Debugging SwiftUI — Common Errors and How to Fix Them

Welcome to Week 12 of the SwiftUI Blog Series!

As elegant as SwiftUI is, debugging can feel like a black box when things go wrong. A blank preview, an unresponsive button, or a layout loop can quickly leave you scratching your head. The good news? Most SwiftUI bugs are predictable, fixable, and often teach you more about how the framework thinks.

In this post, you’ll learn to:

• Decode common SwiftUI error messages
• Fix layout, preview, and state bugs
• Use tools and techniques to isolate and resolve issues

Let’s dive in and take the stress out of debugging 🛠️

1. “The compiler is unable to type-check this expression in reasonable time”

var body: some View {
    VStack {
        if isLoggedIn {
            // Complex view logic
        } else {
            // More complex logic
        }
    }
}

Explanation:

This error means Swift can’t infer the types fast enough. It’s often caused by too many nested views or complex inline logic in one body.

✅ Solution: Break the view into smaller pieces.

var body: some View {
    VStack {
        contentView()
    }
}

@ViewBuilder
private func contentView() -> some View {
    if isLoggedIn {
        LoggedInView()
    } else {
        LoggedOutView()
    }
}

Now the compiler only needs to reason about a small part of the UI at a time — problem solved.

2. SwiftUI Preview Is Blank or Not Updating

#Preview {
    ContentView()
        .previewLayout(.sizeThatFits)
        .padding()
        .background(Color.gray)
}

Explanation:

SwiftUI Previews can break if your view does too much on .onAppear() (like hitting APIs) or if it relies on missing dependencies.

✅ Fixes:

• Use .previewLayout(.sizeThatFits)
• Replace @ObservedObject with .constant() or a mock
• Avoid live work in .onAppear()
• Press Cmd + Option…