c# create barcode 6: Coding a Pong Game in Objective-C

Painting QR Code in Objective-C 6: Coding a Pong Game

CHAPTER 6: Coding a Pong Game
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The function of this code segment should be obvious to even the casual iPhone developer. I ll cover it here for clarity, but be warned, speed is picking up so I won t be examining every line of code. This single line of code creates a timer from the NSTimer class that fires (sends a message) every 0.05 seconds. The message is sent to this object (target:self) causing the gameLoop method (selector:@(selector(gameLoop)) to fire. No extra information is passed to gameLoop (userInfo:nil) and this loop runs continuously (repeats:YES). The gameLoop function performs all the necessary functions to make the game update, the primary ones being: See if anyone scored. Player scored Computer scored Has the ball hit the edge of the room Did the player s paddle hit the ball Did the computer s paddle hit the ball Is the game over Much more detail goes into the gameLoop method as you will see shortly, but let s quickly cover the other support methods you ll need to make the game work. The methods I m going to list are not the only way to define the program. In fact, I suspect there are many better ways of doing things. In fact, in many cases I purposefully did some things counter to the way they should be done, just to see what happens and make things more interesting. NOTE: While I included sections of the code for discussions here, the complete code with comments can be found at the end of the chapter. I included the method setServePosition to position the ball after a point has been scored. The ball is placed at roughly the center of the table so it can be served towards the computer. A fairly obvious upgrade to this would be to set the ball s starting position back near the server s paddle. This would also require keeping track of the scores so that the serve can be alternated every two points.
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-(void) setServePosition { ball.center = CGPointMake(BALL_STARTING_X, BALL_STARTING_Y); ballSpeed = CGPointMake(BALL_DELTA_X, -BALL_DELTA_Y); }
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The compPlay method is the basic artificial intelligence of the whole program. It does nothing more than move the computer s paddle towards the ball along the horizontal axis, i.e., no Y-axis movement. There are two speeds (reaction time) of the computer; it moves slower when the ball is on the player s side of the table and faster when the ball is on the computer s side. The constant (#define) COMP_REACTION_TIME sets the
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CHAPTER 6: Coding a Pong Game
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computer s performance. A higher number provides a faster response and therefore a better player.
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-(void) compPlay { if(ball.center.y <= self.view.center.y + COMP_SETUP_TIME) { if(ball.center.x < compPaddle.center.x) { CGPoint compLocation = CGPointMake(compPaddle.center.x COMP_REACTION_TIME, compPaddle.center.y); compPaddle.center = compLocation; } if(ball.center.x > compPaddle.center.x) { CGPoint compLocation = CGPointMake(compPaddle.center.x + COMP_REACTION_TIME, compPaddle.center.y); compPaddle.center = compLocation; } } }
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Interestingly enough, with all the code and things that even this simple game does, the player (user) only interacts with the game (exclusive of starting or stopping) at two different times. When he presses the SERVE button, you know that the serveAction method fires. This is what you set up with IB. What about the second condition when he moves his paddle When a user touches the iPhone screen, the sendEvent method of the UIApplication gets called, which is transparent to you (unless you ve overridden it which requires subclassing UIApplication). What you do see is the touchesBegan:withEvent: method that is part of the UIResponder class. This provides objects (your code) the interface needed to handle user touches. NOTE: Although you can handle multiple touch events as well, you re going to assume that all touches are single touch events. For the Pong game, you use the touchesBegan as a gateway. This version of the method does two things: (1) it checks to see if the game status is IN_PLAY, and if it is, then (2) it calls your own touchesMoved method to handle the movement of the player s paddle.
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-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { if (status == IN_PLAY) { [self touchesMoved:touches withEvent:event]; } }
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Continuing down the chain of touch events, the touchesMoved:withEvent: method updates the position of the player s paddle by repositioning the paddle to the new position of the player s finger. The other thing you decided to do in this method was change the paddle image depending on its position. If it is to the left or right of the centerline, then you flip the left-right orientation of the paddle to simulate forehand or backhand. If it is near the edges, as defined as 101 pixels to the left or right of the
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