跟随手指移动主要用了两个方法
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
self.black.center = [touch locationInView:self];
}
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
self.black.center = [touch locationInView:self];
}
让黑色view的中心点等于手指触摸的点就可以了 但是这样写的话，当手指触摸到圆外或者移动到圆外，黑色view也能跟随出去，那我们想让黑色view显示在圆内，就需要判断黑色view的中心点坐标是否在圆内，如果不在圆内就不让它跟随，于是添加了下面的代码：
UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:self.bounds];
if (CGPathContainsPoint(path.CGPath, NULL, [touch locationInView:self], NO)) {
self.black.center = [touch locationInView:self];
}
这样做还有一个缺点：手指移动出圆范围内黑色view就停止不动了。我们的目标是，黑色view不会出圆的范围，但是能跟随手指有角度的移动。 博主大学没好好学导致把高中数学忘得差不多了，三角函数的用法也一个不会用了。 于是画了一个图来分析：
分析：当手指移动到圆外，让黑色view的中点等于圆心到圆外任意一点的连线与圆的交点。这样当手指移动到圆外，黑色view也能跟随手指有角度的变化。 根据相似三角形来计算交点的坐标，最后代码：
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
UITouch *touch = [touches anyObject];
UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:self.bounds];
if (CGPathContainsPoint(path.CGPath, NULL, [touch locationInView:self], NO)) {
self.black.center = [touch locationInView:self];
}
}
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
CGFloat r = self.bounds.size.width / 2;
CGPoint center = CGPointMake(r, r);
UITouch *touch = [touches anyObject];
CGPoint pointL = [touch locationInView:self];
UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:self.bounds];
if (CGPathContainsPoint(path.CGPath, NULL, [touch locationInView:self], NO)) {
self.black.center = [touch locationInView:self];
}else{
CGFloat juli = sqrt(pow(center.x - pointL.x, 2) + pow(pointL.y - center.y, 2));
CGFloat x = center.x - r / juli * (center.x - pointL.x);
CGFloat y = center.y + r / juli * (pointL.y - center.y);
self.black.center = CGPointMake(x, y);
}
}