Swift UI（Xcode 12 Beta）：ContentView.swift编译失败，预览无法更新

Hey Jason, I feel your pain—SwiftUI previews can be total divas, especially with early Xcode betas. It’s so weird that the simulator works perfectly but previews are stuck failing to update, but let’s walk through some more targeted fixes to get this sorted out:

• Wipe Xcode’s Derived Data
  Corrupted cached data is one of the most common culprits here. Head to Xcode > Preferences > Locations, click the tiny arrow next to "Derived Data" to open the folder. Find the folder tied to your project, delete it entirely, then quit Xcode cold and restart it. This forces Xcode to rebuild all its cached files from scratch, which often fixes those random preview glitches.

• Double-Check Preview Target Configuration
  Sometimes the preview target gets out of sync with your main app target:

  1. Open your project settings and select the preview target (usually named something like YourAppNamePreview)
  2. Make sure the Deployment Info matches your main target (same iOS version, device type)
  3. Verify the Signing & Capabilities tab has valid settings—even if the simulator doesn’t care, previews can throw fits over tiny signing mismatches.

• Recreate Your Preview File
  It’s possible your existing preview file got corrupted somehow. Try deleting your current preview code, create a brand new SwiftUI file with a super simple test preview like this:

  struct TestPreview: View {
      var body: some View {
          Text("Hello Preview")
      }
  }
  
  struct TestPreview_Previews: PreviewProvider {
      static var previews: some View {
          TestPreview()
      }
  }
  

  If this loads correctly, you can slowly migrate your original code into this new file.

• Update to the Latest Xcode 12 Beta
  Xcode 12’s early betas had tons of preview-related bugs. If you’re not running the very last beta release of Xcode 12, grab the update from Apple’s developer portal—they fixed a lot of these annoying preview issues in later builds.

• Disable Xcode Extensions
  Third-party extensions (like code linters or color pickers) can quietly interfere with previews. Quit Xcode, go to Xcode > Preferences > Extensions, and disable all extensions. Restart Xcode and test the preview again.

Here’s a screenshot of the error I’m encountering:

If none of these steps work, try creating a brand new SwiftUI project from scratch. If previews work there, the problem is likely tied to a wonky setting in your current project—you can then compare the two project configurations to spot the difference.

内容的提问来源于stack exchange，提问作者Jason