Line is erased when drawing shapes - iphone

Question

I am trying to make an application for drawing shapes on screen by touching it.
I can draw a line from one point to another- but it erases on each new draw.
Here is my code:
CGPoint location;
CGContextRef context;
CGPoint drawAtPoint;
CGPoint lastPoint;
-(void)awakeFromNib{
//[self addSubview:noteView];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [[event allTouches] anyObject];
location = [touch locationInView:touch.view];
[self setNeedsDisplayInRect:CGRectMake(0, 0, 320, 480)];
}
- (void)drawRect:(CGRect)rect {
context = UIGraphicsGetCurrentContext();
[[UIColor blueColor] set];
CGContextSetLineWidth(context,10);
drawAtPoint.x =location.x;
drawAtPoint.y =location.y;
CGContextAddEllipseInRect(context,CGRectMake(drawAtPoint.x, drawAtPoint.y, 2, 2));
CGContextAddLineToPoint(context,lastPoint.x, lastPoint.y);
CGContextStrokePath(context);
lastPoint.x =location.x;
lastPoint.y =location.y;
}
Appreciate your help-
Nir.

Answer 1

As you have discovered, -drawRect is where you display the contents of a view. You will only 'see' on screen what you draw here.
This is much more low-level than something like Flash where you might add a movieclip containing a line to the stage and some time later add another movieclip containing a line to the stage and now you see - two lines!
You will need to do some work and prdobably set up something like..
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [[event allTouches] anyObject];
location = [touch locationInView:touch.view];
[self addNewLineFrom:lastPoint to:location];
lastPoint = location;
[self setNeedsDisplayInRect:CGRectMake(0, 0, 320, 480)];
}
- (void)drawRect:(CGRect)rect {
context = UIGraphicsGetCurrentContext();
for( Line *eachLine in lineArray )
[eachLine drawInContext:context];
}
I think you can see how to flesh this out to what you need.
Another way to approach this is to use CALayers. Using this approach you dont draw inside - - (void)drawRect at all - you add and remove layers, draw what you like inside them, and the view will handle compositing them together and drawing to the screen as needed. Probably more what you are looking for.

Answer 2

Every time that drawRect is called, you start with a blank slate. If you don't keep track of everything you have drawn before in order to draw it again, then you end up only drawing the latest swipe of your finger, and not any of the old ones. You will have to keep track of all of your finger swipes in order to redraw them every time drawRect is called.

Answer 3

Instead of redrawing every line, you can draw into an image and then just display the image in your drawRect: method. The image will accumulate the lines for you. Of course, this method makes undo more difficult to implement.
From the iPhone Application Programming Guide:
Use the UIGraphicsBeginImageContext
function to create a new image-based
graphics context. After creating this
context, you can draw your image
contents into it and then use the
UIGraphicsGetImageFromCurrentImageContext
function to generate an image based on
what you drew. (If desired, you can
even continue drawing and generate
additional images.) When you are done
creating images, use the
UIGraphicsEndImageContext function to
close the graphic context. If you
prefer using Core Graphics, you can
use the CGBitmapContextCreate function
to create a bitmap graphics context
and draw your image contents into it.
When you finish drawing, use the
CGBitmapContextCreateImage function to
create a CGImageRef from the bitmap
context. You can draw the Core
Graphics image directly or use this it
to initialize a UIImage object.