iPhone车轮（财富）旋转势头

Question

我在向我的纺车添加动力时遇到困难。
我有这个轮子（类似于this），我用一个触摸事件围绕它的中心旋转它。
这里没有问题，但是当触摸（又名拖动）结束时;我想让车轮保持动力并缓解它的运动。

任何人谁可以给我一些指针，它不一定要在objective-c。 AS3，javascript或JAVA也足够了。

* UPDATE（用于旋转轮代码）*

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { 
     rotation = _startAngle = atan2(384 - touchPoint.y, 512 - touchPoint.x); 
    }; 

    -(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { 
     CGPoint touchPoint = [[touches anyObject] locationInView:self.view]; 
     rotation = atan2(384 - touchPoint.y, 512 - touchPoint.x); 
     rotation = fmod(rotation - _startAngle, M_PI * 2.0f); 
     [wheel setTransform:CGAffineTransformMakeRotation(_circleRotationOffset + rotation)]; 
    }; 

    -(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event { 
     _circleRotationOffset = fmod(_circleRotationOffset + rotation, M_PI * 2); 
    }; 

Answer 1

你想要的势头，以减少因摩擦;摩擦力是速度的函数。所以在技术上你有一个微分方程。但这并不值得投入太多的思考，因为解决方案可能更容易通过挥手来达成。

因此：存储当前角度和当前角速度。 ñ每秒（可能通过NSTimer或CADisplayLink）将角速度添加到角度，然后乘以角速度使其更小 - 例如0.995。接近1.0的常量会使其放慢速度需要更长的时间;如果你高于1.0，它显然会加速。这实际上是欧拉整合的一种形式，但同样不值得担忧。

它可能也值得在角速度上设置一个最小值，这样如果它下降到0.01弧度/秒以下，那么你将它降低到0.这就有效地修改了你的摩擦模型，从动态跳跃到静摩擦在适当的时候，并充当浮点精度缓冲区。

要从拖动中获得初始速度，您只需计算出从轮子中心到拖动起点的矢量，将该矢量旋转90度，使用该矢量做点积，拖动矢量和比例根据距离中心的距离。

Answer 2

如果您正在使用的代码纺的UIImageView像：
[UIView beginAnimations:@"rotateImage" context:nil];
[UIView setAnimationDuration:4.0];
wheelImageView.transform = CGAffineTransformMakeRotation(3.14159265*5);
[UIView commitAnimations];

你可以使用[UIView setAnimationCurve:UIViewAnimationCurveEaseOut];
这使得这样的动画将快速启动关闭，并开始放缓随着时间的推移。

Click Here for More info on UIViewAnimationCurve

Answer 3

我设法得到了一些不错的成绩。
我怎么做的，应该进行调整更多，但是这是最基础的：

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { 
     CGPoint touchPoint = [[touches anyObject] locationInView:self.view]; 
     rotation = _startAngle = atan2(384 - touchPoint.y, 512 - touchPoint.x); 
     [_history removeAllObjects]; 
    }; 

    -(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { 
     CGPoint touchPoint = [[touches anyObject] locationInView:self.view]; 
     [_history insertObject:[NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithDouble:CFAbsoluteTimeGetCurrent()], @"time", [NSValue valueWithCGPoint:touchPoint], @"point", [NSNumber numberWithFloat: _circleRotationOffset + rotation], @"rotation", nil] atIndex:0]; 
     if ([_history count] == 3) { 
      [_history removeLastObject]; 
     } 

     rotation = atan2(384 - touchPoint.y, 512 - touchPoint.x) - _startAngle; 
     [circleImage setTransform:CGAffineTransformMakeRotation(_circleRotationOffset +   rotation)]; 
    }; 

    -(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event { 
     CGPoint tp = [[touches anyObject] locationInView:self.view]; 
     _circleRotationOffset += rotation; 

     NSDictionary *lo = [_history lastObject]; 
     CGPoint pp = [[lo objectForKey:@"point"] CGPointValue]; 
     double timeDif = CFAbsoluteTimeGetCurrent() - [[lo objectForKey:@"time"] doubleValue]; 
     float lastRotation = [[lo objectForKey:@"rotation"] floatValue]; 

     // Calculate strength 
     float dist = sqrtf(((pp.x - tp.x) * (pp.x - tp.x)) + ((pp.y - tp.y) * (pp.y - tp.y))); 
     float strength = MIN(1.0f, dist/80.0f) * (timeDif/.025) * M_PI; 

     float p = _circleRotationOffset; 
     float dif = _circleRotationOffset - lastRotation; 
     BOOL inc = dif > 0; 
     if (dif > 3 || dif < -3) { // Some correction 
      inc = !inc; 
     } 

     if (inc) { 
      _circleRotationOffset += strength; 
     } else { 
      _circleRotationOffset -= strength; 
     } 

     [circleImage.layer removeAllAnimations]; 
     CAKeyframeAnimation *animation = [CAKeyframeAnimation animationWithKeyPath:@"transform.rotation.z"]; 
     animation.duration = MAX(strength/2.5, 1.0f); 
     animation.cumulative = YES; 
     animation.repeatCount = 1; 
     animation.values = [NSArray arrayWithObjects:   
        [NSNumber numberWithFloat:p], 
        [NSNumber numberWithFloat: _circleRotationOffset], nil]; 
     animation.keyTimes = [NSArray arrayWithObjects:  
         [NSNumber numberWithFloat:0], 
         [NSNumber numberWithFloat:1.0], nil]; 
     animation.timingFunctions = [NSArray arrayWithObjects: 
          [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut], nil]; 
     animation.removedOnCompletion = YES; 
     animation.delegate = self; 
     animation.fillMode = kCAFillModeForwards; 

     [circleImage.layer addAnimation:animation forKey:@"rotate"]; 
    };