Create rule that requires SwiftUI state properties to be private

Make private state property rule opt-in

Update CHANGELOG.md

CHANGELOG.md

@@ -18,6 +18,11 @@
 
 #### Enhancements
 
+* Add new `private_swiftui_state_property` opt-in rule to encourage setting 
+  SwiftUI `@State` properties to private.  
+  [mt00chikin](https://github.com/mt00chikin)
+  [#3173](https://github.com/realm/SwiftLint/issues/3173)
+
 * Add local links to rule descriptions to every rule listed
   in `Rule Directory.md`.  
   [kattouf](https://github.com/kattouf)

Source/SwiftLintFramework/Extensions/SwiftSyntax+SwiftLint.swift

@@ -113,6 +113,21 @@ extension TokenKind {
     }
 }
 
+extension AttributeListSyntax? {
+    var hasStateAttribute: Bool {
+        guard let attributes = self else { return false }
+
+        return attributes.contains { attr in
+            guard let stateAttr = attr.as(CustomAttributeSyntax.self),
+                  let identifier = stateAttr.attributeName.as(SimpleTypeIdentifierSyntax.self) else {
+                return false
+            }
+
+            return identifier.name.text == "State"
+        }
+    }
+}
+
 extension ModifierListSyntax? {
     var containsLazy: Bool {
         contains(tokenKind: .contextualKeyword("lazy"))

Source/SwiftLintFramework/Models/PrimaryRuleList.swift

@@ -1,4 +1,4 @@
-// Generated using Sourcery 2.0.0 — https://github.com/krzysztofzablocki/Sourcery
+// Generated using Sourcery 2.0.1 — https://github.com/krzysztofzablocki/Sourcery
 // DO NOT EDIT
 
 /// The rule list containing all available rules built into SwiftLint.
@@ -149,6 +149,7 @@ let builtInRules: [Rule.Type] = [
     PrivateOutletRule.self,
     PrivateOverFilePrivateRule.self,
     PrivateSubjectRule.self,
+    PrivateSwiftUIStatePropertyRule.self,
     PrivateUnitTestRule.self,
     ProhibitedInterfaceBuilderRule.self,
     ProhibitedSuperRule.self,

Source/SwiftLintFramework/Rules/Lint/PrivateSwiftUIStatePropertyRule.swift

@@ -0,0 +1,74 @@
+import SwiftSyntax
+
+/// Rule to require that any state properties in SwiftUI be declared as private.
+/// State properties should only be accessible from inside the View's body, or from methods called by it
+struct PrivateSwiftUIStatePropertyRule: SwiftSyntaxRule, OptInRule, ConfigurationProviderRule {
+    var configuration = SeverityConfiguration(.warning)
+
+    static let description = RuleDescription(
+        identifier: "private_swiftui_state",
+        name: "Private SwiftUI @State Properties",
+        description: "SwiftUI's state properties should be private",
+        kind: .lint,
+        nonTriggeringExamples: [
+            Example(
+                """
+                struct ContentView: View {
+                    @State private var isPlaying: Bool = false
+                }
+                """
+            ),
+            Example(
+                """
+                struct ContentView: View {
+                    @State fileprivate var isPlaying: Bool = false
+                }
+                """
+            ),
+            Example(
+                """
+                struct ContentView: View {
+                    var isPlaying = false
+                }
+                """
+            ),
+            Example(
+                """
+                struct ContentView: View {
+                    @StateObject var foo = Foo()
+                }
+                """
+            )
+        ],
+        triggeringExamples: [
+            Example(
+                """
+                struct ContentView: View {
+                    @State var isPlaying: Bool = false
+                }
+                """
+            )
+        ])
+
+    init() {}
+
+    func makeVisitor(file: SwiftLintFile) -> ViolationsSyntaxVisitor {
+        Visitor(viewMode: .sourceAccurate)
+    }
+}
+
+private extension PrivateSwiftUIStatePropertyRule {
+    final class Visitor: ViolationsSyntaxVisitor {
+        override func visitPost(_ node: MemberDeclListItemSyntax) {
+            guard
+                let decl = node.decl.as(VariableDeclSyntax.self),
+                decl.attributes.hasStateAttribute,
+                !decl.modifiers.isPrivateOrFileprivate
+            else {
+                return
+            }
+
+            violations.append(decl.letOrVarKeyword.positionAfterSkippingLeadingTrivia)
+        }
+    }
+}

Tests/GeneratedTests/GeneratedTests.swift

@@ -1,4 +1,4 @@
-// Generated using Sourcery 2.0.0 — https://github.com/krzysztofzablocki/Sourcery
+// Generated using Sourcery 2.0.1 — https://github.com/krzysztofzablocki/Sourcery
 // DO NOT EDIT
 @_spi(TestHelper)
 @testable import SwiftLintFramework
@@ -877,6 +877,12 @@ class PrivateSubjectRuleGeneratedTests: XCTestCase {
     }
 }
 
+class PrivateSwiftUIStatePropertyRuleGeneratedTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(PrivateSwiftUIStatePropertyRule.description)
+    }
+}
+
 class PrivateUnitTestRuleGeneratedTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(PrivateUnitTestRule.description)