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Listing 6-14. Update_Input in EAGLView.m with Old Value Tracking
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Vector2D TouchPos; int TouchOldValue; int TouchValue; void Update_Input() { /* Copy the Active Touches */ TouchPos.x = ActiveTouchPos.x; TouchPos.y = ActiveTouchPos.y; TouchOldValue = TouchValue; TouchValue = ActiveTouchValue; }
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CHAPTER 6: Multiplatform Game Development: iPhone Games for Linux and Windows
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By tracking the old value and the current value and comparing the two, you can learn many things. Listing 6-15 includes a few functions you can call instead of sharing the variable.
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Listing 6-15. Several Functions to Use for Testing Specific Combinations of TouchValue and TouchOldValue
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int TouchChanged() { return (TouchValue ^ TouchOldValue); } int TouchIsDown() { return (TouchValue ^ TouchOldValue) & TouchValue; } int TouchIsUp() { return (TouchValue ^ TouchOldValue) & TouchOldValue; } int Touching() { return TouchValue; }
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You may have noticed that I m doing something clever here. Because I treat a touch as TouchValue becoming 1, I can use bit operations to learn about it. I use the XOR (^) bit operation to detect change. Whenever TouchValue and TouchOldValue are different, the XOR will result in 1. I can then use an AND (&) bit operation to test what type of change it was. Finally, I rely on the C Boolean logic that any value not zero is true, so I can write easy-to-read code like that shown in Listing 6-16 in game.c.
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Listing 6-16. Example Usage of the TouchIsDown Function
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if ( TouchIsDown() ) { /* Finger has (this frame) touched the screen */ ... }
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These functions could live outside EAGLView.m, since they rely entirely on the snapshots of data. But to keep the example simpler, I ll leave them there. A modified game.h in Listing 6-17 hides the TouchValue variable behind functions.
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Listing 6-17. Sharing Functions Instead of TouchValue Inside game.h
#ifndef __GAME_H__ #define __GAME_H__ typedef struct { float x, y; } Vector2D;
CHAPTER 6: Multiplatform Game Development: iPhone Games for Linux and Windows
extern Vector2D TouchPos; int TouchChanged(); int TouchIsDown(); int TouchIsUp(); int Touching(); void Work(); void Draw(); #endif /* __GAME_H__ */
When writing user interface code, touch down and touch up events are especially useful. If you remember which user interface element was touched (down event), you can test for the same element when released (up event). If they are the same, you activate the action. This is how you make a button that users can touch, while giving them the option of dragging their finger off the button to not activate it. Most standard buttons on Mac OS X and Windows systems work this way.
Frame Skipping
Frame skipping is a visual compromise. It s the idea that if the previous frame took too long, you can skip drawing until you ve caught up. This relies on the fact that most work done during a frame is less intensive than actually drawing. As a rule of thumb, you should be able to perform two or more frames of work when not drawing. Frame skipping code can handle the occasional heavy frame of work, but for best results, it should be avoided if possible. There are two common types of frame skipping: Fixed frame-rate skipping: This is as easy as it sounds. In a game that normally runs at 60 fps, you can run at half the frame rate by changing the timer to tick at 30 fps, and by calling your Work code twice per frame. That s all there is to it. In practice, fixed frame-rate is usually not what you want. Floating or dynamic frame-rate frame skipping: This is the most common kind. It s more complicated, but it is the one you want. To do it, you need to check system time to learn about the actual time that has passed. Rather than hit you with all the code for floating frame skipping at once, I ll show you the code in steps, as follows: Prepare drawView to make multiple Work function calls in a single frame. Build a library of system time operations. Use the library to implement frame skipping.
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