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Using constraints on a Layer

Subject: Using constraints on a Layer
From: Neil Clayton <email@hidden>
Date: Sun, 16 Mar 2008 13:37:11 +1300
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Not sure if I'm reading and understanding the documentation correctly. It's my understanding that I could use constraints on a layer B, such that it could "track" the size and position of another layer (for example).

As an example assuming I have a layer (it will be a sibling) that I want to track called 'clickedLayer', I might do this:

- (void) selectLayer:(CALayer*)clickedLayer { currentSelectedThing.frame = clickedLayer.frame; NSMutableArray *constraints = [NSMutableArray array]; currentSelectedThing.borderWidth = 30; currentSelectedThing.borderColor = CGColorGetConstantColor(kCGColorWhite); currentSelectedThing.opacity = 1.0; currentSelectedThing.zPosition = 1;

[constraints addObject:[CAConstraint constraintWithAttribute:kCAConstraintMidX relativeTo:clickedLayer.name attribute:kCAConstraintMidX]]; [constraints addObject:[CAConstraint constraintWithAttribute:kCAConstraintMidY relativeTo:clickedLayer.name attribute:kCAConstraintMidY]]; [constraints addObject:[CAConstraint constraintWithAttribute:kCAConstraintWidth relativeTo:clickedLayer.name attribute:kCAConstraintWidth]]; [constraints addObject:[CAConstraint constraintWithAttribute:kCAConstraintHeight relativeTo:clickedLayer.name attribute:kCAConstraintHeight]]; currentSelectedThing.constraints = constraints;

	self.selectedLayer = clickedLayer;
}

This currentSelectedThing layer has been previously added to the root layer by:

	self.currentSelectedThing = [CALayer layer];
	currentSelectedThing.opacity = 0;
	currentSelectedThing.bounds = CGRectZero;
	currentSelectedThing.name = @"currentSelection";
	currentSelectedThing.needsDisplayOnBoundsChange = YES;
	[self.layer addSublayer:currentSelectedThing];

The part I don't understand is that I cannot get this to work unless I set the frame on the layer. It just doesn't show up. currentSelectedThing.frame = clickedLayer.frame;

If I don't set the frame it never shows (presumably it's not getting resized). Is my understanding of constraints incorrect here? Or must I do something else in order have the constraint apply?

--
Regards,
Neil Clayton,  http://shinywhitebox.com

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