SwiftUI Why Can't pass a publisher between Views? - swift

Question

I just want to do some test like this ↓
Create one publisher from first view
Pass it to second view
Bind the publisher with some property in second view and try to show it on screen
The code is ↓ (First View)
struct ContentView: View {
let publisher = URLSession(configuration: URLSessionConfiguration.default)
.dataTaskPublisher(for: URLRequest(url: URL(string: "https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8")!))
.map(\.data.description)
.replaceError(with: "Error!")
.receive(on: DispatchQueue.main)
.eraseToAnyPublisher()
var body: some View {
NavigationView {
List {
NavigationLink(destination: ResponseView(publisher: publisher)) {
Text("Hello, World!")
}
}.navigationBarTitle("Title", displayMode: .inline)
}
}
}
(Second View)
struct ResponseView: View {
let publisher: AnyPublisher<String, Never>
#State var content: String = ""
var body: some View {
HStack {
VStack {
Text(content)
.font(.system(size: 12))
.onAppear { _ = self.publisher.assign(to: \.content, on: self) }
Spacer()
}
Spacer()
}
}
}
But the code is not working. The request failed with message blow ↓
2020-11-11 11:08:04.657375+0800 PandaServiceDemo[83721:1275181] Task <6B53516E-5127-4C5E-AD5F-893F1AEE77E8>.<1> finished with error [-999] Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLStringKey=https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8, NSLocalizedDescription=cancelled, NSErrorFailingURLKey=https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8}
Can someone tell me what happened and what is the right approach to do this?

Answer 1

The issue here is, the Subscription isn't stored anywhere. You have to store it in a AnyCancellable var and retain the subscription.
Use .print() operator whenever you are debugging combine related issues. I find it really useful.
The right approach is to extract the publisher and subscription into an ObservableObject and inject it into the View or use #StateObject
class DataProvider: ObservableObject {
#Published var content: String = ""
private var bag = Set<AnyCancellable>()
private let publisher: AnyPublisher<String, Never>
init() {
publisher = URLSession(configuration: URLSessionConfiguration.default)
.dataTaskPublisher(for: URLRequest(url: URL(string: "https://v.juhe.cn/joke/content/list.php?sort=asc&page=&pagesize=&time=1418816972&key=aa73ebdd8672a2b9adc9dbb2923184c8")!))
.map(\.data.description)
.print()
.replaceError(with: "Error!")
.receive(on: DispatchQueue.main)
.eraseToAnyPublisher()
}
func loadData() {
publisher.assign(to: \.content, on: self).store(in: &bag)
}
}
struct ContentView: View {
#StateObject var dataProvider = DataProvider()
var body: some View {
NavigationView {
List {
NavigationLink(destination: ResponseView(dataProvider: dataProvider)) {
Text("Hello, World!")
}
}.navigationBarTitle("Title", displayMode: .inline)
}
}
}
struct ResponseView: View {
let dataProvider: DataProvider
var body: some View {
HStack {
VStack {
Text(dataProvider.content)
.font(.system(size: 12))
.onAppear {
self.dataProvider.loadData()
}
Spacer()
}
Spacer()
}
}
}
Please note that we have used #StateObject to make sure that DataProvider instance does not get destroyed when the view updates.

Answer 2

You need to store the subscription, otherwise it would be de-initialized and automatically cancelled.
Typically, this is done like this:
var cancellables: Set<AnyCancellable> = []
// ...
publisher
.sink {...}
.store(in: &cancellables)
So, you can create a #State property like the above, or you can use .onReceive:
let publisher: AnyPublisher<String, Never>
var body: some View {
HStack {
// ...
}
.onReceive(publisher) {
content = $0
}
}
You should be careful with the above approaches, since if ResponseView is ever re-initialized, it would get a copy of the publisher (most publishers are value-types), so it would start a new request.
To avoid that, add .share() to the publisher:
let publisher = URLSession(configuration: URLSessionConfiguration.default)
.dataTaskPublisher(...)
//...
.share()
.eraseToAnyPublisher()

Answer 3

In terms of SwiftUI, you are doing something fundamentally wrong: creating the publisher from the View. This means a new publisher will be created every time ContentView is instantiated, and for all means and purposes this can happen a lot of times, SwiftUI makes no guarantees a View will be instantiated only once.
What you need to do is to extract the published into some object, which is either injected from upstream, or managed by SwiftUI, via #StateObject.

Answer 4

Well there is 2 way for this Job: way one is better
Way 1:
import SwiftUI
struct ContentView: View {
#State var urlForPublicsh: URL?
var body: some View {
VStack
{
Text(urlForPublicsh?.absoluteString ?? "nil")
.padding()
Button("Change the Publisher") {urlForPublicsh = URL(string: "https://stackoverflow.com")}
.padding()
SecondView(urlForPublicsh: $urlForPublicsh)
}
.onAppear()
{
urlForPublicsh = URL(string: "https://www.apple.com")
}
}
}
struct SecondView: View {
#Binding var urlForPublicsh: URL?
var body: some View {
Text(urlForPublicsh?.absoluteString ?? "nil")
.padding()
}
}
Way 2:
import SwiftUI
class UrlForPublicshModel: ObservableObject
{
static let shared = UrlForPublicshModel()
#Published var url: URL?
}
struct ContentView: View {
#StateObject var urlForPublicshModel = UrlForPublicshModel.shared
var body: some View {
VStack
{
Text(urlForPublicshModel.url?.absoluteString ?? "nil")
.padding()
Button("Change the Publisher") {urlForPublicshModel.url = URL(string: "https://stackoverflow.com")}
.padding()
SecondView()
}
.onAppear()
{
urlForPublicshModel.url = URL(string: "https://www.apple.com")
}
}
}
struct SecondView: View {
#ObservedObject var urlForPublicshModel = UrlForPublicshModel.shared
var body: some View {
Text(urlForPublicshModel.url?.absoluteString ?? "nil")
.padding()
}
}