Question posted in Xcode
Whether you're new to Xcode or an experienced developer, our archive has everything you need to know about this integrated development environment (IDE). From basic functionalities to advanced features, our archive covers a wide range of Xcode-related questions and answers. Browse our archive now and find solutions to your Xcode questions, and take your app development skills to the next level

Xcode – SwiftUI preview crashes when accessing assets from a nested Swift Package

MojtabaHosseini
May 17, 2021
197 views
1 vote
2 Answers

A Module

I have a module (A) that has two structs.

One with a color initiated directly in code.
One with a color loaded from the assets.

public struct CodeColor {
    public init() { }
    public let value = SwiftUI.Color(#colorLiteral(red: 0.8549019694, green: 0.250980407, blue: 0.4784313738, alpha: 1))
}

public struct AssetColor {
    public init() { }
    public let value = SwiftUI.Color("Legacy/Material/Gold", bundle: .module)
}

The preview is working like a charm:

B Module

The second module (B) should use the previous one (A) as a dependency to load colors from:

import A

public struct CodeColor {
    public init() { }
    public var value: SwiftUI.Color { A.CodeColor().value }
}

public struct AssetColor {
    public init() { }
    public var value: SwiftUI.Color { A.AssetColor().value }
}

But as soon as it touches the assets from module (A), the preview crashes:

🛑 The error:

Can not preview in this file. Failed to update preview.

RemoteHumanReadableError: Failed to update preview.

The preview process appears to have crashed.

Error encountered when sending 'previewInstances' message to agent.

==================================

|  RemoteHumanReadableError: The operation couldn’t be completed. (BSServiceConnectionErrorDomain error 3.)
|  
|  BSServiceConnectionErrorDomain (3):
|  ==BSErrorCodeDescription: OperationFailed

So why is that?

Note: The stranger thing is that the exact B preview code is working if it is in an actual app (not another package)

Here is the full code on github

Answers

I found a solution in the developer forum:

Link

https://developer.apple.com/forums/thread/664295?login=true#reply-to-this-question

NOTE – LOCAL

The solution is for local package. You define

let bundleNameIOS = "LocalPackages_TargetName"
let bundleNameMacOs = "PackageName_TargetName"

REMOTE (Packages from GitHub e.g.)

If you have your package on GitHub and fetch it remotely, you can’t define local packages and you have to slightly change it to:

let bundleNameIOS = "TargetName_TargetName"
let bundleNameMacOs = "TargetName_TargetName"

Example

Here is a full implementation example, you have to put it inside your package with the resources:

// Inside your package with the resources:
// Extend Bundle to access it in other packages
extension Bundle {

    // public static let assets = Bundle.module
    // Updated with workaround
    public static let assets = Bundle.myModule
    
}

Define your bundle

private class CurrentBundleFinder {}
extension Foundation.Bundle {
    static var myModule: Bundle = {
        /* The name of your package. You may have same PackageName and TargetName*/
        let bundleNameIOS = "TargetName_TargetName"
        let bundleNameMacOs = "TargetName_TargetName"
        
        let candidates = [
            /* Bundle should be present here when the package is linked into an App. */
            Bundle.main.resourceURL,
            /* Bundle should be present here when the package is linked into a framework. */
            Bundle(for: CurrentBundleFinder.self).resourceURL,
            // -> Optional UI Tests
            /* Bundle should be present here when the package is used in UI Tests. */
            Bundle(for: CurrentBundleFinder.self).resourceURL?.deletingLastPathComponent(),
            /* For command-line tools. */
            Bundle.main.bundleURL,
            /* Bundle should be present here when running previews from a different package (this is the path to "…/Debug-iphonesimulator/"). */
            Bundle(for: CurrentBundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent(),
            Bundle(for: CurrentBundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent(),
        ]
        
        for candidate in candidates {
            let bundlePathiOS = candidate?.appendingPathComponent(bundleNameIOS + ".bundle")
            let bundlePathMacOS = candidate?.appendingPathComponent(bundleNameMacOs + ".bundle")
            if let bundle = bundlePathiOS.flatMap(Bundle.init(url:)) {
                return bundle
            } else if let bundle = bundlePathMacOS.flatMap(Bundle.init(url:)) {
                return bundle
            }
        }
        fatalError("unable to find bundle")
    }()
}

Accessing package A resource in package B

You can now access the resource from package A inside package B like:

let example = UIImage(named: "Exampe", in: Bundle.assets, with: nil)!

This workaround worked for me in Xcode 14.2

private extension Bundle {
    private static let packageName = "PACKAGE_NAME"
    private static let moduleName = "MODULE_NAME"
    
    #if targetEnvironment(simulator)
    static var swiftUIPreviewsCompatibleModule: Bundle {
        final class CurrentBundleFinder {}

        let isPreview = ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1"
        
        guard isPreview else {
            return Bundle.module
        }
        
        // This is a workaround for SwiftUI previews
        // previews crash when accessing other package view using assets from Bundle.module
        
        let bundleName = "(packageName)_(moduleName).bundle"
        
        func bundle(stepsBack: Int) -> Bundle? {
            var bundleURL = Bundle(for: CurrentBundleFinder.self).bundleURL
            for _ in 1...stepsBack { bundleURL.deleteLastPathComponent() }
            bundleURL.appendPathComponent(moduleName)
            bundleURL.appendPathComponent("Products")
            bundleURL.appendPathComponent("Debug-iphonesimulator")
            bundleURL.appendPathComponent("PackageFrameworks")
            
            let directories: [String]
            do {
                directories = try FileManager.default.contentsOfDirectory(atPath: bundleURL.path)
            } catch {
                return nil
            }
            
            guard let matchingDir = directories.first(where: { $0.hasSuffix(".framework") }) else {
                return nil
            }
            
            bundleURL.appendPathComponent(matchingDir)
            bundleURL.appendPathComponent(bundleName)
            
            return Bundle(url: bundleURL)
        }
        
        // Steps back 5 is a workaround for crashes
        // when another module is importing this module
        return bundle(stepsBack: 5) ?? .module
    }
    #else
    static var swiftUIPreviewsCompatibleModule: Bundle { .module }
    #endif
}

Please signup or login to give your own answer.

Click here to cancel reply.