Objective c 如何围绕中心旋转CGContext？

我有以下代码来创建多色ImageContext：

UIGraphicsBeginImageContext(self.view.frame.size);
[self.imageToDelete drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);

CGColorRef mycolor;

mycolor = [[UIColor blueColor] CGColor];

CGPoint mid = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);

CGContextMoveToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y);


CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y+50);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(),3);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeCopy);
CGContextSetStrokeColorWithColor(UIGraphicsGetCurrentContext(), mycolor);
CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());

mycolor = [[UIColor whiteColor] CGColor];
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y+50);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y+75);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(),3);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeCopy);
CGContextSetStrokeColorWithColor(UIGraphicsGetCurrentContext(), mycolor);
CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());

mycolor = [[UIColor cyanColor] CGColor];
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y+75);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), mid.x, mid.y+100);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(),3);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeCopy);
CGContextSetStrokeColorWithColor(UIGraphicsGetCurrentContext(), mycolor);

CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());

self.imageToDelete = UIGraphicsGetImageFromCurrentImageContext();

[self.mainImage drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) blendMode:kCGBlendModeCopy alpha:1.0];
[self.imageToDelete drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) blendMode:kCGBlendModeNormal alpha:1];
self.mainImage = UIGraphicsGetImageFromCurrentImageContext();

SKSpriteNode *node = [SKSpriteNode spriteNodeWithTexture:[SKTexture textureWithImage:self.mainImage]];
node.zPosition = 100;
node.position = CGPointMake(self.frame.size.width/2, self.frame.size.height/2);
node.size = self.mainImage.size;
[self addChild:node];

UIGraphicsEndImageContext();

结果是：

---

我想围绕屏幕中心（或这条线的顶部）旋转这条线，并保持原来的线。有没有办法复制并围绕中心点旋转？为了澄清，

mid

是一个CGPoint，定义为

（self.view.frame.size.width/2，self.view.frame.size.height/2）

，这是在精灵套件、iOS、Objective-C中完成的。

旋转围绕原点。若要围绕另一个点旋转，请将该点平移到原点，然后旋转并向后平移。

旋转围绕原点。若要围绕另一个点旋转，请将该点平移到原点，然后旋转并向后平移。

旋转围绕原点。若要围绕另一个点旋转，请将该点平移到原点，然后旋转并向后平移。

旋转围绕原点。若要围绕另一个点旋转，请将该点平移到原点，然后旋转并向后平移。

此处示例代码：此处示例代码：此处示例代码：此处示例代码：此处示例代码：