IOS 特定于设备的开发：基于加速计的滚动视图

    倾斜滚轮使用设备的内置加速计来控制在UIScrollView的内容周围移动。当用户调增设备时，材料会相应的下落，他不会把视图定位在屏幕上，而是把内容视图滚动到一个新的偏移位置。

   创建这个界面的挑战在于：确定设备在什么地方应该具有他的静止轴，大多数人最初建议当显示屏靠在他的背部应该是稳定的，并且z轴方向笔直的指向上方。事实证明：这实际是一个相当糟糕的设计选择。要使用那根轴，就意味着导航期间屏幕必须实际的偏离观看者。随着设备旋转离开视图，用户将不能完全看到屏幕上所发生的事情，尤其是在固定的位置使用设备时，站在高处看设备有时会产生这种效果。

   作为代替，下面程序假定稳定的位置是通过z轴指向大约45度的方向，即用户把iPhone或iPad窝在手中的自然位置，这处于正面朝上和正面朝前方的中间位置。从这个歪斜的位置来回倾斜，使屏幕在调整期间保持最大的可见性。

   这个程序中的另一处改变是低得多加速常量。这使屏幕上的运动能够更慢的发生，让用户更容易降低速度并恢复导航。

#define SIGN(_NUM_) ((_NUM_ < 0) ? (-1) : 1) 
#define MIN(A,B) __NSMIN_IMPL__(A,B,__COUNTER__) 
#define MAX(A,B) __NSMAX_IMPL__(A,B,__COUNTER__)

@implementation TestBedViewController
{
    UIScrollView *sv;
    
    float xoff;
    float xaccel;
    float xvelocity;
    
    float yoff;
    float yaccel;
    float yvelocity;
}

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration
{
    // extract the acceleration components
    float xx = -acceleration.x;
    float yy = (acceleration.z + 0.5f) * 2.0f; // between face up and face forward
    
    // 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) + 0.005f) * SIGN(xaccel);
    if (accelDirY == newDirY) yaccel = (abs(yaccel) + 0.005f) * SIGN(yaccel);
    
    // Apply acceleration changes to the current velocity
    xvelocity = -xaccel * xx;
    yvelocity = -yaccel * yy;
}

- (void) tick
{
    xoff += xvelocity;
    xoff = MIN(xoff, 1.0f);
    xoff = MAX(xoff, 0.0f);
    
    yoff += yvelocity;
    yoff = MIN(yoff, 1.0f);
    yoff = MAX(yoff, 0.0f);
    
    CGFloat xsize = sv.contentSize.width - sv.frame.size.width;
    CGFloat ysize = sv.contentSize.height - sv.frame.size.height;
    sv.contentOffset = CGPointMake(xoff * xsize, yoff * ysize);
}

- (void) loadView
{
    [super loadView];
    sv = [[UIScrollView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]];
    self.view = sv;
}

- (void) viewDidAppear:(BOOL)animated
{
    NSString *map = @"http://maps.weather.com/images/maps/current/curwx_720x486.jpg";
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperationWithBlock:
     ^{
         // Load the weather data
         NSURL *weatherURL = [NSURL URLWithString:map];
         NSData *imageData = [NSData dataWithContentsOfURL:weatherURL];
         
         // Update the image on the main thread using the main queue
         [[NSOperationQueue mainQueue] addOperationWithBlock:^{
             UIImage *weatherImage = [UIImage imageWithData:imageData];
             UIImageView *imageView = [[UIImageView alloc] initWithImage:weatherImage];
             CGSize initSize = weatherImage.size;
             CGSize destSize = weatherImage.size;
             
             while ((destSize.width < (self.view.frame.size.width * 4)) ||
                    (destSize.height < (self.view.frame.size.height * 4)))
             {
                 destSize.width += initSize.width;
                 destSize.height += initSize.height;
             }
             
             imageView.userInteractionEnabled = NO;
             imageView.frame = (CGRect){.size = destSize};
             sv.contentSize = destSize;
             
             [sv addSubview:imageView];
             
             // Activate the accelerometer
             [[UIAccelerometer sharedAccelerometer] setDelegate:self];
             
             // Start the physics timer
             [NSTimer scheduledTimerWithTimeInterval: 0.03f target: self selector: @selector(tick) userInfo: nil repeats: YES];
         }];
     }];
}