iOS 7 剛出來時，大家第一個會注意到的事在 Lock Screen 有一個會動的背景；那就是 Motion Effects。
其實它很簡單，我試著在一個UIView 加上一個 UIButton，接著讓 Button 是浮動的。看程式碼就懂了。
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
aButton.layer.mask.frame = aButton.bounds;
aButton.layer.masksToBounds = YES;
UIInterpolatingMotionEffect *xAxis = [[UIInterpolatingMotionEffect alloc]
initWithKeyPath:@"center.x"
type:UIInterpolatingMotionEffectTypeTiltAlongHorizontalAxis];
xAxis.minimumRelativeValue = [NSNumber numberWithFloat:-90.0];
xAxis.maximumRelativeValue = [NSNumber numberWithFloat:90.0];
UIInterpolatingMotionEffect *yAxis = [[UIInterpolatingMotionEffect alloc]
initWithKeyPath:@"center.y"
type:UIInterpolatingMotionEffectTypeTiltAlongVerticalAxis];
yAxis.minimumRelativeValue = [NSNumber numberWithFloat:-90.0];
yAxis.maximumRelativeValue = [NSNumber numberWithFloat:90.0];
UIMotionEffectGroup *group = [[UIMotionEffectGroup alloc]init];
group.motionEffects = @[xAxis, yAxis];
[aButton addMotionEffect:group];
}
