How to run a function first and then navigation afterwards in swiftUI

First, you cannot have a Button in a NavigationLink when a Label is required. So, when you use a NavigationLink where a Label is required, you need to provide a label. Simple.

Unfortunately, when you go creative and provide a button, the compiler does not emit a warning or error, and if you tap it, no navigation happens.

Second, as @workingdog support Ukraine already pointed out correctly, a NavigationLink needs to be embedded in a NavigationStack or a NavigationSplitView.

So, a minimal example would look like this:

struct ContentView: View {
    var body: some View {
        NavigationStack {
            NavigationLink("Show Detail View") {
                Text("The Detail View")
            }
        }
    }
}

This little snippet contains all the functionality provided by Navigation Stack, Split View, master/detail, back-button and works on multiple platforms.

This approach has a catch, though: as mentioned, you need to provide a Label, but in your use case you want to have a button that triggers the navigation. Luckily, there's a solution, too:

One way to achieve this, without tricks is to use the "programmatic navigation link" form. That is, there are forms to create navigation links where to set the navigation stack programmatically. An example may make this clear:

What you need is a data structure representing "paths" on a navigation stack. This is in essence a sequence of items, say an Array, where each item represents a view on the stack. The actual type of this item is kept intentionally abstract, which enables a great deal of versatility. Keep on reading.

This "paths" variable is defined as a @State variable in a view whose body contains your NavigationStack. Again, a few lines of code demonstrate this:

struct ContentView2: View {
    typealias Item = String
    @State private var paths: [Item] = []
    
    var body: some View {
        NavigationStack(path: $paths) {
            ... 
        }
    }
}

As you can see, the NavigationStack receives a parameter, an Array of "Items". Note also, that this is a Binding and its value can (and will) be modified by the NavigationStack (for example, when tapping the "Back" button).

Each item represents a view pushed on the stack. Initially, the array is empty, means there is no view pushed onto the navigation stack.

Also, the type of the items is rather abstract. You can make it what suits your needs, and this is usually, defining the data you want to render.

Now, what you need to do is, controlling this array (with your button), and secondly, declare what the pushed view should look like:

struct ContentView2: View {
    typealias Item = String
    @State private var paths: [Item] = []
    
    var body: some View {
        NavigationStack(path: $paths) {
            Button("Show View") {
                self.paths = ["Other View"]
            }
            .navigationDestination(for: Item.self) { item in
                OtherView(item: item)
            }
        }
    }
}

So, here you have your button, and when pressed, it says, that the stack – which represents your pushed views – should contain one item.

So, this gets rendered by the NavigationStack. When the back button will be tapped, the NavigationStack removes the top item, which actually makes the array empty.

This brief example shows the principle way how you may accomplish your goal. I intentionally omitted much of your code just to demonstrate the essence. Your task is till to separate the business logic from the presentation logic and combine all required peaces.

Note also, that when your logic gets more complex, you may consider to compute the "paths" variable outside of a view – say, in a "Model". You have great control about when a view will navigate and what the other view will display. For example, if the action starts an asynchronous task and you want to navigate only after the task returns a result, it can be easily achieved.

Keep in mind though, that the semantic should be "navigation". If it's not "navigation", you may want to present the other view differently, for example as a sheet.