What does "Receiver type 'CALayer' for instance message is a forward declaration" mean here?

What does "Receiver type 'CALayer' for instance message is a forward declaration" mean here?

objective-c ios core-animation calayer automatic-ref-counting

objective-c

CALayer with transparent hole in it

ios objective-c calayer quartz-core

objective-c

Rounded UIView using CALayers - only some corners - How?

iphone objective-c xcode uiview calayer

objective-c

Why masksToBounds = YES prevents CALayer shadow?

ios objective-c uiview calayer

objective-c

CALayers didn't get resized on its UIView's bounds change. Why?

iphone ios objective-c uiview calayer

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CALayer: add a border only at one side

iphone objective-c core-animation calayer

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remove all subLayers from a view

ios objective-c calayer

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AVPlayer layer inside a view does not resize when UIView frame changes

objective-c uiview calayer avplayer

objective-c

UIImage from CALayer in iOS

ios objective-c uiimageview uiimage calayer

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iOS Core Animation: CALayer bringSublayerToFront?

iphone objective-c ios core-animation calayer

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Animate CALayer border change

ios objective-c calayer

objective-c

Rotating a CALayer 90 degrees?

objective-c core-animation calayer

objective-c

When do I need to set the contentsScale property of a CALayer?

ios objective-c macos calayer

objective-c

SpringWithDamping for CALayer animations?

ios objective-c animation uiview calayer

objective-c

Observing animated property changes in a CALayer

iphone objective-c macos core-animation calayer

objective-c