To develop Use Cases in iOS you must first identify what your app must do. In techie-speak this is known as capturing the app’s functional requirements. Functional requirements are typically captured as use cases. A use case is a description of a specific interaction between an actor (an entity external to the system, such as a user or another system) and the system being designed.

Developing use cases begins with creating a written description of the app. Here is a simple description of Tic-Tac-Toe:

Tic-tac-toe, also spelled tick-tack-toe, or noughts and crosses, as it's known in Britain and some of the Commonwealth countries, is a pencil-and-paper game for two players. Two players take turns marking the empty spaces in a three-by-three grid.

Next, one player using the symbols X and the other player using the symbol O begin to mark spaces in the grid.The X player usually goes first. The player who succeeds in placing three of his marks to fill a horizontal, vertical, or diagonal row wins the game.

Now, here's a narrative that extends the preceding description about playing Tic-Tac-Toe on an iOS device.

Tic-Tac-Toe for iOS implements the Tic-Tac-Toe paper game as an iOS app. Users can play the game against the computer. Multiple games can be played in each session, with either the computer playing first or the user playing first on an electronic board shown on the device’s touchscreen. Scores for each session are accumulated. If the user quits the session, scores are reset.