End the game when new pieces can't be placed

Tetris Mac/ContentView.swift

@@ -60,6 +60,10 @@ struct ContentView: View {
     
     func startTimer() {
         self.timer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true, block: { (_) in
+            guard case .playing(_) = self.controller.state else {
+                self.stopTimer()
+                return
+            }
             self.controller.step()
         })
     }

Tetris/ContentView.swift

@@ -123,6 +123,10 @@ struct ContentView: View {
 
     func startTimer() {
         self.timer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { (_) in
+            guard case .playing(_) = self.controller.state else {
+                self.stopTimer()
+                return
+            }
             self.controller.step()
         }
     }

TetrisKit/GameController.swift

@@ -32,10 +32,6 @@ public class GameController: ObservableObject {
     @Published public var score = 0
     var previousPieceWasTetris = false
 
-    public var ended: Bool {
-        return (0..<width).first(where: { board[$0, 0] }) != nil
-    }
-
     public init() {
         self.board = GameBoard(width: width, height: height)
         self.currentPiece = nil
@@ -48,13 +44,20 @@ public class GameController: ObservableObject {
     }
     
     func nextPiece() {
-        let tetromino = nextTetrominoes.removeFirst()
-        currentPiece = GamePiece(tetromino: tetromino, topLeft: ((width - tetromino.shape.count) / 2, 0))
+        let tetromino = nextTetrominoes.first!
+        let nextPiece = GamePiece(tetromino: tetromino, topLeft: ((width - tetromino.shape.count) / 2, 0))
         
+        if overlapsAny(nextPiece) {
+            state = .ended
+        } else {
+            nextTetrominoes.removeFirst()
             if currentBag.isEmpty {
                 generateBag()
             }
             nextTetrominoes.append(currentBag.removeFirst())
+            
+            currentPiece = nextPiece
+        }
     }
     
     func generateBag() {
@@ -70,8 +73,10 @@ public class GameController: ObservableObject {
         clearLines()
         nextPiece()
         
+        if case .playing(.dropped) = state {
             state = .playing(.normal)
         }
+    }
     
     func clearLines() {
         var cleared = 0
@@ -194,6 +199,7 @@ public class GameController: ObservableObject {
 public enum GameState {
     case waitingForStart
     case playing(PlayState)
+    case ended
 }
 
 public enum PlayState {