Rule Request: Enforce private properties in SwiftUI View's

[sindresorhus]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

Apple recommends that state properties in SwiftUI View's are private:

You should only access a state property from inside the view’s body, or from methods called by it. For this reason, declare your state properties as private, to prevent clients of your view from accessing it. - https://developer.apple.com/documentation/swiftui/state

2. Provide several examples of what would and wouldn't trigger violations.

Would trigger:

struct ContentView: View {
	@State var foo = ""
}

struct ContentView: View {
	@ObservedObject var foo = Foo()
}

struct ContentView: View {
	@EnvironmentObject var foo
}

struct ContentView: View {
	@StateObject var foo = Foo()
}

struct ContentView: View {
	@Environment(\.colorScheme) var colorScheme
}

Would NOT trigger:

struct ContentView: View {
	var foo = ""
}

3. Should the rule be configurable, if so what parameters should be configurable?

I would like to see an option to enforce that all properties that are not initializers to be private.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

Yes, since it's what Apple recommends.

[christiankm]

I created a simple custom rule for this, as a starting point. I will try to make a PR for an official rule.

state_private:
    name: "Private SwiftUI State"
    regex: "\\@State\\s*var"
    message: "SwiftUI @State variables should always be marked private."
    severity: warning

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. Please comment to prevent this issue from being closed. Thank you for your contributions!

[sindresorhus]

Please keep it open.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. Please comment to prevent this issue from being closed. Thank you for your contributions!

[sindresorhus]

Please keep it open.

[mt00chikin]

FYI, I created #4769 as a starting point to try and address a subset of these concerns. My team was also looking at a custom rule to detect @State properties and require them to be private. It doesn't cover every concern addressed in here, but it could definitely be updated or configured to cover additional cases if desired.