A flexible way to handle async errors in SwiftUI
In this post, let’s take a look at how to handle async errors in a flexible and scalable way in SwiftUI. We’ll cover both completion block- and async/await-based use cases.
TL;DR
In this post, we’ll create an observable AlertContext that can be passed around in an app to present alerts from anywhere, using a single binding. We’ll then create an ErrorAlertConvertible protocol that can be implemented by any Error that can generate an Alert, and an ErrorAlerter protocol that provides convenient functionality to any view that has an AlertContext instance.
Although the post contains a lot of text, the total amount of final code is actually not that much. You can have a look at the already implemented types in SwiftUIKit and use them in your own apps.
The traditional way to show alerts
Consider the case where we have a standard alert modifier and want to show an alert when something goes wrong. In it’s easiest form, such a setup could look something like this:
struct MyView: View {
@State
private var isAlertPresented = false
enum MyError: Error {
case somethingWentWrong
}
var body: some View {
Button(action: doSomething) {
Text("Do something")
}.alert(isPresented: $isAlertPresented) {
Alert(title: Text("Something went wrong"))
}
}
}
private extension MyView {
func doSomething() {
doSomethingAsync { error in
isAlertPresented = error != nil
}
}
/**
Fake an async call to test what happens.
*/
func doSomethingAsync(completion: (Error?) -> Void) {
completion(MyError.somethingWentWrong)
}
}
Here, we fake an async call and sets an isPresented flag to true to simulate that the operation fails. It’s a super-simple and not that applicable example, but you get the idea.
To create a more flexible and robust setup, let’s first come up with a way to have a single alert that can be used within the entire app, then find a way to trigger that alert in a single way.
Setting up the alert
I have alreaty written about an easier way to handle alerts in SwiftUI in this old post. You can read it for a more thorough discussion, but let’s quickly just put it to work here as well.
Although we’ll only use alerts in this post, let’s create an observable class that can be used to manage any presentable content, such as alerts, full-screen covers, sheets etc:
open class PresentationContext<Content>: ObservableObject {
public init() {}
@Published
public var isActive = false
@Published
public internal(set) var content: (() -> Content)? {
didSet { isActive = content != nil }
}
public var isActiveBinding: Binding<Bool> {
.init(get: { self.isActive },
set: { self.isActive = $0 }
)
}
public func dismiss() {
isActive = false
}
public func presentContent(_ content: @autoclosure @escaping () -> Content) {
self.content = content
}
}
As you can see, this context holds the active/presented state as well as a generic content builder that generates the content view that should be presented. Calling presentContent(...) sets the content builder, which in turn sets isActive to true, while calling dismiss() sets isActive to false.
This class is not as clean. For instance, the binding is called isActiveBinding instead of isActive and the presentation function is called presentContent(...) instead of present(...). You’ll soon see the reason for this.
We can now subclass PresentationContext to create a clean, explicit context for handling alerts:
public class AlertContext: PresentationContext<Alert> {
public func present(_ alert: @autoclosure @escaping () -> Alert) {
presentContent(alert())
}
}
This class is super simple. By inheriting PresentationContext and binding it to Alert, we get a context that can be used to present alerts and alerts alone.
The class also has a cleaner present(...) function that calls presentContent(...). This is the function that is meant to be used, so depending on how you setup these contexts, you could make presentContent internal to avoid exposing it altogether. We’ll also create a view modifier soon, that will use the isActiveBinding, which means that we don’t have to expose that either.
Having a non-generic presentContent function in the base class also lets us avoid generic types by creating generic functions instead, for instance to present any view as a sheet:
public class SheetContext: PresentationContext<AnyView> {
public func present<Sheet: View>(_ sheet: @autoclosure @escaping () -> Sheet) {
presentContent(sheet().any())
}
}
With these things in mind, I’m happy to have a noisy base class, while the subclasses that are meant to be used are cleaner and tighter.
