Handle KeepAlive as a marker protocol

Sources/Atoms/Atom/Atom.swift

@@ -23,14 +23,6 @@ public protocol Atom {
     /// with other atoms.
     typealias UpdatedContext = AtomUpdatedContext
 
-    /// A boolean value indicating whether the atom value should be preserved even if
-    /// no longer watched to.
-    ///
-    /// It's recommended to conform the ``KeepAlive`` to this atom, instead of overriding
-    /// this property to return `true`.
-    /// The default is `false`.
-    static var shouldKeepAlive: Bool { get }
-
     /// A unique value used to identify the atom internally.
     ///
     /// This key don't have to be unique with respect to other atoms in the entire application
@@ -70,10 +62,6 @@ public protocol Atom {
 }
 
 public extension Atom {
-    static var shouldKeepAlive: Bool {
-        false
-    }
-
     func makeCoordinator() -> Coordinator where Coordinator == Void {
         ()
     }

Sources/Atoms/Core/AtomCache.swift

@@ -4,7 +4,6 @@ internal protocol AtomCacheProtocol: CustomStringConvertible {
 
     var atom: Node { get set }
     var value: Node.Loader.Value? { get set }
-    var shouldKeepAlive: Bool { get }
 
     func reset(with store: StoreContext)
 }
@@ -19,10 +18,6 @@ internal struct AtomCache<Node: Atom>: AtomCacheProtocol {
     var atom: Node
     var value: Node.Loader.Value?
 
-    var shouldKeepAlive: Bool {
-        Node.shouldKeepAlive
-    }
-
     func reset(with store: StoreContext) {
         store.reset(atom)
     }

Sources/Atoms/Core/StoreContext.swift

@@ -423,11 +423,10 @@ private extension StoreContext {
         //     1. It's not marked as `KeepAlive`.
         //     2. It has no downstream atoms.
         //     3. It has no subscriptions from views.
-        let shouldKeepAlive = store.state.caches[key]?.shouldKeepAlive ?? false
-        let shouldRelease =
-            !shouldKeepAlive
-            && (store.graph.children[key]?.isEmpty ?? true)
-            && (store.state.subscriptions[key]?.isEmpty ?? true)
+        let shouldKeepAlive = store.state.caches[key].map { $0 is any KeepAlive } ?? false
+        let isChildrenEmpty = store.graph.children[key]?.isEmpty ?? true
+        let isSubscriptionEmpty = store.state.subscriptions[key]?.isEmpty ?? true
+        let shouldRelease = !shouldKeepAlive && isChildrenEmpty && isSubscriptionEmpty
 
         guard shouldRelease else {
             return

Sources/Atoms/KeepAlive.swift

@@ -12,9 +12,3 @@
 /// ```
 ///
 public protocol KeepAlive where Self: Atom {}
-
-public extension KeepAlive {
-    static var shouldKeepAlive: Bool {
-        true
-    }
-}

Tests/AtomsTests/Core/AtomCacheTests.swift

@@ -4,20 +4,6 @@ import XCTest
 
 @MainActor
 final class AtomCacheTests: XCTestCase {
-    func testShouldKeepAlive() {
-        struct KeepAliveAtom: ValueAtom, KeepAlive, Hashable {
-            func value(context: Context) -> Int {
-                0
-            }
-        }
-
-        let cache0 = AtomCache(atom: TestValueAtom(value: 0))
-        let cache1 = AtomCache(atom: KeepAliveAtom())
-
-        XCTAssertFalse(cache0.shouldKeepAlive)
-        XCTAssertTrue(cache1.shouldKeepAlive)
-    }
-
     func testReset() {
         let store = AtomStore()
         let context = StoreContext(store)