objective c - Drawing incrementally in a UIView (iPhone)

Question

As far as I have understood so far, every time I draw something in the drawRect: of a UIView, the whole context is erased and then redrawn.

So I have to do something like this to draw a series of dots:

Method A: drawing everything on every call

- (void)drawRect:(CGRect)rect { 

    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextDrawImage(context, self.bounds, maskRef);      //draw the mask
    CGContextClipToMask(context, self.bounds, maskRef);     //respect alpha mask
    CGContextSetBlendMode(context, kCGBlendModeColorBurn);  //set blending mode

    for (Drop *drop in myPoints) {
        CGContextAddEllipseInRect(context, CGRectMake(drop.point.x - drop.size/2, drop.point.y - drop.size/2, drop.size, drop.size));
    }

    CGContextSetRGBFillColor(context, 0.5, 0.0, 0.0, 0.8);
    CGContextFillPath(context);
}

Which means, I have to store all my Dots (that's fine) and re-draw them all, one by one, every time I want to add a new one. Unfortunately this gives my terrible performance and I am sure there is some other way of doing this, more efficiently.

EDIT: Using MrMage's code I did the following, which unfortunately is just as slow and the color blending doesn't work. Any other method I could try?

Method B: saving the previous draws in a UIImage and only drawing the new stuff and this image

- (void)drawRect:(CGRect)rect
{
    //draw on top of the previous stuff
    UIGraphicsBeginImageContext(self.frame.size);
    CGContextRef ctx = UIGraphicsGetCurrentContext(); // ctx is now the image's context
    [cachedImage drawAtPoint:CGPointZero];
    if ([myPoints count] > 0)
    {
        Drop *drop = [myPoints objectAtIndex:[myPoints count]-1];
        CGContextClipToMask(ctx, self.bounds, maskRef);         //respect alpha mask
        CGContextAddEllipseInRect(ctx, CGRectMake(drop.point.x - drop.dropSize/2, drop.point.y - drop.dropSize/2, drop.dropSize, drop.dropSize));
        CGContextSetRGBFillColor(ctx, 0.5, 0.0, 0.0, 1.0);
        CGContextFillPath(ctx);
    }
    [cachedImage release];
    cachedImage = [UIGraphicsGetImageFromCurrentImageContext() retain];
    UIGraphicsEndImageContext();

    //draw on the current context   
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, self.bounds, maskRef);          //draw the mask
    CGContextSetBlendMode(context, kCGBlendModeColorBurn);      //set blending mode
    [cachedImage drawAtPoint:CGPointZero];                      //draw the cached image
}

EDIT: After all I combined one of the methods mention below with redrawing only in the new rect. The result is: FAST METHOD:

- (void)addDotAt:(CGPoint)point
{
    if ([myPoints count] < kMaxPoints) {
        Drop *drop = [[[Drop alloc] init] autorelease];
        drop.point = point;
        [myPoints addObject:drop];
        [self setNeedsDisplayInRect:CGRectMake(drop.point.x - drop.dropSize/2, drop.point.y - drop.dropSize/2, drop.dropSize, drop.dropSize)];      //redraw
    }
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextDrawImage(context, self.bounds, maskRef);                                              //draw the mask
    CGContextClipToMask(context, self.bounds, maskRef);                                             //respect alpha mask
    CGContextSetBlendMode(context, kCGBlendModeColorBurn);                                          //set blending mode

    if ([myPoints count] > 0)
    {
        Drop *drop = [myPoints objectAtIndex:[myPoints count]-1];
        CGPathAddEllipseInRect (dotsPath, NULL, CGRectMake(drop.point.x - drop.dropSize/2, drop.point.y - drop.dropSize/2, drop.dropSize, drop.dropSize));
    }
    CGContextAddPath(context, dotsPath);

    CGContextSetRGBFillColor(context, 0.5, 0.0, 0.0, 1.0);
    CGContextFillPath(context);
}

Thanks everyone!

Answer 1

If you are only actually changing a small portion of the UIView's content every time you draw (and the rest of the content generally stays the same), you can use this. Rather than redraw all the content of the UIView every single time, you can mark only the areas of the view that need redrawing using -[UIView setNeedsDisplayInRect:] instead of -[UIView setNeedsDisplay]. You also need to make sure that the graphics content is not cleared before drawing by setting view.clearsContextBeforeDrawing = YES;

Of course, all this also means that your drawRect: implementation needs to respect the rect parameter, which should then be a small subsection of your full view's rect (unless something else dirtied the entire rect), and only draw in that portion.