iOS, relation entre CALayer et UIView

21 décembre 2011
c ios objective-c
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...
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