Printing CALayers

Question

I have a NSView which contains many CALayers. When a user is editing a document, these CALayers animate all edits. I am attempting to implement printing for my app, but I am having some problems printing these CALayers correctly.

Some CALayers bounds occupy the entire NSView, and do not need to be laid out, because their position never changes. However, I also have one CALayer which contains about 20 small CALayers. These CALayers animate their position changes during normal editing. However, when attempting to print the NSView, these small CALayers never get laid out correctly. I am wondering if there is something special I have to do to ensure that these layers are positioned correctly, and allow the NSView to be drawn/printed correctly.

Does anyone have experience printing a Core Animation backed NSView? Any suggestions are appreciated.

Answer 1

Last I looked into it, it was not possible to properly print CALayers. It seemed to me at the time that Core Animation was designed for screen only and not for print (which seems consistent with the fact that it was designed initially for iPhone).

I'd love to know I'm wrong.

Answer 2

In order to work around layout issues, as well as the fact that using -renderInContext: to draw a layer hierarchy does not preserve vector elements, we subclassed CALayer in the Core Plot framework. The CPLayer subclass overrides the default -drawInContext: method to call our custom -renderAsVectorInContext: method (where we do all of our Core Graphics drawing for a layer). To generate a PDF context (or similar) for printing, we then call a custom method with the following code:

-(void)recursivelyRenderInContext:(CGContextRef)context
{
    // render self
    CGContextSaveGState(context);

    [self applyTransform:self.transform toContext:context];

    self.renderingRecursively = YES;
    if ( !self.masksToBounds ) {
        CGContextSaveGState(context);
    }
    [self renderAsVectorInContext:context];
    if ( !self.masksToBounds ) {
        CGContextRestoreGState(context);
    }
    self.renderingRecursively = NO;

    // render sublayers
    for ( CALayer *currentSublayer in self.sublayers ) {
        CGContextSaveGState(context);

        // Shift origin of context to match starting coordinate of sublayer
        CGPoint currentSublayerFrameOrigin = currentSublayer.frame.origin;
        CGRect currentSublayerBounds = currentSublayer.bounds;
        CGContextTranslateCTM(context,
                              currentSublayerFrameOrigin.x - currentSublayerBounds.origin.x, 
                              currentSublayerFrameOrigin.y - currentSublayerBounds.origin.y);
        [self applyTransform:self.sublayerTransform toContext:context];
        if ( [currentSublayer isKindOfClass:[CPLayer class]] ) {
            [(CPLayer *)currentSublayer recursivelyRenderInContext:context];
        } else {
            if ( self.masksToBounds ) {
                CGContextClipToRect(context, currentSublayer.bounds);
            }
            [currentSublayer drawInContext:context];
        }
        CGContextRestoreGState(context);
    }
    CGContextRestoreGState(context);
}

This goes through and renders each layer onto a flat Core Graphics context, preserving position, rotation, and other transforms while rendering all elements as sharp vectors.

One other thing to watch out for when trying to render layers is that the state of your presentation layer hierarchy may not be the same as your internal layer hierarchy. You may have animations that have been applied to move your layers, but the layers' position properties may not have been changed to match. In that case, you should make sure that you either animate the properties themselves, so that the values always stay in sync, or set the values in your layer once the animations have completed.