core graphics, how to draw lines at runtime? - iphone

Question

The task is, to draw paths at runtime on custom maps which im using in a Scrollview, and then i will have to draw paths at runtime whenever the location coordinates (lat, long) updates. The problem what im trying to solve here is that i have made a class 'graphics' which is a subclass of UIView, in which i code the drawing in the 'drawrect:' method. So when im adding the graphics as subview of the scrollview over image, the line draws, but i need to keep drawing the line as though it were paths. I need to draw the lines at runtime, need to keep updating the points(x,y) of 'CGContextStrokeLineSegments' method. The code:
ViewController:
- (void)loadView {
[[UIApplication sharedApplication] setStatusBarHidden:YES withAnimation:UIStatusBarAnimationNone];
CGRect fullScreenRect=[[UIScreen mainScreen] applicationFrame];
scrollView=[[UIScrollView alloc] initWithFrame:fullScreenRect];
graph = [[graphics alloc] initWithFrame:fullScreenRect];
scrollView.contentSize=CGSizeMake(320,480);
UIImageView *tempImageView2 = [[UIImageView alloc] initWithImage:[UIImage imageNamed:#"fortuneCenter.png"]];
self.view=scrollView;
[scrollView addSubview:tempImageView2];
scrollView.userInteractionEnabled = YES;
scrollView.bounces = NO;
[scrollView addSubview:graph];
}
Graphics.m:
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
// Initialization code
self.backgroundColor = [UIColor clearColor];
}
return self;
}
- (void)drawRect:(CGRect)rect
{
CGContextRef context = UIGraphicsGetCurrentContext();
CGPoint point [2] = { CGPointMake(160, 100), CGPointMake(160,300)};
CGContextSetRGBStrokeColor(context, 255, 0, 255, 1);
CGContextStrokeLineSegments(context, point, 2);
}
So how can i draw the lines at runtime. Im just simulating right now, so im not using the realtime data (coordinates). Just want to simulate by using dummy data (coordinates of x,y). Lets say have a button, whenever i press it it updates the coordinates so path extends.

Answer 1

The easiest way would be to add an instance variable representing the points to the UIView subclass.
Then, every time the path changes, update the ivar appropriately and call -setNeedsDisplay or setNeedsDisplayInRect on the custom UIView (or even on its superview). The runtime will then redraw the new path.

Answer 2

You just need to make CGPoint point[] dynamically resizable, from the looks of it.
You can use malloc, a std::vector, or even NSMutableData to store the points you add. Then you pass that array to CGContextStrokeLineSegments.
If 2 points is all you will need, move CGPoint point[2] to an ivar so you may store the positions, then (as Rich noted) invalidate rects appropriately when these values (or the array) are changed.

Answer 3

This subject comes up every now and then, so I created a longer blog post on the general concepts involved with one potential solution, creating and using your own graphics context, here: http://www.musingpaw.com/2012/04/drawing-in-ios-apps.html