借助一点编程工作,iPhone的机载加速计就可以使对象在屏幕上四处“移动”,实时响应用户倾斜手机的方式。下面的代码就是创建一个动画式的蝴蝶,用户可以使之快速移过屏幕。
使之工作的秘密在于:向程序中添加一个所谓的"物理计时器“。他不是直接响应加速中的变化,而是加速计回调用于测量当前的力。它取决于计时器例程随着 时间的推移通过改变他的画面对蝴蝶应用那些力。下面是列出要记住的关键点。
(1).只要力的方向仍然保持相同,蝴蝶就会加速。他的速度会依据加速力在x或y方向上的量度成比例的提高。
(2).由计时器用的rick例程将通过蝴蝶的原点添加速度向量来移动蝴蝶。
(3).蝴蝶移动的范围是有界限的,因此,当他撞到某个边缘时,将会停止那个方向的移动。还可以一直讲蝴蝶保留在屏幕中。tick方法将会检查界限条件。
(4).蝴蝶会改变它自身的方向,使之总是下落。可以在tick方法中应用一个简单的旋转变换来实现这一点。在使用变换时,还要关注画面或中心偏移。在应用偏移之前,总是要重置数学处理,然后重新应用任何角度改变。不这样的话,可能导致画面出人意料的放大,收缩或扭曲。
代码如下:
#define SIGN(_NUM_) ((_NUM_ < 0) ? (-1) : 1)
#define RECTCENTER(RECT) CGPointMake(CGRectGetMidX(RECT), CGRectGetMidY(RECT)) @interface ViewController ()
{
UIImageView *butterfly; float xaccel;
float xvelocity;
float yaccel;
float yvelocity; float mostRecentAngle;
} @end @implementation ViewController - (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor greenColor];
[self initButterfly];
} - (void) initButterfly
{
CGSize size; // Load the animation cells
NSMutableArray *butterflies = [NSMutableArray array];
for (int i = ; i <= ; i++)
{
NSString *fileName = [NSString stringWithFormat:@"bf_%d.png", i];
UIImage *image = [UIImage imageNamed:fileName];
size = image.size;
[butterflies addObject:image];
} // Begin the animation
butterfly = [[UIImageView alloc] initWithFrame:(CGRect){.size=size}];
[butterfly setAnimationImages:butterflies];
butterfly.animationDuration = 0.75f;
[butterfly startAnimating]; // Set the butterfly's initial speed and acceleration
xaccel = 2.0f;
yaccel = 2.0f;
xvelocity = 0.0f;
yvelocity = 0.0f; // Add the butterfly
butterfly.center = RECTCENTER(self.view.bounds);
[self.view addSubview:butterfly]; // Activate the accelerometer
[[UIAccelerometer sharedAccelerometer] setDelegate:self]; // Start the physics timer
[NSTimer scheduledTimerWithTimeInterval: 0.03f target: self selector: @selector(tick) userInfo: nil repeats: YES];
} - (void) tick
{
butterfly.transform = CGAffineTransformIdentity; // Move the butterfly according to the current velocity vector
CGRect rect = CGRectOffset(butterfly.frame, xvelocity, 0.0f);
if (CGRectContainsRect(self.view.bounds, rect))
butterfly.frame = rect; rect = CGRectOffset(butterfly.frame, 0.0f, yvelocity);
if (CGRectContainsRect(self.view.bounds, rect))
butterfly.frame = rect; butterfly.transform = CGAffineTransformMakeRotation(mostRecentAngle + M_PI_2);
} - (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration
{
// extract the acceleration components
float xx = -acceleration.x;
float yy = acceleration.y;
mostRecentAngle = atan2(yy, xx); // Has the direction changed?
float accelDirX = SIGN(xvelocity) * -1.0f;
float newDirX = SIGN(xx);
float accelDirY = SIGN(yvelocity) * -1.0f;
float newDirY = SIGN(yy); // Accelerate. To increase viscosity lower the additive value
if (accelDirX == newDirX) xaccel = (abs(xaccel) + 0.85f) * SIGN(xaccel);
if (accelDirY == newDirY) yaccel = (abs(yaccel) + 0.85f) * SIGN(yaccel); // Apply acceleration changes to the current velocity
xvelocity = -xaccel * xx;
yvelocity = -yaccel * yy;
}