Command and State Patterns. Curt Clifton Rose-Hulman Institute of Technology

Transcription

1 Command and Patterns Curt Clifton Rose-Hulman Institute of Technology

2 Final Exam me by Tuesday, Feb. 16, to sign up. Monday, Feb. 22, 8am Optional If you don t take the exam, we ll use your exam 1 grade as your final exam grade Sign-up for exam during 10th week If you sign-up, you have to take the exam Taking the exam can lower your grade

3 Plan for Today Short survey on projects Please bring laptops tomorrow for course evaluations. Pattern Command Pattern Design Studio Concurrent Poker

4 Checkers Handling Turn Taking and Undo

5 Turn Taking in Checkers Simple move: slide a piece diagonally to adjacent, open square Jump move: move a piece diagonally over an adjacent opponents piece landing in an open square Multiple s: must again if another is available after landing

6 It s Good to be King A piece reaching the far side of the board is kinged Only kings may move backward (toward the player) A player s turn ends when a piece is kinged (i.e., can t into king row, become a king, and back out)

7 Machines Good way to represent the behavior of a system when future actions depend on previous actions

8 Basic Turn Taking One's Turn simple move simple move Two's Turn

9 Jumping One's Turn simple move no more s Two's Turn After Jump One's Turn After Jump no more s simple move Two's Turn

10 Winners and Losers no legal moves One Wins One's Turn simple move no more s Two's Turn After Jump player two out of pieces player one out of pieces One's Turn After Jump no more s simple move Two's Turn no legal moves Two Wins What does this have to do with objects?

11 Winners and Losers no legal moves players 2 Game getlegalmoves() makemove() board One Wins player two out of pieces One's Turn One's Turn After Jump simple move no more s no more s simple move Two's Turn After Jump Two's Turn player one out of pieces Two Wins Board no legal moves

12 Pattern Problem: When the behavior of an object, obj, changes depending on its state, how can we avoid complicated conditional statements? Solution: Create state classes implementing a common interface. Delegate state-dependent methods from obj to the current state object. Example Q1,2

13 Handling Simple Moves players 2 Game getlegalmoves() makemove() player game updates board, then calls turn.moved() or turn.ed(x) turn Turn getlegalmoves() ed(piece x) moved() board Board Winner StartTurn Already Jumped Piece er moved() throws an exception moved(x) creates a new StartTurn for the other player and sets game.turn moved(x) throws an exception

14 Handling Jump Moves players 2 Game getlegalmoves() makemove() player game updates board, then calls turn.moved() or turn.ed(x) turn Turn getlegalmoves() ed(piece x) moved() board Board Winner StartTurn Already Jumped Piece er ed() throws an exception ed(x) creates a new AlreadyJumped(x) and sets game.turn ed(x) throws an exception if x!= er, otherwise does nothing

15 Getting Legal Moves players 2 Game getlegalmoves() makemove() Board getlegalmoves() returns empty list board player Just delegates: return turn.getlegalmoves() game turn Winner Turn getlegalmoves() ed(piece x) moved() StartTurn getlegalmoves() returns all moves for player using er, unless no more s. In that case, it sets game.turn and delegates to new turn. Already Jumped Piece er getlegalmoves() returns all legal moves for player, unless no legal moves. In that case, it sets game.turn and delegates to new turn.

16 Applying the Pattern player players 2 Game getlegalmoves() makemove() game turn Turn getlegalmoves() ed(piece x) moved() board Board Winner StartTurn Already Jumped Piece er Q3

17 Cartoon of the Day Used by permission.

18 Suppose we want to be able to undo moves

19 Command Pattern Problem: When we need to record operations so we can undo them, or execute them later, what should we do? Solution: Define a Command interface that represents all possible operations. Create subclasses of it for each kind of operation and instances for each actual operation. Example Q4,5

20 Adding Undo to Checkers Update board appropriately { SimpleMove apply() undo() Move from : Position to : Position board : Board apply() undo() * movesmade {Stack} * movesundone {Stack} JumpMove removedfrom : Position removedpiece : Piece apply() undo() Kinds of operations Command interface MoveHistory undo() redo() canundo() canredo() void undo() { Move c = movesmade.pop(); c.undo(); movesundone.push(c); } void redo() { Move c = movesundone.pop(); c.apply(); movesmade.push(c); }

21 Uses for the Command Pattern Undo/redo Prioritizing and Queueing operations Composing multi-part operations Progress bars Macro recording

22 Design Studio: Concurrent Poker Team describes problem and perhaps current solution (if any) ~5 min. Class thinks about questions, alternative approaches. Q7 ~3 min. On-board design ~12 min.