iPhone車輪（財富）旋轉勢頭

Question

我在向我的紡車添加動力時遇到困難。
我有這個輪子（類似於this），我用一個觸摸事件圍繞它的中心旋轉它。
這裏沒有問題，但是當觸摸（又名拖動）結束時;我想讓車輪保持動力並緩解它的運動。

任何人誰可以給我一些指針，它不一定要在objective-c。 AS3，javascript或JAVA也足夠了。

* UPDATE（用於旋轉輪代碼）*

-(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

你想要的勢頭，以減少因摩擦;摩擦力是速度的函數。所以在技術上你有一個微分方程。但這並不值得投入太多的思考，因爲解決方案可能更容易通過揮手來達成。

因此：存儲當前角度和當前角速度。 ñ每秒（可能通過NSTimer或CADisplayLink）將角速度添加到角度，然後乘以角速度使其更小 - 例如0.995。接近1.0的常量會使其放慢速度需要更長的時間;如果你高於1.0，它顯然會加速。這實際上是歐拉整合的一種形式，但同樣不值得擔憂。

它可能也值得在角速度上設置一個最小值，這樣如果它下降到0.01弧度/秒以下，那麼你將它降低到0.這就有效地修改了你的摩擦模型，從動態跳躍到靜摩擦在適當的時候，並充當浮點精度緩衝區。

要從拖動中獲得初始速度，您只需計算出從輪子中心到拖動起點的矢量，將該矢量旋轉90度，使用該矢量做點積，拖動矢量和比例根據距離中心的距離。

Answer 2

如果您正在使用的代碼紡的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];
這使得這樣的動畫將快速啓動關閉，並開始放緩隨着時間的推移。

Click Here for More info on UIViewAnimationCurve

Answer 3

我設法得到了一些不錯的成績。
我怎麼做的，應該進行調整更多，但是這是最基礎的：

-(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"]; 
    };