SwiftUI previews stopped working: build error with @main

Question

Created Feb ’21

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I've been working for the past day or so to add a very minor feature to one of our UIKit based apps using SwiftUI, as an experiment. Suddenly, this morning, previews stopped working, citing a build error that only occurs when building for preview:

Compiling failed: 'main' attribute cannot be used in a module that contains top-level code

I was editing the view when this started happening, but no fiddling I've done with it since has fixed the issue.

I have tried:

Reverting all changes in Git
Cleaning the build folder (repeatedly)
Touching both the file with the SwiftUI view, and the AppDelegate that's annotated with @main
Closing the project, and Xcode

The project builds and runs just fine: this error only occurs with previews.

I had been working in Xcode 12.5 beta 2 when the problem started, but it persists when I open the project in Xcode 12.3.

Is there some extra build cache that I can go purge?

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Answer 1

Developer Tools Engineer OP

Apple

Feb ’21

Sorry to hear you are having problems getting previews working. Best next step will be to file a feedback with diagnostics.
We will need the diagnostics Xcode Previews generates in order to make sure we understand the error the previews system is encountering.

When you get an error in Xcode Previews, an error banner appears in the canvas
Click the "Diagnostics" button in that banner
In the sheet that appears, click "Generate Report" in the bottom left of the sheet
Attach (or make from the folder) the resulting zip file to the bug (will be named something like previews-diagnostics-0123456789.zip)

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Answer 2

610 OP

Feb ’21

OK. Filed FB9017523.

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Answer 3

ast3150 OP

Jun ’21

I was able to solve this error by moving the App.swift file (containing the @main statement) to trash, then re-creating the file and its contents.

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