Binding the alert to the view
With the AlertContext in place, we can now create a view modifier that can bind a context to a view:
public extension View {
func alert(_ context: AlertContext) -> some View {
alert(
isPresented: context.isActiveBinding,
content: context.content ?? { Alert(title: Text("")) }
)
}
}
This lets us use myView.alert(...) just like before, but instead of a binding and a view or an item that was later introduced, we can provide a context to get a more flexible setup.
Presenting an alert
With the AlertContext and the alert(...) view modifier in place, we could clean up the initial code to look like this:
struct MyView: View {
@StateObject
private var alert = AlertContext()
enum MyError: String, Error {
case somethingWentWrong
}
var body: some View {
Button(action: doSomething) {
Text("Do something")
}.alert(alert)
}
}
private extension MyView {
func doSomething() {
doSomethingAsync { error in
guard let error = error else { return }
let title = Text(error.localizedDescription)
alert.present(Alert(title: title))
}
}
/**
Fake an async call to test what happens.
*/
func doSomethingAsync(completion: (Error?) -> Void) {
completion(MyError.somethingWentWrong)
}
}
Here, we create a StateObject context and apply it to our button, then call alert.present(...) to present an alert when our fake operation fails.
Although this is already more flexible, having this context gives us even more freedom. We could for instance inject a context from the outside, using .environmentObject(...):
struct ParentView: View {
@StateObject
private var alert = AlertContext()
var body: some View {
MyView()
.environmentObject(alert)
}
}
which could then be access by replacing the @StateObject property with @EnvironmentObject:
struct MyView: View {
@EnvironmentObject
private var alert: AlertContext
We could also inject the context with the MyView initializer and set it up as an observed object instead:
struct ParentView: View {
@StateObject
private var alert = AlertContext()
var body: some View {
MyView(alert: alert)
}
}
struct MyView: View {
init(alert: AlertContext) {
self._alert = ObservedObject(wrappedValue: alert)
}
@ObservedObject
private var alert: AlertContext
...
}
As you can see, having the context instead of private state gives us a lot more flexibility.
In fact, we don’t even have to bind the context in MyView. We could bind it within ParentView and just pass the context down the view hierarchy to let any view use it to present an alert:
struct ParentView: View {
@StateObject
private var alert = AlertContext()
var body: some View {
MyView(alert: alert)
.alert(alert)
}
}
struct MyView: View {
init(alert: AlertContext) {
self._alert = ObservedObject(wrappedValue: alert)
}
...
func alert(_ text: String) {
alert.present(
Alert(title: text)
)
}
}
With this rather long detour, and with this setup in place, we can now start looking at some interesting ways to present error alerts when operations fail.
Alerting errors
We can now use AlertContext to present any error as an alert. Given a general Error, it could look something like this:
func alert(_ error: Error) {
alert.present(
Alert(title: error.localizedDescription)
)
}
However, we can do better. Let’s create a few protocols to setup a more flexible way to handle alerts.
First, let’s define a protocol that can be implemented by any error that can be presented as an alert:
protocol ErrorAlertConvertible: Error {
var errorTitle: String { get }
var errorMessage: String { get }
}
With the required properties, the protocol can be extended to create an alert like this:
extension ErrorAlertConvertible {
var errorAlert: Alert {
Alert(
title: Text(errorTitle),
message: Text(errorMessage),
dismissButton: .default(Text("OK")) // Use localized strings though
)
}
}
We can now use this protocol to define app- or domain-specific errors, for instance:
enum MyError: ErrorAlertConvertible {
case general
var errorTitle: String {
switch self {
case .general:
return "Something went wrong"
}
}
var errorMessage: String {
switch self {
case .general:
return "Please try again"
}
}
}
Next, let’s define a protocol that can be implemented by types that can present errors using a context:
protocol ErrorAlerter {
var alert: AlertContext { get }
}
With this single requirement, we can extend any type that implements ErrorAlerter with functions that try to perform async operations and automatically alert any errors that occur.
For instance, adding this function lets an alerter alert any errors:
@MainActor
extension ErrorAlerter {
/**
Alert the provided error.
