Debugging SwiftUI: Trials and Tribulations

Assumed audience: people interested SwiftUI or UI programming in general.

I have been using SwiftUI since day one, on both client and side projects. It’s an incredibly refreshing way of coding interfaces for Apple devices, but it’s far from being wrinkle-free. The framework’s bold ambitions are sometimes hampered by subpar debugging tools and arcane, seemingly random limitations.

I previously wrote about a SwiftUI bug that gave me quite a run for my money:

The stack trace isn’t of much help and no amount of view hierarchy tweaks got rid of it entirely. Initially I was under the impression that either my code or one of my external dependencies were doing something horribly wrong, but this post from Peter Steinberger made me realize that it is a rather widespread issue.

Since I didn’t manage to reproduce it on a particular test device at the time, I assumed it’s yet another simulator quirk. But a little over a month later, the same bug reared its ugly head on a test device. Fun times!

The app in question uses tab navigation where the first tab is always accessible, while the second tab shows different content depending on whether the user is signed in to their account or not. The view code looked roughly like this:

TabView {
  Home()
    .tabItem { /* ... */ }
    .tag { /* ... */ }

  if isSignedIn {
    Profile()
      .tabItem { /* ... */ }
      .tag { /* ... */ }
  } else {
    SignIn()
      .tabItem { /* ... */ }
      .tag { /* ... */ }
  }
}

To avoid the isSignedIn boolean check in multiple modifiers, I opted for an if-else block with separate views and tabItem modifiers.1 Except that didn’t seem to sit right with SwiftUI for some reason. The app launches and displays the content of the first tab exactly as intended. But when I switch tabs—or god forbid, sign in or out—the app crashes with this cryptic error message:

[error] precondition failure: invalid size for indirect attribute: <number> vs <number>

The call stack points exclusively to SwiftUI internals, with the last call referring to a certain AG::Graphresponsible for holding the view tree and diffing it. As far as debugging goes, this is very, very little to work with.

With my hands tied, I briefly considered resurrecting the UITabBarController wrapper I used prior to WWDC, especially since I didn’t manage to make a reproducible test case or submit a feedback to Apple. At first, I spent an inordinate amount of time tweaking the view hierarchies of the profile and sign-in screens themselves, with varying—and rather inconsistent—levels of success. I ran out of both time and patience, so I moved on to other things to not stall the project any further. Weeks later, I came back to the problematic screen—pitchfork in hand—ready to gut the entire tab navigation and replace it with a custom-made solution. I had had enough.

But then came the proverbial aha moment: what if the conditional check took place inside the second tab, instead of around it?

TabView {
  Home()
    .tabItem { /* ... */ }
    .tag { /* ... */ }

  ProfileOrSignIn(isSignedIn: $isSignedIn)
    .tabItem { /* ... */ }
    .tag { /* ... */ }
}

And just like that, the crasher was gone. Squashed into oblivion.

If this sounds dumb, it’s because it is. At no point did the compiler chastise me for doing something I wasn’t supposed to do. And to make matters worse, the documentation doesn’t mention anything related to… wait, what documentation?

It’s a bittersweet feeling when you fix a bug in a non-deterministic, haphazard way. On one hand you are elated you’ve managed to fix it at all. On the other, you feel somewhat powerless and insecure about your ability to handle similar situations in the future. I adore SwiftUI, but moments like these give me a good dose of anticipatory anxiety—the kind that erodes trust and dampen the thrill of being at the bleeding edge.


  1. As an aside, apps that greet users with a sign up screen are generally trash. You don’t ask prospective tenants to sign the contract at the door before they’ve even seen the apartment.

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