#pragma mark - UIControl functions
//開始跟蹤觸控
-(BOOL) beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    [super beginTrackingWithTouch:touch withEvent:event];
    
    return YES;
}

//繼續跟蹤觸控
-(BOOL) continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    [super continueTrackingWithTouch:touch withEvent:event];
    
    CGPoint lastPoint = [touch locationInView:self];
    [self moveHandle:lastPoint];
    [self sendActionsForControlEvents:UIControlEventValueChanged];
    
    return YES;
}

//結束跟蹤觸控
-(void)endTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
    [super endTrackingWithTouch:touch withEvent:event];
    
    if(_snapToLabels && labelsEvenSpacing != nil) {
        CGPoint bestGuessPoint;
        float minDist = 360;    //距離
        for (int i=0; i<[labelsEvenSpacing count]; i++) {
            //在圓上得百分比
            CGFloat percentageAlongCircle = i/(float)[labelsEvenSpacing count];
            //度數
            CGFloat degreesForLabel = percentageAlongCircle * 360;
            if(abs(fixedAngle - degreesForLabel) < minDist) {
                minDist = abs(fixedAngle - degreesForLabel);
                bestGuessPoint = [self pointFromAngle:degreesForLabel + 90 + 180];
            }
        }
        CGPoint centerPoint = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);
        angle = floor(AngleFromNorth(centerPoint, bestGuessPoint, NO));
        _currentValue = [self valueFromAngle];
        [self setNeedsDisplay];
    }
}