When a layer needs to be displayed and has no valid backing store (perhaps because the layer received a setNeedsDisplay message), the system sends the display message to the layer.
The -[CALayer display] method looks roughly like this:
- (void)display {
if ([self.delegate respondsToSelector:@selector(displayLayer:)]) {
[[self.delegate retain] displayLayer:self];
[self.delegate release];
return;
}
CABackingStoreRef backing = _backingStore;
if (!backing) {
backing = _backingStore = ... code here to create and configure
the CABackingStore properly, given the layer size, isOpaque,
contentScale, etc.
}
CGContextRef gc = ... code here to create a CGContext that draws into backing,
with the proper clip region
... also code to set up a bitmap in memory shared with the WindowServer process
[self drawInContext:gc];
self.contents = backing;
}
So, if you override display, none of that happens unless you call [super display]. And if you implement displayLayer: in FooView, you have to create your own CGImage somehow and store it in the layer's contents property.
The -[CALayer drawInContext:] method looks roughly like this:
- (void)drawInContext:(CGContextRef)gc {
if ([self.delegate respondsToSelector:@selector(drawLayer:inContext:)]) {
[[self.delegate retain] drawLayer:self inContext:gc];
[self.delegate release];
return;
} else {
CAAction *action = [self actionForKey:@"onDraw"];
if (action) {
NSDictionary *args = [NSDictionary dictionaryWithObject:gc forKey:@"context"];
[action runActionForKey:@"onDraw" object:self arguments:args];
}
}
}
The onDraw action is not documented as far as I know.
The -[UIView drawLayer:inContext:] method looks roughly like this:
- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)gc {
set gc's stroke and fill color spaces to device RGB;
UIGraphicsPushContext(gc);
fill gc with the view's background color;
if ([self respondsToSelector:@selector(drawRect:)]) {
[self drawRect:CGContextGetClipBoundingBox(gc)];
}
UIGraphicsPopContext(gc);
}
...I used Hopper to disassemble the IOS simulator libraries...
