昨天寫了重力感應的例子，我覺得這個例子比較有用處，我分享出來：

1 ）顯然ios4 之後可以使用coreMotion的framework 為了向下兼容加上UIAccelerator，

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#import <Foundation/Foundation.h> 
#import <CoreMotion/CoreMotion.h> 
@protocol  IFAccelerometerDelegate<NSObject> 
- (void)accelerateWithX:(NSNumber*)x withY:(NSNumber*)y withZ:(NSNumber*)z withTimeInterval:(NSTimeInterval)timeInterval; 
@end 
@interface IFAccelerometer : NSObject<UIAccelerometerDelegate> 
{ 
    UIAccelerometer *_accelerometer;  
    CMMotionManager *_motionManager; 
    id<IFAccelerometerDelegate>  _delegate; 
} 
+ (id)shareAccelerometer; 
- (void)addOberser:(id)oberserer; 
- (void)removeObserver; 
 
@end 

CMMotionManager 將是我們使用的Object，可以用來監測重力！
同時，咱們不能在需要監測重力感應的地方直接使用這個類，這樣耦合比較嚴重，也不利於重用。所以抽離出來，在代碼中您可以看到，我將定義一個signleton，同時將重力變化的事件回調給其代理。

2.接著往下是定義其函數，這個很簡單，直接貼代碼。

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#import "IFAccelerometer.h" 
static IFAccelerometer *accelerometerInstance = nil; 
@implementation IFAccelerometer 
+ (id)shareAccelerometer 
{ 
    if (!accelerometerInstance) { 
        static dispatch_once_t onceToken; 
        dispatch_once(&onceToken, ^{ 
            accelerometerInstance = [[[self class]alloc]init]; 
        }); 
    } 
    return accelerometerInstance; 
} 
 
- (id)init 
{ 
    self = [super init]; 
    if (self) { 
#ifdef __IPHONE_5_0 
        _motionManager = [[CMMotionManager alloc]init]; 
        if (_motionManager.accelerometerAvailable) { 
            [_motionManager setAccelerometerUpdateInterval:1/60.f]; 
            NSOperationQueue *operationQueue = [NSOperationQueue mainQueue]; 
            [_motionManager startAccelerometerUpdatesToQueue:operationQueue withHandler:^(CMAccelerometerData *data,NSError *error) 
             { 
                 if ([_delegate respondsToSelector:@selector(accelerateWithX:withY:withZ:withTimeInterval:)]) 
                 { 
                     NSNumber *x  = [NSNumber numberWithDouble:data.acceleration.x]; 
                     NSNumber *y  = [NSNumber numberWithDouble:data.acceleration.y]; 
                     NSNumber *z  = [NSNumber numberWithDouble:data.acceleration.z]; 
                     [_delegate accelerateWithX:x withY:y withZ:z withTimeInterval:data.timestamp]; 
                 } 
             } 
             ]; 
 
        } 
#else 
#ifdef __IPHONE_4_0 
               _accelerometer = [UIAccelerometer sharedAccelerometer]; 
               [_accelerometer setUpdateInterval:(1/60.0f)]; 
               _accelerometer.delegate = self; 
#endif 
#endif 
 
    } 
    return self; 
} 
 
- (void)addOberser:(id)oberserer 
{ 
    _delegate = oberserer; 
} 
 
- (void)removeObserver 
{ 
    _delegate = nil; 
} 
 
- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration 
{ 
    if ([_delegate respondsToSelector:@selector(accelerateWithX:withY:withZ:withTimeInterval:)]) 
    { 
        NSNumber *x  = [NSNumber numberWithDouble:acceleration.x]; 
        NSNumber *y  = [NSNumber numberWithDouble:acceleration.y]; 
        NSNumber *z  = [NSNumber numberWithDouble:acceleration.z]; 
        [_delegate accelerateWithX:x withY:y withZ:z withTimeInterval:acceleration.timestamp]; 
    } 
 
} 
3.以ViewController 為例介紹如何使用重力感應
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- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
    [[IFAccelerometer shareAccelerometer]addOberser:self]; 
    _imageView = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"33.png"]]; 
    _imageView.frame = CGRectMake(0, 0, KIMAGEWIDTH, KIMAGEHEIGHT); 
    _imageView.center = self.view.center; 
    [self.view addSubview:_imageView]; 
     
} 

_imageView 是一個UIImageView的成員，其寬高是一個分別是我定義的宏。
注意，我在頭文件添加了重力感應的代理，此時這行

[[IFAccelerometer shareAccelerometer]addOberser:self];

表明我在這個viewController中使用這個重力感應的數據。

好，現在是完成回調的時候了，繼續貼代碼

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- (void)accelerateWithX:(NSNumber *)x withY:(NSNumber *)y withZ:(NSNumber *)z withTimeInterval:(NSTimeInterval)timeInterval 
{ 
 
    float deceleration = 0.4f; 
    float sensitivity  = 6.0f; 
    float maxVelocity  = 100.0f; 
     
    velocity.x = velocity.x * deceleration + [x doubleValue] * sensitivity; 
    velocity.y = velocity.y * deceleration + [y doubleValue] * sensitivity; 
     
    if(velocity.x > maxVelocity){ 
        velocity.x = maxVelocity; 
    }else if(velocity.x < -maxVelocity){ 
        velocity.x = -maxVelocity; 
    } 
     
    if(velocity.y > maxVelocity){ 
        velocity.y = maxVelocity; 
    }else if(velocity.y < -maxVelocity){ 
        velocity.y = -maxVelocity; 
    } 
     
    CGPoint pos = _imageView.center; 
    pos.x += velocity.x; 
    pos.y -= velocity.y; 
     
    float imageWidthHalved =  KIMAGEWIDTH   * 0.5f; 
    float leftBorderLimit  = 0.0f; 
    float rightBorderLimit = 0.0f; 
    if (imageWidthHalved>self.view.frame.size.width/2.0f) { 
         leftBorderLimit  =   self.view.frame.size.width - imageWidthHalved; 
         rightBorderLimit =  imageWidthHalved; 
    } 
    else 
    { 
        leftBorderLimit  =   imageWidthHalved ; 
        rightBorderLimit =  self.view.frame.size.width - imageWidthHalved; 
    } 
     
     
 
    float imageHeightHalved = KIMAGEHEIGHT * 0.5f; 
    float topBorderLimit    = 0.0f; 
    float bottomBorderLimit = 0.0f; 
    if (imageHeightHalved>self.view.frame.size.height/2.0f) { 
         topBorderLimit    = self.view.frame.size.height - imageHeightHalved; 
         bottomBorderLimit =  imageHeightHalved ; 
    } 
    else 
    { 
        topBorderLimit    = imageHeightHalved ; 
        bottomBorderLimit = self.view.frame.size.height - imageHeightHalved  ; 
    } 
     
     
    if(pos.x < leftBorderLimit){ 
        pos.x = leftBorderLimit; 
        velocity = CGPointZero; 
    }else if(pos.x > rightBorderLimit){ 
        pos.x = rightBorderLimit; 
        velocity = CGPointZero; 
    } 
     
    if(pos.y < topBorderLimit){ 
        pos.y = topBorderLimit; 
        velocity = CGPointZero; 
    }else if(pos.y > bottomBorderLimit){ 
        pos.y = bottomBorderLimit; 
        velocity = CGPointZero; 
    } 
     
    _imageView.center = pos; 
} 

上面是對於邊界的處理等等操作，都很簡單，不一一介紹了。