Swift version: 5.6
All UIView
subclasses have a layer
property, which is responsible for drawing their contents efficiently. These layers are powered by Core Animation, which handles all the drawing and animation that UIKit requests. The layer is separate from the view because drawing is separate from placement – you can move a UIView
around freely, change its alpha
, add or remove it all you want, and so on, but that doesn’t affect the underlying way it’s drawn.
Although most views use a plain CALayer
behind them, there are specialized subclasses that do more advanced things. CATiledLayer
is responsible for tiling images in the way you see in the Maps app, CAEmitterLayer
handles creating particles, CAGradientLayer
draws color gradients, and more – there’s a selection depending on your need.
Broadly speaking, iOS developers mostly work with CALayer
through the layer
property of their views – things like rounding corners, adding borders, and drawing shadows can all be done there.
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This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
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