how to draw continuous lines by QuartzCore in iOS - iphone

Question

I have got a question about quartz core on iOS
When I use Quartz to draw lines, the following happens and I can't find the reason:
when I draw the second line the first line would disappear
when I draw the third line the second would the disappear, and the 1st and 3rd would show
..
when I draw the 2n+1th line, 1,3,5,...2n-1th lines shows, and 2,4,6,8... 2n dissappears
see the code below. I don't save contexts and paths
as my understanding, I think it should be one of the two cases
display all lines I drawn
display the last line I drawn and the previous lines should disappear
But the two cases don't happen.
- (void)drawInContext:(CGContextRef)context {
// 橡皮擦
//CGContextClearRect(context, CGRectMake(0, 0, 320, 480));
CGContextSetLineWidth(context, 4.0);
CGContextMoveToPoint(context, previousPoint.x, previousPoint.y);
CGContextSetRGBStrokeColor(context, 0, 1.0, 1.0, 1.0);
CGContextAddLineToPoint(context, nextPoint.x,nextPoint.y);
CGContextStrokePath(context);
//NIF_TRACE(#"began : %# moved : %#", NSStringFromCGPoint(previousPoint),NSStringFromCGPoint(nextPoint));
}
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
previousPoint = [touch locationInView:self];
nextPoint = [touch locationInView:self];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
previousPoint = nextPoint;
nextPoint = [touch locationInView:self];
[self setNeedsDisplay];
}
I saw a demo that use NSMutableArray to save all UIBezierPaths it drawn, when the View redraw, it transfer paths saved in array, restores them in drawRect:
UIBezierPath is a wrapper of Objective C, and it only works in 3.2+
I need do something make it work in 3.0+
I think it must be exists a better method to save contexts and paths (Colors,Paths,strokeWidths)
Anybody have ideas?

Answer 1

Every time you "drawInContext" you are clearing the drawing area and placing the new line.... This clear code:
CGContextClearRect(context, CGRectMake(0, 0, 320, 480));
Needs to be somewhere else in your code... somewhere where it will only be activated 1 time (or if you need to clear the whole drawing (perhaps an erase all function))

Answer 2

You need to save the image on canvas:
CGImageRef imageRef;
- (void)drawRect:(CGRect)rect {
CGContextRef context = UIGraphicsGetCurrentContext();
if (imageRef) {
// Restore the screen that was previously saved
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, rect, imageRef);
CGImageRelease(imageRef);
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
}
// Your drawing code here...
imageRef = CGBitmapContextCreateImage(context);
}