwift is overkill for my need? Thanks for any help.
Easiest way to quickly make this observable with RxSwift would probably be to use the RxSwift class Variable (all code here is untested off the top of my head):
import RxSwift
class Car {
var miles = Variable<Int>(0)
var name = Variable<String>("Turbo")
}
This enables you to observe the values by subscribing to them:
let disposeBag = DisposeBag()
let car = Car
car.name.asObservable()
.subscribeNext { name in print("Car name changed to \(name)") }
.addToDisposeBag(disposeBag) // Make sure the subscription disappears at some point.
Now you've lost the old value in each event. There are of course numerous ways to solve this, the RxSwifty way would probably be to add a scan operation to your element sequence, which works a lot like reduce does on a normal Array:
car.name.asObservable()
.scan(seed: ("", car.name.value)) { (lastEvent, newElement) in
let (_, oldElement) = lastEvent
return (oldElement, newElement)
}
.subscribeNext { (old, new) in print("Car name changed from \(old) to \(new)") }
.addToDisposeBag(disposeBag)