Fix a recycling bug that purges indirect subviews

LayoutKitTests/ViewRecyclerTests.swift

@@ -91,6 +91,30 @@ class ViewRecyclerTests: XCTestCase {
         XCTAssertNotNil(one.superview)
     }
 
+    func testRecycledViewWithoutPurgingIndirectSubviews() {
+        let root = View()
+        let headerView = View()
+        headerView.isLayoutKitView = true
+        root.addSubview(headerView)
+        let titleView = View(viewReuseId: "1")
+        root.addSubview(titleView)
+        let collectionView = View()
+        let cellView = View()
+        cellView.isLayoutKitView = true
+        root.addSubview(collectionView)
+        collectionView.addSubview(cellView)
+
+        let recycler = ViewRecycler(rootView: root)
+        let _ = recycler.makeOrRecycleView(havingViewReuseId: nil, viewProvider: {
+            return View()
+        })
+
+        recycler.purgeViews()
+        XCTAssertNil(headerView.superview)
+        XCTAssertNil(titleView.superview)
+        XCTAssertNotNil(cellView.superview)
+    }
+
     #if os(iOS) || os(tvOS)
     /// Test that a reused view's frame shouldn't change if its transform and layer anchor point
     /// get set to the default values.
@@ -148,5 +172,7 @@ extension View {
     convenience init(viewReuseId: String) {
         self.init(frame: .zero)
         self.viewReuseId = viewReuseId
+        // All views created in the ViewRecycler should be marked as LayoutKitView.
+        self.isLayoutKitView = true
     }
 }

Sources/ViewRecycler.swift

@@ -28,7 +28,7 @@ class ViewRecycler {
     private let defaultTransform = CGAffineTransform.identity
     #endif
 
-    /// Retains all subviews of rootView for recycling.
+    /// Retains all LayoutKit subviews of rootView for recycling.
     init(rootView: View?) {
         rootView?.walkSubviews { (view) in
             if let viewReuseId = view.viewReuseId {
@@ -108,9 +108,14 @@ private var isLayoutKitViewKey: UInt8 = 0
 
 extension View {
 
-    /// Calls visitor for each transitive subview.
+    /// Calls visitor for each transitive LayoutKit subview.
     func walkSubviews(visitor: (View) -> Void) {
-        for subview in subviews {
+        /*
+         Fix a recycling bug that purges indirect subviews by only walking subviews that are created by LayoutKit.
+         If a subview isn't a LayoutKit subview, then there is no need to walk subviews of it,
+         as they must not be created by this layout.
+         */
+        for subview in subviews where subview.isLayoutKitView {
             visitor(subview)
             subview.walkSubviews(visitor: visitor)
         }