How to navigate independently using @ObservableObjects and @StateObjects

Creating a Mother View 👩‍👧‍👦

The first step is to create a mother view that hosts both ContentViews as its subviews. For this purpose, create a new File-New-File-SwiftUI View and name it MotherView. In this view, we want to show either ContentViewA or ContentViewB depending on where the user navigated to. 

Important: Since our MotherView will “contain” the ContentViews, it must be the root view when the app launches. To set the MotherView as the root view, open the NavigatinInSwiftUIApp.swift file and replace the ContentViewA inside the WindowGroup with an instance of our MotherView.

@main
struct NavigatingInSwiftUIApp: App {
    var body: some Scene {
        WindowGroup {
            MotherView()
        }
    }
}

Back to our MotherView.swift file: To keep track of the selected main view, we need to declare a State property first. By default, we want to present ContentViewA as the first “page”. For this purpose, create a new Swift file named “Helper” and insert the following enum.

enum Page {
    case page1
    case page2
}

Now, we can declare the State property in our MotherView and assign it to the page1 option of our Page enum.

struct MotherView: View {
    
    @State var currentPage: Page = .page1
    
    var body: some View {
        Text("Hello World")
    }
}

Depending on the Page assigned to the currentPage State, we want to either host ContentViewA or ContentViewB. Let’s implement this logic by inserting a switch-statement inside a VStack.

var body: some View {
    switch currentPage {
    case .page1:
        ContentViewA()
    case .page2:
        ContentViewB()
    }
}

Let’s run a preview and take a look at it. Since our State is currently assigned to .page1, our first switch case is met and the ContentViewA gets hosted. Let’s change the page State to .page2 and see what happens.

Here we go! When our State property changes, the whole MotherView gets updated and the switch block gets executed again, this time showing us ContentViewB. So far, so good.

But we want to let the user change the currentPage State by tapping on the Buttons inside of ContentViewA and ContentViewB, respectively. Note that the Buttons are not part of the MotherView itself, so we need to create a “bridge” for accessing the MotherView’s currentPage State from the outside; meaning that when, for instance, tapping on the “Next” Button of ContentViewA, we alter the Page assigned to the currentPage State of the MotherView for eventually navigating to ContentViewB.

We can achieve this by interjecting something called an ObservableObject into our MotherView – ContentViews hierarchy. 

Observable Objects ?! 🤯

At this point, you are probably asking yourself: “What the heck are ObservableObjects?!”. Well, understanding this can be pretty tough but don’t worry, we will explain it to you in a simple way.

ObservableObjects are similar to State properties which you should already know. But instead of just refreshing the body of the related view when the data assigned to the State changes, ObservableObjects are capable of the following things: 

• Instead of variables, ObservableObjects are classes that can contain data, for example, a String assigned to a variable 
• We can bind multiple views to the ObservableObject (in other words: we can make these views observe the ObservableObject).
• Because an ObservableObject is just a class, the observing views can access and manipulate the data inside the ObservableObject
• When a change happens to the ObservableObject’s data, all observing views get automatically notified and refreshed similar to when the value assigned to a State changes

So, how can we utilize this functionality? Well, we can create an ObservableObject class that contains a variable indicating the current Page that should be displayed. Then we can bind our MotherView, our ContentViewA, and our ContentViewB to it. Then, we can tell our MotherView to show the corresponding ContentView depending on the Page assigned to the ObservableObject’s variable.

From the Buttons inside the ContentViews, we can update the Page assigned to the ObservableObject’s variable. This would cause all three observing views to update their bodies, including the MotherView. With this functionality, we can achieve that the MotherView will present show the correct ContentView depending on the selected Page!

Let’s create such an ObservableObject. To do this, create a new Swift file and name it “ViewRouter”. Make sure you import the SwiftUI framework. Then create a class called “ViewRouter” conforming to the ObservableObject protocol.

import SwiftUI

class ViewRouter: ObservableObject {
    
}

As said, an ObservableObject notifies and causes all of its observing views to update themselves when a change happens. But what exactly do we mean by “when a change happens”? As said, the main task of our ViewRouter should be to stay tracked on which Page (meaning which ContentView) should be currently shown – whether it’s on the launch of the app or when the user taps on a specific Button. For this purpose, we declare a variable called currentPage inside our ViewRouter class and assign .page1 to it as its default value.

class ViewRouter: ObservableObject {
    
    var currentPage: Page = .page1
    
}

The views that will observe the ViewRouter, especially the MotherView, should get notified and updated when the Page assigned to the currentPage changes. 

To do this, we use the @Published property wrapper.

@Published var currentPage: Page = .page1

The @Published property wrapper works very similarly to the @State property wrapper. Every time the value assigned to the wrapped property changes, every observing view rerenders. In our case, we want our MotherView to observe the ViewRouter and to navigate to the right Page depending on the currentPage’s updated value.

Updating the MotherView 🔁

To make the MotherView observe the ViewRouter, we need to declare a @StateObject property, which is used for binding views to ObservableObjects.

struct MotherView: View {
    
    @State var currentPage: Page = .page1
    
    @StateObject var viewRouter: ViewRouter

    var body: some View {
        //...
    }
}

When doing this, we also need to provide our MotherView_Previews struct with an instance of the ViewRouter.

struct MotherView_Previews: PreviewProvider {
    static var previews: some View {
        MotherView(viewRouter: ViewRouter())
    }
}

In the NavigatingInSwiftUIApp.swift file, we defined the MotherView as the root view when the app launches. Thus, not only does our Preview simulator need to be provided with a ViewRouter instance, but also the actual App hierarchy when the app gets executed on a real device or in the standard simulator.

