Xcode 12 - SwiftUI preview doesn't work on Swift Package when have another Swift Package as dependencies - 'previewInstances' message to agent

Question

I have an issue on SwiftUI preview in a view located in a Swift Package when my code imports a control or value from an other swift package.

import Foundation
import SwiftUI
import Common

struct AppointmentListItem: View {
    var appointment: Appointment
    var body: some View {
        VStack{
            HStack(spacing: 10){
               //Client Info
                Image(self.appointment.client.profilePicture)
                   .resizable()
                   .aspectRatio(contentMode: .fill)
                   .frame(width: 35, height: 35)
                   .clipShape(Circle())
                   .shadow(radius: 10)
                   .overlay(Circle().stroke(Color.white, lineWidth: 1.5))
                Text(self.appointment.client.fullName)
                   .font(.system(size: 18))
                   .bold()
                   .frame(maxWidth: .infinity, alignment: .leading)
                Text(self.appointment.getHourAndMinutes()).bold()
               //Detail info
               Button(action: {
                   withAnimation{
                       print("Go to details")
                   }
               }){
                   Image(systemName: "ellipsis")
                       .font(.system(size: 18))
                       .frame(width: 20, height: 20)
                    .rotationEffect(Angle.init(degrees: 90))
               }
            }
            .padding()
        }
        .foregroundColor(Color.white)
        .background(RoundedRectangle(cornerRadius: 20)
                        .fill(Color.hippoPrimary)// <- this color is part of Common package
        )
    }
}

If I remove or change .fill(Color.hippoPrimary) the preview is available.

The error provided by Xcode is the following:

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

This is my Package.swift file:

// swift-tools-version:5.3
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "TodayAppointments",
    platforms: [
        .iOS(.v13)
    ],
    products: [
        // Products define the executables and libraries a package produces, and make them visible to other packages.
        .library(
            name: "TodayAppointments",
            targets: ["TodayAppointments"]),
    ],
    dependencies: [
        // Dependencies declare other packages that this package depends on.
        // .package(url: /* package url */, from: "1.0.0"),
        .package(path: "../Common")
    ],
    targets: [
        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
        // Targets can depend on other targets in this package, and on products in packages this package depends on.
        .target(
            name: "TodayAppointments",
            dependencies: ["Common"]),
        .testTarget(
            name: "TodayAppointmentsTests",
            dependencies: ["TodayAppointments"]),
    ]
)

In the Common Package, the Colors are defined this way:

public extension Color {
    static let hippoPrimary = Color("Primary", bundle: .module)
    static let progressBarBackground = Color("ProgressBarBackground", bundle: .module)
    static let textBackground = Color("TextBackground", bundle: .module)
    static let textColor = Color("TextColor", bundle: .module)
    static let appleSignInBackground = Color("AppleSignInBackground", bundle: .module)
    static let buttonActionText = Color("Text", bundle: .module)
}

The build hasn't errors so I understand that the dependencies are ok, sounds like a IDE.

Thanks in Advance.

Answer 1

Workaround for both iOS and macOS (not tested with Catalyst):

extension Foundation.Bundle {
    static var swiftUIPreviewsCompatibleModule: Bundle {
        final class CurrentBundleFinder {}

        /* The name of your local package, prepended by "LocalPackages_" for iOS and "PackageName_" for macOS. You may have same PackageName and TargetName*/
        let bundleNameIOS = "LocalPackages_TargetName"
        let bundleNameMacOs = "PackageName_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,

            /* 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")
    }
}

Then replace calls to Bundle.module with Bundle.swiftUIPreviewsCompatibleModule.

Answer 2

Update 1:

Sorry my last post was not accurate. This is a bug in Xcode with no work around. Just submitted to Apple a bug report (FB8880328). Also, posted a details write up with example code w/ repro steps here. Direct link to GitHub project: https://github.com/ryanholden8/SwiftUI-Preview-Failing-Test-Project

Old Post:

Got this exact error doing the same thing, putting colors in a separate package. This post helped me get to the bottom of it. I deleted the default class that is generated in the colors package. However, I did not delete the unit test that was based on that default class.

In short: Delete the auto-generated unit test in your Common package. Or make sure all the unit tests pass.