Fix an invalid behavior after switching to a new layout object (#611)

* Added a test for the use case, ControllerTests.test_switching_layout()

Sources/Core.swift

@@ -209,6 +209,9 @@ class Core: NSObject, UIGestureRecognizerDelegate {
             contentOffset = scrollView?.contentOffset
         }
 
+        if layoutAdapter.validStates.contains(state) == false {
+            state = layoutAdapter.initialState
+        }
         layoutAdapter.updateStaticConstraint()
         layoutAdapter.activateLayout(for: state, forceLayout: forceLayout)
 
@@ -725,7 +728,6 @@ class Core: NSObject, UIGestureRecognizerDelegate {
         }
 
         guard shouldAttract(to: target) else {
-            self.state = target
             self.updateLayout(to: target)
             self.unlockScrollView()
             // The `floatingPanelDidEndDragging(_:willAttract:)` must be called after the state property changes.

Sources/Layout.swift

@@ -782,12 +782,6 @@ class LayoutAdapter {
             NSLayoutConstraint.activate(constraint: self.fitToBoundsConstraint)
         }
 
-        var state = state
-
-        if validStates.contains(state) == false {
-            state = layout.initialState
-        }
-
         // Recalculate the intrinsic size of a content view. This is because
         // UIView.systemLayoutSizeFitting() returns a different size between an
         // on-screen and off-screen view which includes

Tests/ControllerTests.swift

@@ -334,6 +334,36 @@ class ControllerTests: XCTestCase {
         fpc.move(to: .tip, animated: false)
         XCTAssertEqual(fpc.surfaceView.frame.height, fpc.view.bounds.height - fpc.surfaceLocation(for: .tip).y)
     }
+
+    func test_switching_layout() {
+        final class FirstLayout: FloatingPanelLayout {
+            let position: FloatingPanelPosition = .bottom
+            let initialState: FloatingPanelState = .half
+            let anchors: [FloatingPanelState : FloatingPanelLayoutAnchoring] = [
+                .full: FloatingPanelLayoutAnchor(absoluteInset: 16.0, edge: .top, referenceGuide: .safeArea),
+                .half: FloatingPanelLayoutAnchor(absoluteInset: 262, edge: .top, referenceGuide: .safeArea),
+                .tip: FloatingPanelLayoutAnchor(absoluteInset: 44.0, edge: .bottom, referenceGuide: .safeArea)
+            ]
+        }
+        final class SecondLayout: FloatingPanelLayout {
+            let position: FloatingPanelPosition = .bottom
+            let initialState: FloatingPanelState = .half
+            let anchors: [FloatingPanelState : FloatingPanelLayoutAnchoring] = [
+                .half: FloatingPanelLayoutAnchor(absoluteInset: 262, edge: .top, referenceGuide: .safeArea)
+            ]
+        }
+        let fpc = FloatingPanelController()
+        fpc.layout = FirstLayout()
+        fpc.showForTest()
+
+        fpc.move(to: .tip, animated: false)
+
+        // Switch to another layout
+        fpc.layout = SecondLayout()
+        fpc.invalidateLayout()
+
+        XCTAssertEqual(fpc.state, .half)
+    }
 }
 
 private class MyZombieViewController: UIViewController, FloatingPanelLayout, FloatingPanelBehavior, FloatingPanelControllerDelegate {