UIAccelerometer 加速计

UIAccelerometer加速计是用来检测iphone手机在x.y.z轴三个轴上的加速度。要获得此类调用:

UIAccelerometer *accelerometer = [UIAccelerometer sharedAccelerometer];

同时，你需要设置它的delegate。

UIAccelerometer *accelerometer = [UIAccelerometer sharedAccelerometer];
accelerometer.delegate = self;
accelerometer.updateInterval = 1.0/60.0;

委托方法：- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration中的UIAcceleration是表示加速度类。包含了来自加速计UIAccelerometer的真是数据。它有3个属性的值x、y、z。iphone的加速计支持最高以每秒100次的频率进行轮询。此时是60次。

1) 应用程序可以通过加速计来检测摇动，如：用户可以通过摇动iphone擦除绘图。
也可以用户连续摇动几次iphone，执行一些特殊的代码：

- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration
{
static NSInteger shakeCount = 0;
static NSDate *shakeStart;

NSDate *now = [[NSDate alloc] init];
NSDate *checkDate = [[NSDate alloc] initWithTimeInterval:1.5f sinceDate:shakeStart];

if ([now compare:checkDate] == NSOrderedDescending || shakeStart == nil)
{
  shakeCount = 0;
  [shakeStart release];
  shakeStart = [[NSDate alloc] init];
}

[now release];
[checkDate release];

if (fabsf(acceleration.x) > 2.0 || fabsf(acceleration.y) > 2.0 || fabsf(acceleration.z) > 2.0)
{
  shakeCount++;
  if (shakeCount > 4)
  {
   // -- DO Something
   shakeCount = 0;
   [shakeStart release];
   shakeStart = [[NSDate alloc] init];
  }
}
}

2）加速计最常见的是用作游戏控制器。在游戏中使用加速计控制对象的移动！在简单情况下，可能只需获取一个轴的值，乘上某个数（灵敏度），然后添加到所控制对象的坐标系中。在复杂的游戏中，因为所建立的物理模型更加真实，所以必须根据加速计返回的值调整所控制对象的速度。
在cocos2d中接收加速计输入input.使其平滑运动，一般不会去直接改变对象的position.通过：

- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration
{
// -- controls how quickly velocity decelerates(lower = quicker to change direction)
float deceleration = 0.4; 

// -- determins how sensitive the accelerometer reacts(higher = more sensitive)
float sensitivity = 6.0;

// -- how fast the velocity can be at most
float maxVelocity = 100;

// adjust velocity based on current accelerometer acceleration
playerVelocity.x = playerVelocity.x * deceleration + acceleration.x * sensitivity;


// -- we must limit the maximum velocity of the player sprite, in both directions
if (playerVelocity.x > maxVelocity)
{
  playerVelocity.x = maxVelocity;
}
else if (playerVelocity.x < - maxVelocity)
{
  playerVelocity.x = - maxVelocity;
}
}

上面deceleration是减速的比率，sensitivity是灵敏度。maxVelocity是最大速度，如果不限制则一直加大就很难停下来。

playerVelocity.x = playerVelocity.x * deceleration + acceleration.x * sensitivity;
中 playervelocity是一个速度向量。是累积的。

- (void) update: (ccTime)delta
{
// -- keep adding up the playerVelocity to the player's position
CGPoint pos = player.position;
pos.x += playerVelocity.x;

// -- The player should also be stopped from going outside the screen
CGSize screenSize = [[CCDirector sharedDirector] winSize];
float imageWidthHalved = [player texture].contentSize.width * 0.5f;
float leftBorderLimit = imageWidthHalved;
float rightBorderLimit = screenSize.width - imageWidthHalved;

// -- preventing the player sprite from moving outside the screen
if (pos.x < leftBorderLimit)
{
  pos.x = leftBorderLimit;
  playerVelocity = CGPointZero;
}
else if (pos.x > rightBorderLimit)
{
  pos.x = rightBorderLimit;
  playerVelocity = CGPointZero;
}

// assigning the modified position back
player.position = pos;
}