Why Swift Package Manager does not support weak linking (-weak_framework SwiftUI)

Using Swift
1

Currently in SwiftPM with Swift5.1, it has only these methods in linker flags control:

public struct LinkerSetting : Encodable {
    public static func linkedLibrary(_ library: String, _ condition: PackageDescription.BuildSettingCondition? = nil) -> PackageDescription.LinkerSetting

    public static func linkedFramework(_ framework: String, _ condition: PackageDescription.BuildSettingCondition? = nil) -> PackageDescription.LinkerSetting

    public static func unsafeFlags(_ flags: [String], _ condition: PackageDescription.BuildSettingCondition? = nil) -> PackageDescription.LinkerSetting
}

None of these 3 options is useful for real world App use case.

The unsafeFlags can not be used in a framework, because Xcode will complain that:

The package product 'SDWebImageSwiftUI' cannot be used as a dependency of this target because it uses unsafe build flags.
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2

You don't need to explicitly weak link system frameworks like SwiftUI. If your package has a deployment target lower than macOS 10.15 / iOS 13.0 / tvOS 13.0 / watchOS 6.0, then SwiftUI will automatically be weak linked.

If you find some configuration where this does NOT occur, please file a bug report and attach a sample project which reproduces the issue if possible.

3

The problem is not only SwiftUI. The problem exist on all the cases, like Combine, like LinkPresentation, like RealityKit.

I’ve already says, using the min deployment target version have Side-Effect. which is not a framework author want and have to avoid. I don’t think mixing these two things (weak linking and OS compatibility), for a Package Manager, is a good design

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4

And currently in Xcode 11.2.1, Yes it’s s Bug that SwiftUI is automatically weak linking if the min deployment target is iOS 12. But Combine framework is not.

If you only change that, you will still get a dyld crash when startup because Combine symbol is not loaded on iOS 12. You have a manually specify -weak_framework Combine in Other Linker Flags.

I think I’ve already fired Radars, if not, I can fire again. Another anooying problem on Xcode is that the Linked Libaraies and Framework does not showing Combine, which menas I can not add Combine and choose thr framework into Optional(this is the same effect as using -weak_framework Combine). Not a big point but just annoying if some new developers don’t know what a linker do.

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