I would like to know if the behavior of onAppear and onDisappear in SwiftUI (Xcode 11 beta 6 when I wrote this) is what a developer would find more useful or it is just being more a problem than a feature.
Right now, if we use a cascaded navigation as you will find in the sample code I attach (that compiles and runs fine in Xcode 11b6), a console output of a user navigating back and forth would just trigger onAppear only in the case of a new view load in the forward direction (meaning going deeper).
In the navigation: Root -> NestedView1 -> NestedView2 -> NestedView3 , when adding a debug helper to each view stage,
.onAppear(perform: {print("onAppear level N")})
.onDisappear(perform: {print("onDisappear level N")})
debug console would show
onAppear root level 0
onAppear level 1
onAppear level 2
onAppear level 3
(No onDisappear triggering)
but travelling back Root <- NestedView1 <- NestedView2 <- NestedView3
debug console would show ... nothing
(No onAppear or onDisappear triggering)
struct NestedViewLevel3: View {
@Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Spacer()
Text("Level 3")
Spacer()
Button(action: {
self.presentationMode.wrappedValue.dismiss()
}) {
Text("Back")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.blue)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
}
Spacer()
}
.navigationBarBackButtonHidden(false)
.navigationBarTitle("Level 3", displayMode: .inline)
.onAppear(perform: {print("onAppear level 3")})
.onDisappear(perform: {print("onDisappear level 3")})
}
}
struct NestedViewLevel2: View {
@Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Spacer()
NavigationLink(destination: NestedViewLevel3()) {
Text("To level 3")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.gray)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
.shadow(radius: 10)
}
Spacer()
Text("Level 2")
Spacer()
Button(action: {
self.presentationMode.wrappedValue.dismiss()
}) {
Text("Back")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.blue)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
}
Spacer()
}
.navigationBarBackButtonHidden(false)
.navigationBarTitle("Level 2", displayMode: .inline)
.onAppear(perform: {print("onAppear level 2")})
.onDisappear(perform: {print("onDisappear level 2")})
}
}
struct NestedViewLevel1: View {
@Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Spacer()
NavigationLink(destination: NestedViewLevel2()) {
Text("To level 2")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.gray)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
.shadow(radius: 10)
}
Spacer()
Text("Level 1")
Spacer()
Button(action: {
self.presentationMode.wrappedValue.dismiss()
}) {
Text("Back")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.blue)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
}
Spacer()
}
.navigationBarBackButtonHidden(false)
.navigationBarTitle("Level 1", displayMode: .inline)
.onAppear(perform: {print("onAppear level 1")})
.onDisappear(perform: {print("onDisappear level 1")})
}
}
struct RootViewLevel0: View {
var body: some View {
NavigationView {
VStack {
Spacer()
NavigationLink(destination: NestedViewLevel1()) {
Text("To level 1")
.padding(.horizontal, 15)
.padding(.vertical, 2)
.foregroundColor(Color.white)
.clipped(antialiased: true)
.background(
RoundedRectangle(cornerRadius: 20)
.foregroundColor(Color.gray)
.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: 40, alignment: .center)
)
.shadow(radius: 10)
}
Spacer()
}
}
.navigationBarTitle("Root level 0", displayMode: .inline)
.navigationBarBackButtonHidden(false)
.navigationViewStyle(StackNavigationViewStyle())
.onAppear(perform: {print("onAppear root level 0")})
.onDisappear(perform: {print("onDisappear root level 0")})
}
}
struct ContentView: View {
var body: some View {
RootViewLevel0()
}
}
}
Now, would a developer rather have onAppear and onDisappear:
1) Triggered for the purpose of launching actions that need to be performed only once and only when the user travels forward, like in the current observed behavior.
2) Triggered each time the view appears, more like the action name seems to mean, be it backwards, forwards and any number of times.
I'd take the option 2, simple and brutal (and what I currently need), but I am quite a naive newbie at NavigationView, and option 2 may break a lot of established paradigms I'm not taking into account.
Your feedback will help me find out if the corresponding Feedback Assistant case for SwiftUI is on legitimate grounds.
