How to draw to context without losing it? - iphone

Question

I am drawing UIImages to/from a grid. I am currently only drawing the changes... and only the changes... but the old images are supposed to stay where they are... however right now they vanish after the next call of drawRect:(CGRect)rect:
- (void)drawRect:(CGRect)rect
{
int cellSize = self.bounds.size.width / WIDTH;
double xOffset = 0;
CGRect cellFrame = CGRectMake(0, 0, cellSize, cellSize);
NSUInteger cellIndex = 0;
cellFrame.origin.x = xOffset;
for (int i = 0; i < WIDTH; i++)
{
cellFrame.origin.y = 0;
for (int j = 0; j < HEIGHT; j++, cellIndex++)
{
if([[self.state.boardChanges objectAtIndex:(i*HEIGHT)+j] intValue]==1){
if (CGRectIntersectsRect(rect, cellFrame)) {
NSNumber *currentCell = [self.state.board objectAtIndex:cellIndex];
if (currentCell.intValue == 1)
{
[image1 drawInRect:cellFrame];
}
else if (currentCell.intValue == 0)
{
[image2 drawInRect:cellFrame];
}
}
}
cellFrame.origin.y += cellSize;
}
cellFrame.origin.x += cellSize;
}
}
I tried mixtures of the following without any results:
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
//CGContextAddRect(context, originalRect);
CGContextClip(context);
[image drawInRect:rect];
CGContextRestoreGState(context);

Answer 1

Cocoa/UIKit may discard the previously-drawn content of views whenever it likes. When your view is asked to draw, it must draw everything within the provided rect. You can't just decide to not draw some stuff. (Well, you can do whatever you like, but you get the results that you're seeing.)

Answer 2

Found another solution-> Just take a screenshot and draw it with drawRect as a background.
-(void)createContent{
CGRect rect = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
UIGraphicsBeginImageContext(rect.size);
[self.layer renderInContext:UIGraphicsGetCurrentContext()];
CGImageRef screenshotRef = CGBitmapContextCreateImage(UIGraphicsGetCurrentContext());
_backgroundImage = [[UIImage alloc] initWithCGImage:screenshotRef];
CGImageRelease(screenshotRef);
UIGraphicsEndImageContext();
}
- (void)drawRect:(CGRect)rect
{
[_backgroundImage drawInRect:CGRectMake(0.0f, 0.0f, self.bounds.size.width, self.bounds.size.height)];
....
....
....
}
######EDIT
Found a much more sophisticated way to do the above :
http://www.cimgf.com/2009/02/03/record-your-core-animation-animation/
#### EDIT
Another solution would be to use CALayers and only update those parts that change...
####EDIT
OR:
self.layer.contents = (id) [_previousContent CGImage];