Transforming iOS device coordinates to user coordinates outside drawRect - ios

Question

I'm using CoreGraphics in my UIView to draw a graph and I want to be able to interact with the graph using touch input. Since touches are received in device coordinates, I need to transform it into user coordinates in order to relate it to the graph, but that has become an obstacle since CGContextConvertPointToUserSpace doesn't work outside of the graphics drawing context.
Here's what I've tried.
In drawRect:
CGContextScaleCTM(ctx,...);
CGContextTranslateCTM(ctx,...); // transform graph to fit the view nicely
self.ctm = CGContextGetCTM(ctx); // save for later
// draw points using user coordinates
In my touch event handler:
CGPoint touchDevice = [gesture locationInView:self]; // touch point in device coords
CGPoint touchUser = CGPointApplyAffineTransform(touchDevice, self.ctm); // doesn't give me what I want
// CGContextConvertPointToUserSpace(touchDevice) <- what I want, but doesn't work here
Using the inverse of ctm doesn't work either. I'll admit I'm having trouble getting my head around the meaning and relationships between device coordinates, user coordinates, and the transformation matrix. I think it's not as simple as I want it to be.
EDIT: Some background from Apple's documentation (iOS Coordinate Systems and Drawing Model).
"A window is positioned and sized in screen coordinates, which are defined by the coordinate system for the display."
"Drawing commands make reference to a fixed-scale drawing space, known as the user coordinate space. The operating system maps coordinate units in this drawing space onto the actual pixels of the corresponding target device."
"You can change a view’s default coordinate system by modifying the current transformation matrix (CTM). The CTM maps points in a view’s coordinate system to points on the device’s screen."

Answer 1

I discovered that the CTM already included a transformation to map view coordinates (with origin at the top left) to screen coordinates (with origin at the bottom left). So (0,0) got transformed to (0,800), where the height of my view was 800, and (0,2) mapped to (0,798) etc. So I gather there are 3 coordinate systems we're talking about: screen coordinates, view/device coordinates, user coordinates. (Please correct me if I am wrong.)
The CGContext transform (CTM) maps from user coordinates all the way to screen coordinates. My solution was to maintain my own transform separately which maps from user coordinates to view coordinates. Then I could use it to go back to user coordinates from view coordinates.
My Solution:
In drawRect:
CGAffineTransform scale = CGAffineTransformMakeScale(...);
CGAffineTransform translate = CGAffineTransformMakeTranslation(...);
self.myTransform = CGAffineTransformConcat(translate, scale);
// draw points using user coordinates
In my touch event handler:
CGPoint touch = [gesture locationInView:self]; // touch point in view coords
CGPoint touchUser = CGPointApplyAffineTransform(touchPoint, CGAffineTransformInvert(self.myTransform)); // this does the trick
Alternate Solution:
Another approach is to manually setup an identical context, but I think this is more of a hack.
In my touch event handler:
#import <QuartzCore/QuartzCore.h>
CGPoint touch = [gesture locationInView:self]; // view coords
CGSize layerSize = [self.layer frame].size;
UIGraphicsBeginImageContext(layerSize);
CGContextRef context = UIGraphicsGetCurrentContext();
// as in drawRect:
CGContextScaleCTM(...);
CGContextTranslateCTM(...);
CGPoint touchUser = CGContextConvertPointToUserSpace(context, touch); // now it gives me what I want
UIGraphicsEndImageContext();