detect direction of UIKIt gesture recognizer

gestureRecognizerShouldBegin.m

- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer {
	// スワイプのジェスチャを開始するかを判定する
	if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]]) {
		UIPanGestureRecognizer *panGesture = (UIPanGestureRecognizer *)gestureRecognizer;
		CGPoint velocity = [panGesture velocityInView:panGesture.view];
		
		double radian = atan(velocity.y/velocity.x);
		double degree = radian * 180 / M_PI;
		
		// 指定した角度よりも大きく（斜めに）スワイプした場合は，ジェスチャは開始されない
		if (fabs(degree) > SwipeDirectionThreadholdAsDegree) {
			return NO;
		}
	}
	return YES;
}