iOS 6 Programming Cookbook

17.13. Rotating Shapes Drawn on Graphic Contexts

Problem

You want to be able to rotate the contents that you have drawn on a graphics context without changing your drawing code.

Solution

Use the CGAffineTransformMakeRotation function to create an affine rotation transformation.

Discussion

Note

I strongly suggest that you read the material in Recipes 17.11and in 17.12before proceeding with this section. To avoid redundancy, I have tried to keep material that has been taught in earlier sections out of later sections.

Just like scaling and translation, you can apply rotation translation to shapes drawn on paths, and graphics contexts. You can use the CGAffineTransformMakeRotation function and pass the rotation value in radians to get back a rotation transformation, of type CGAffineTransform. You can then apply this transformation to paths and shapes. If you want to rotate the whole context by a specific angle, you must use the CGContextRotateCTM procedure.

Let’s rotate the same rectangle we had in Figure 17-27 45 degrees clockwise (see Figure 17-37). The values you supply for rotation must be in radians. Positive values cause clockwise rotation, while negative values cause counterclockwise rotation:

/* Rotate the rectangle 45 degrees clockwise */
CGAffineTransform transform =
CGAffineTransformMakeRotation((45.0f * M_PI) / 180.0f);

/* Add the rectangle to the path */
CGPathAddRect(path,
              &transform,
              rectangle);

As we saw in Recipe 17.12, we can also apply a transformation directly to a graphics ...