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CALayer's draw(in:) is not called if frame.size is zero

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iOS
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Core Animation
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One way to perform custom drawing using CGContext in CALayer is to override draw(in:). However, this method will not be called at all if the layer's frame.size contains a zero. Before calling draw(in:), make sure both the width and height of the frame.size are 1 or greater.

Basic steps for CALayer drawing using CGContext:

  1. Create a custom subclass of the layer, override draw(in:), and implement the desired drawing logic.
  2. Determine the layer's frame (especially its size) or set it to at least (1, 1).
  3. Call setNeedsDisplay() on the layer to mark it for a drawing update by the system.

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