How to do print debugging in SwiftUI

14 Aug 2021 ⋅ 3 min read ⋅ SwiftUI Debugging

Table of Contents

Print debugging[1] is the simplest form of debugging. It is a debugging technique that we can easily carry over to any language and IDE.

In UIKit, we use a print statement to trace the flow of execution or stored value. In this example, we print out view variable in a viewDidLoad.

override func viewDidLoad() {
    super.viewDidLoad()

print(view)
}

Coming from the UIKit world, it might tempting to use print inside a SwiftUI view. But if you tried to put a print statement inside a SwiftUI view, you will get "Type '()' cannot conform to 'View'" compile error.

struct ContentView: View {
    var body: some View {
        print("Update body")
        Text("Hello, World!")
    }
}

Type '()' cannot conform to 'View' error.

Practical Combine: Learn everything you need to know about Combine and how you can use it in your projects with Practical Combine. It contains:

Thirteen chapters worth of content. Playgrounds and sample projects that use the code shown in the chapters. Free updates for future iOS versions.

Use print statement inside SwiftUI view

The solution to the problem is easy. You need to assign print() to a variable (var) or a constant (let).

Here is an example of how we can use print() in a SwiftUI view.

struct ContentView: View {
    var body: some View {
        var x = print("Update body") // 1
        let y = print("Update body") // 2
        let _ = print("Update body") // 3
        Text("Hello, World!")
    }
}

1 and 2 We don't use the returning result (x and y) anywhere. The compiler will give a warning in these cases.
3 Since there is no use for print's return type, using an underscore to ignore the value and silence the warning is preferred.

Result:

Update body
Update body
Update body

Here is an example where we use print to detect whether the view body is reevaluated.

struct ContentView: View {
    @State private var isTrue = true

var body: some View {
        VStack {
            // 1
            let _ = print("Update ContentView")
            Text(isTrue ? "True": "False")
            Button("Toggle") {
                isTrue.toggle()
            }
            SpyView()
        }
    }
}

struct SpyView: View {
    var body: some View {
        // 2
        let _ = print("Update SpyView")
        Text("Spy")
    }
}

1 We use this print to trace whether the body of ContentView is executed or not.
2 We use this print to trace whether the body of SpyView is executed or not.

We learn from the result that only the body of the ContentView is reevaluated when we toggle the isTrue. The SpyView don't need to reevaluate since it doesn't depend on the isTrue value.

Print debuggin in SwiftUI.

Practical Combine: Learn everything you need to know about Combine and how you can use it in your projects with Practical Combine. It contains:

Thirteen chapters worth of content. Playgrounds and sample projects that use the code shown in the chapters. Free updates for future iOS versions.

Use _printChanges statement to understand what is causing a view to be reevaluated

If you use a print statement to trace view lifecycle, you can use _printChanges for better understanding. _printChanges print out what is causing a view to be reevaluated.

In the following example, we change from print to _printChanges.

struct ContentView: View {
    @State private var isTrue = true

var body: some View {
        VStack {
            let _ = Self._printChanges()
            Text(isTrue ? "True": "False")
            Button("Toggle") {
                isTrue.toggle()
            }
            SpyView()
        }
    }
}

struct SpyView: View {
    var body: some View {
        let _ = Self._printChanges()
        Text("Spy")
    }
}

You can see in the debug console what is causing a view to be reevaluated.

_printChanges print out what is causing a view reevaluated.

We need Self in Self._printChanges() since _printChanges is a static method.
::

Print debugging (or tracing) is the act of watching (live or recorded) trace statements, or print statements, that indicate the flow of execution of a process. https://en.wikipedia.org/wiki/Debugging ↩︎

Enjoy the read?

If you enjoy this article, you can subscribe to the weekly newsletter.
Every Friday, you'll get a quick recap of all articles and tips posted on this site. No strings attached. Unsubscribe anytime.

Feel free to follow me on Twitter and ask your questions related to this post. Thanks for reading and see you next time.

If you enjoy my writing, please check out my Patreon https://www.patreon.com/sarunw and become my supporter. Sharing the article is also greatly appreciated.

Become a patron

Buy me a coffee

Pop-Up Buttons in SwiftUI

Learn how to create macOS pop-up buttons in SwiftUI.

How to present an alert in SwiftUI in iOS 13/14

Learn how to show an alert (UIAlertController) in SwiftUI.

← Home