Measure distance of line drawn on iPhone screen - ios

Question

I am trying to creating an application that allows the user to draw a line on the screen and it measures the distance of the drawn line. I have been able to successfully draw the line but I don't know how to measure it. The line does not have to be perfectly straight either. It is basically a squiggle. If someone could please point me in the right direction or help guide me that would be awesome. I am using Xcode 5.1.1 and objective-c. I only just started dabbling in the language this summer.
EDIT: I am looking to measure the distance in either inches or cm. I would like the measurement to be the entire line, to follow the curve of the line. The distance not the displacement.
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
mouseSwipe = YES; //swipe declared in header
UITouch *touch = [touches anyObject];
currentPoint = [touch locationInView:self.view]; //tracking finger movement on screen
UIGraphicsBeginImageContext(CGSizeMake(320, 568)); // 568 iphone 5, 480 is iphone 4 (320,525)
[drawImage.image drawInRect:CGRectMake(0, 0, 320, 568)]; // 0,0 centered in corner
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); //round line end
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0); // width of line
CGContextSetStrokeColorWithColor(UIGraphicsGetCurrentContext(), [[UIColor redColor] CGColor]); //sets color to red (change red to any color for that color)
//CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0,1,0,1); //green color
CGContextBeginPath(UIGraphicsGetCurrentContext()); //start of when drawn path
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
[drawImage setFrame:CGRectMake(0, 0, 320, 568)]; //(320, 568)
drawImage.image = UIGraphicsGetImageFromCurrentImageContext(); //importnant
UIGraphicsEndImageContext(); //finished drawing for time period
lastPoint = currentPoint;
[self.view addSubview:drawImage]; //adds to page
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [[event allTouches] anyObject]; //touch fires of touch
location = [touch locationInView:touch.view];
lastClick = [NSDate date];
lastPoint = [touch locationInView:self.view]; //stops connecting to previous line
lastPoint.y -= 0;
[super touchesEnded:touches withEvent: event];
}

Answer 1

First add a property that cumulates the length of the path.
#property(nonatomic, assign) CGFloat pathLength;
Initialize it to 0.0 when user begins drawing the path. Maybe do this in touchesBegan, or the same place elsewhere in your code where you realize you're beginning to draw. Add a method that computes the cartesian distance between points:
- (CGFloat)distanceFrom:(CGPoint)p1 to:(CGPoint)p2 {
CGFloat x = (p2.x - p1.x);
CGFloat y = (p2.y - p1.y);
return sqrt(x*x + y*y);
}
As you get touches moved, you are already handling the current and last touch positions. All you must do now is cumulate the distance between successive points:
// in touches moved, after you have lastPoint and currentPoint
self.pathLength += [self distanceFrom:currentPoint to:lastPoint];
There are quite a few refs here and elsewhere for converting these points to inches or cm. As far as I can see all are fraught with the inability to get the device resolution at runtime from the SDK. If you're willing to add a (dangerous) constant to the code, you can get PPI here, and divide that into the pathLength computed above.