When SwiftUI View is Equatable - questions...

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Created Mar ’23

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I need to optimize my SwiftUI code, and views conforming to Equatable seem to a good way forward. However, I have some questions, that could be also an interest of others.

View can conform to Equatable, and NOT encapsulated in EquatableView nor .equatable() view modifier, and modern SwiftUI implementations still can use the Equatable functionality, right?
When view contains @ObservedObject or @EnvironmentObject, then static func == (...) have to handle the changes of the observed objects, otherwise the view is not updated after the observed object published change. At least this is my experience. Correct?
Does view, that is Equatable, need to care about equality of its subviews? I hope no. I hope that view can evaluate equality only based on properties available to it, and SwiftUI takes responsibility for its subviews. I. e.: Even when some view returns true on ==, but something changes in its subview (detail hidden to the view), SwiftUI is able to recognize the change and takes appropriate actions. Correct?
When view returns true on equality check (==), but some environment value changes (not visible to the view), the view still reflects the change. Correct?
When the observed object conforms to Equatable, it is being used by SwiftUI. For what purpose? In most (if not all) cases, lhs and rhs are the same instance, so == returns true.

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Answer 1

malc OP

Nov ’23

∙View can conform to Equatable...

True, .equatable() is no longer required, not sure what version that changed in.

.

∙When view contains @ObservedObject...

False, regardless of the outcome of ==, body is always called when any @Published property of the object changes or objectWillChange is published.

.

∙Does view, that is Equatable, need to care about equality of its subviews..

No, a subview will just follow the same algorithm. Best to only init a View with the data it actually needs and uses in its body to keep invalidation tight.

.

∙When view returns true on equality check...

True, if you returned true from == but had @Environment(\.calendar) var calendar and the calendar changed the body would still be called. This is the same behaviour as @ObservedObject in the 2nd question.

.

∙When the observed object conforms to Equatable...

In my testing the if an ObservedObject conformed to Equatable the == func was never called.

My test code:

import SwiftUI
import Combine

struct ViewTest {

    class MyObject: ObservableObject, Equatable {
        static func == (lhs: ViewTest.MyObject, rhs: ViewTest.MyObject) -> Bool {
            // not called
            let x = lhs.counter1 == rhs.counter1 && lhs.counter2 == rhs.counter2
            return x
        }
        
        @Published var counter1 = 0
        @Published var counter2 = 0
        
        func inc() {
            counter1 += 1
        }
        
    }
    
    struct ContentView: View {
        @State var counter = 0
        @State var counter2 = 0
        @StateObject var object = MyObject()
        
        var body: some View {
            Form {
                Button("Increment \(counter)") {
                    counter += 1
                }
                Button("Increment \(counter2)") {
                    counter2 += 1
                }
//                ContentView2(x: counter) { [c = counter] in
//                    print("\(c)") // body is called once when counter2 is changed the first time (once after every time if it has the onReceive
//                }

                ContentView2(x: counter2)
                    .environment(\.calendar, ((counter % 2 == 0) ? Calendar.current : Calendar(identifier: .chinese)))
                //{
                  //  print("\(self.counter)") // body called every time counter2 is changed
               // }
            //    .equatable()
                                  }
        }
    }
    
    
    
    struct ContentView2: View, Equatable {
      //  @Environment(\.managedObjectContext) var context
        //@State var counter = 0
        //@ObservedObject var object: MyObject
        @Environment(\.calendar) var calendar
        let x: Int
       // let y: () -> () // if the closure passed in does somethiung with self then body is always called.
        
        
        static func == (lhs: Self, rhs: Self) -> Bool {
            let result = lhs.x == rhs.x 
            //&& lhs.object.counter2 == rhs.object.counter2
            return result
        }
        
        var body: some View {
            let _ = Self._printChanges()
            HStack {
                Text("ff")
                Button("Button") {
                    //y()
                }
            }
//                .onReceive(Just(0)) { _ in // causes body to be called
//                    print("")
//                }
//            .task {
//                    counter = 0
//                }.onChange(of: x) { a in
//                    
//                }
//                .onAppear {
//                    counter = 0
//                }
        }
    }
    
}

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