I'm trying to build an app using SwiftUI and an MVVM architecture. I'd like to have my View present an alert whenever its ViewModel deems it necessary—say, when it has a new result of some sort available from the Model. So suppose whenever the VM detects a new result it sets its status
accordingly:
The ViewModel:
enum Status {
case idle
case computing
case newResultAvailable
}
class MyViewModel: ObservableObject {
@Published var status = Status.idle
...
}
The View:
struct ContentView: View {
@ObservedObject var vm = MyViewModel()
@State private var announcingResult = false {
didSet {
// reset VM status when alert is dismissed
if announcingResult == false {
vm.status = .idle
}
}
}
var body: some View {
Text("Hello")
.alert(isPresented: $announcingResult) {
Alert(title: Text("There's a new result!"),
message: nil,
dismissButton: .default(Text("OK")))
}
}
}
Apple has designed the .alert()
modifier to take a binding as its first argument, so that the alert is displayed whenever the bound property becomes true
. Then, when the alert is dismissed, the bound property is automatically set to false
.
My question is:
How can I have the alert appear whenever the VM's status
becomes .newResultAvailable
? It seems to me that that's how proper MVVM should function, and it feels very much in the spirit of all the SwiftUI WWDC demos, but I can't find a way…
Here is possible approach (tested & works with Xcode 11.3+)
struct ContentView: View {
@ObservedObject var vm = MyViewModel()
var body: some View {
let announcingResult = Binding<Bool>(
get: { self.vm.status == .newResultAvailable },
set: { _ in self.vm.status = .idle }
)
return Text("Hello")
.alert(isPresented: announcingResult) {
Alert(title: Text("There's a new result!"),
message: nil,
dismissButton: .default(Text("OK")))
}
}
}
also sometimes the following notation can be preferable
var body: some View {
Text("Hello")
.alert(isPresented: Binding<Bool>(
get: { self.vm.status == .newResultAvailable },
set: { _ in self.vm.status = .idle }
)) {
Alert(title: Text("There's a new result!"),
message: nil,
dismissButton: .default(Text("OK")))
}
}