Therefore, let’s go to the NavigatingInSwiftUIApp file and declare a @StateObject property. Then, pass the initialized @StateObject to the viewRouter of our MotherView.

@main
struct NavigatingInSwiftUIApp: App {
    
    @StateObject var viewRouter = ViewRouter()
    
    var body: some Scene {
        WindowGroup {
            MotherView(viewRouter: viewRouter)
        }
    }
}

Our MotherView router is now able to observe and access the viewRouter‘s OberservableObject. So, let’s show the corresponding ContentView depending on the Page assigned to the viewRouter‘s currentPage property.

var body: some View {
    switch viewRouter.currentPage {
    case .page1:
        ContentViewA()
    case .page2:
        ContentViewB()
    }
}

You can delete the currentPage State of the MotherView, since we won’t need it anymore.

Let’s take a look at the simulator of our MotherView: The MotherView reads the value of the ViewRouter’s currentPage variable and hosts the corresponding ContentView. You can check this by changing the default value assigned to the ViewRouter’s currentPage property to .page2. Go back to the MotherView Preview simulator and see what happens! The @Published property of our ObservableObject told the MotherView to update its body.

Because we want ContentViewA to be the default view, assign .page1 to the currentPage property again.

Great! We accomplished a lot so far! We initialized a ViewRouter instance and bound it to the MotherView by using a @StateObject. Every time the values assigned to the currentPage property of the ViewRouter instance gets updated, the MotherView refreshes its body with eventually showing the right ContentView!

Bind the ContentViews to the ViewRouter ⛓

Our MotherView is now able to show the correct ContentView depending on the Page assigned to the currentPage property of the ViewRouter. But until now, the user is not able to change this value by tapping on the respective Button of ContentViewA and ContentViewB.

Let’s start with ContentViewA. To let it access the currentPage and manipulate its value we have to bind it to the ViewRouter. So, let’s create an @StateObject again. 

struct ContentViewA: View {
    
    @StateObject var viewRouter: ViewRouter
    
    var body: some View {
        //...
    }
}

We need to update the ContentViewA_Previews struct again.

struct ContentViewA_Previews: PreviewProvider {
    static var previews: some View {
        ContentViewA(viewRouter: ViewRouter())
    }
}

ContentViewA should observe the ViewRouter instance we created inside the NavigatingInSwiftUI struct and passed to the MotherView. So, let’s assign our new @StateObject to this instance when initializing ContentViewA in our MotherView.

switch viewRouter.currentPage {
case .page1:
    ContentViewA(viewRouter: viewRouter)
case .page2:
    ContentViewB()
}

Great! Now we have access to the currentPage property of our viewRouter. Use the Button’s action closure to assign .page2 to it when tapping on the “Next”-Button.

Button(action: {
    viewRouter.currentPage = .page2
}) {
    NextButtonContent()
}

Okay, let’s see if that works: Run the app in the regular simulator or start a Live preview of the MotherView and tap on the “Next”-Button. Great, we successfully navigate to ContentViewB! 

Here’s what happens when the user taps on the “Next”-Button of ContentViewA: ContentViewA changes the Page assigned to the currentPage property of the viewRouter to .page2. Therefore, the viewRouter tells all bound views to rerender their bodies, including the MotherView. The MotherView updates its body and checks the currentPage‘s value. Because it’s .page2 now, the case for showing ContentViewB is met and we eventually navigate to it!

To be able to navigate back to ContentViewA, repeat this implementation process for ContentViewB:

Declare a @StateObject property as a ViewRouter instance…

struct ContentViewB: View {
    
    @StateObject var viewRouter: ViewRouter
    
    var body: some View {
        //...
    }
}

…and update the related previews struct:

struct ContentViewB_Previews: PreviewProvider {
    static var previews: some View {
        ContentViewB(viewRouter: ViewRouter())
    }
}

Assign this viewRouter property to the initial ViewRouter instance passed by the NavigatingInSwiftUIApp struct to the MotherView.

switch viewRouter.currentPage {
case .page1:
    ContentViewA(viewRouter: viewRouter)
case .page2:
    ContentViewB(viewRouter: viewRouter)
}

Then, update the Button’s action parameter for showing the first page again:

Button(action: {
    viewRouter.currentPage = .page1
}) {
    BackButtonContent()
}

We can now navigate independently between our ContentViews!

Recap 🎊

We just figured out how to navigate between different views using an @ObservableObject. We created a ViewRouter and bound our MotherView and the ContentViews to it. Then, we achieved to manipulate the ViewRouter’s currentPage property when clicking on the ContentViews “Next”-Buttons. Due to the @Published property wrapper’s functionality, this causes the MotherView to rebuild its body with eventually hosting the right ContentView! 

But often, there is an alternative, more efficient way to do this: Using an @EnvironmentObject. EnvironmentObjects provide us with more freedom and independence within our app’s view hierarchy. You will see what we mean by that when reading on.

By the way: we are also going to learn how to apply some nice transition animations to our app!