Draw on a custom context

Question

I'm actually trying to draw lines on a view. In order to not clear the context before every draws, I understood that I have to create my own context in order to draw on it.

I found this way to create a context :

CGContextRef MyCreateBitmapContext (int pixelsWide,
                                int pixelsHigh)
{
    CGContextRef    context = NULL;
    CGColorSpaceRef colorSpace;
    void *          bitmapData;
    int             bitmapByteCount;
    int             bitmapBytesPerRow;

    bitmapBytesPerRow   = (pixelsWide * 4);
    bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

    colorSpace = CGColorSpaceCreateDeviceRGB();
    bitmapData = calloc( bitmapByteCount, sizeof(int) );
    if (bitmapData == NULL)
    {
        fprintf (stderr, "Memory not allocated!");
        return NULL;
    }
    context = CGBitmapContextCreate (bitmapData,
                                     pixelsWide,
                                     pixelsHigh,
                                     8,      // bits per component
                                     bitmapBytesPerRow,
                                     colorSpace,
                                     kCGImageAlphaPremultipliedLast);
    if (context== NULL)
    {
        free (bitmapData);
        fprintf (stderr, "Context not created!");
        return NULL;
    }
    CGColorSpaceRelease( colorSpace );

    return context;
}

But my question is : how can I use this context in order to not have my view cleaned every time ?

EDIT (after the answer of @Peter Hosey) :

I try to do something like :

- (void)drawRect:(CGRect)rect {
    // Creation of the custom context
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    CGContextDrawImage(context, CGRectMake(0, 0, self.frame.size.width, self.frame.size.height), cgImage);
    CGImageRelease(cgImage);
    if (isAuthorizeDrawing) {
         [self drawInContext:context andRect:rect]; // Method which draw all the lines sent by the server
         isAuthorizeDrawing = NO;
    }

    // Draw the line
    [currentDrawing stroke];
}

I also set clearsContextBeforeDrawing to NO for the UIView.

When I zoom (isAuthorizeDrawing is set to YES in order to redraw all the lines correctly scaled), the lines don't disappear but when I try to draw new lines (isAuthorizeDrawing is set to NO in order to not redraw everything at each setNeedsDisplay call), all the lines disappeared and the drawing is going really slow.. :/

Am I doing something wrong ?

EDIT 2

Here are my drawing methods :

-(void)drawInContext:(CGContextRef)context {
    for (int i = 0; i < self.drawings.count; ++i) {
        Drawing* drawing = [self.drawings objectAtIndex:i];

        CGContextSetStrokeColorWithColor(context, drawing.colorTrait.CGColor);
        CGContextSetLineWidth(context, 1.0);

        CGContextMoveToPoint(context, [[drawing.points objectAtIndex:0] CGPointValue].x * self.zoomScale, [[drawing.points objectAtIndex:] CGPointValue].y * self.zoomScale);

        for (int i = 1; i < drawing.points.count; i++) {
            CGContextAddLineToPoint(context, [[drawing.points objectAtIndex:i] CGPointValue].x * self.zoomScale, [[drawing.points objectAtIndex:i] CGPointValue].y * self.zoomScale);
        }

        CGContextStrokePath(context);
    }
}

-(void)drawRect:(CGRect)rect {
    if (isRedrawing) {
        [self drawInContext:UIGraphicsGetCurrentContext()];
        isRedrawing = NO;
    }

    [[UIColor redColor] set];
    [currentPath stroke];
}

Answer 1

Your context holds onto all of your drawing for you.

When you're drawing into a context supplied by a view (i.e., within drawRect:), either the view is creating a new context each time or it has erased the context's contents, like shaking an Etch-a-Sketch. Either way, you're getting a context into which (effectively, at least) nothing has been drawn.

When you're drawing into your context, assuming you're not doing anything to erase it between uses, everything you draw into it just piles up.

One ramification of this is that you need to be careful of drawing state like the current transformation matrix, clipping path, etc., because nothing is resetting those parameters between drawing sessions. That may be useful, depending on what you're doing, but either way, you need to be aware of it and plan accordingly.

Presumably you want to show the user what you've drawn so far from time to time (namely, when drawRect: happens). To do that, ask the bitmap context to create an image of its contents and draw that image into the current (UIKit-supplied) context.

Alternatively, just tell the view not to clear itself before every draw, and don't bother managing your own bitmap context at all.