How to do print debugging in SwiftUI

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!")
    }
}

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

Demo

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.

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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.