*/
func alert(error: Error) {
if let error = error as? ErrorAlertConvertible {
return alert.present(error.errorAlert)
}
alert.present(Alert(
title: Text(error.localizedDescription),
dismissButton: .default(Text(L10n.ok)))) // Or "OK" if you're not using swiftgen
}
}
Notice that since the function will change state within the alert context, we should apply a @MainActor to the extension. This lets us use the function with async/await, since we can just await the alert.
To support block-based operations, we could add a non-async function that just uses the async alert:
extension ErrorAlerter {
func alertAsync(error: Error) {
DispatchQueue.main.async {
alert(error: error)
}
}
}
With the ErrorConvertible and ErrorAlerter protocols in place, we are now ready to put it all together. Let’s start with a block-based example.
Alert errors from block-based functions
Let’s put ErrorAlerter to more work by adding an extension that lets us call any completion block-based operation and automatically alert any errors that occur:
extension ErrorAlerter {
typealias BlockResult<ErrorType: Error> = Result<Void, ErrorType>
typealias BlockCompletion<ErrorType: Error> = (BlockResult<ErrorType>) -> Void
typealias BlockOperation<ErrorType: Error> = (BlockCompletion<ErrorType>) -> Void
func tryWithErrorAlert<ErrorType: Error>(_ operation: @escaping BlockOperation<ErrorType>, completion: @escaping BlockCompletion<ErrorType>) {
operation { result in
switch result {
case .failure(let error): alertAsync(error: error)
case .success: break
}
completion(result)
}
}
}
The code may look a bit confusing, but let’s go through it in steps.
We first define a generic result type, which we then use in a generic completion type, which we then use in a generic operation type. This makes the rest of the code more readable.
We then define a tryWithErrorAlert function that can be used with any parameter-less function. If an error occurs, it calls alertAsync, which as we saw earlier updates the context to present the error. If the error implements ErrorAlertConvertible, it has full control over how it will be presented.
Also note that the provided completion is called as well, to give us a way to handle the result. We can handle an error in any way we want…we just don’t have to care about alerting it to the user.
This means that any view that has an AlertContext can now present any error using the same alert within the entire app, using the same approach by just implementing the ErrorAlerter protocol.
Now let’s look at how to achieve the same with async/await.
Alert errors from async functions
With async/await, our code can become a lot cleaner, since we don’t need result types, completions etc. An async alternative to the block-based tryWithErrorAlert function could look like this:
typealias AsyncOperation = () async throws -> Void
func tryWithErrorAlert(_ operation: @escaping AsyncOperation) {
Task {
do {
try await operation()
} catch {
await alert(error: error)
}
}
}
This means that with this code, we can perform any parameter-less async throwing function and alert any error that is thrown, using the async alert function in the @MainActor extension.
Using this approach, and with MyError implementing ErrorAlertConvertible as we saw before, the initial example could look like this, if it implements ErrorAlerter:
struct MyView: View, ErrorAlerter {
@StateObject
var alert = AlertContext()
var body: some View {
Button(action: doSomething) {
Text("Do something")
}.alert(alert)
}
}
extension MyView {
func doSomething() {
tryWithErrorAlert(doSomethingAsync)
}
/**
Fake an async call to test what happens.
*/
func doSomethingAsync() async throws {
throw(MyError.general)
}
}
As we discussed before, we can also inject the alert context from the outside or into the rest of the view hierarchy, and use tryWithErrorAlert in any view that implements ErrorAlerter.
This is a lot cleaner than the block-based approach, but if your app targets an iOS version that doesn’t support async/await, you may have to stick with the block-based one for now.
Conclusion
In this post, we created an observable AlertContext that can be passed around in an app to present alerts from anywhere using a single view binding. We then created an ErrorAlertConvertible protocol that can be implemented by any Error that can generate an Alert, and an ErrorAlerter protocol that provides convenient functionality to any view that has an AlertContext instance.
Although the post contains a lot of text, the total amount of final code is actually not that much. You can have a look at the already implemented types in SwiftUIKit and use them in your own apps if you want.
I hope this helps and that you find the approach as usable as I do. I’d love to hear your thoughts on this, so don’t hesitate to comment or reach out with any thoughts you may have.
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