CGContext

CGContext is a Core Graphics drawing destination and state container used on Apple platforms to render graphics. It provides the routines and state needed to draw shapes, images, and text into a destination such as a view, layer, bitmap, or PDF document. A context may be created for offscreen rendering (a bitmap or PDF context) or obtained from the system for onscreen drawing during a view’s drawing cycle.

Contexts come in device-based forms provided by the system (for example, during a view’s draw cycle) and offscreen bitmap contexts created with functions like CGBitmapContextCreate. In iOS and macOS, CGContextRef (or its Swift-equivalent) stores the current graphics state, including transformations, clipping paths, colors, line parameters, and alpha settings. The graphics state stack can be saved and restored with CGContextSaveGState and CGContextRestoreGState to manage nested drawing configurations.

The drawing model centers on paths and primitives. Developers construct paths using functions such as CGContextMoveToPoint, CGContextAddLineToPoint, and CGContextAddRect, then render with CGContextStrokePath or CGContextFillPath. The coordinate system can be transformed via the current transformation matrix, allowing flips, translations, and scaling. Clipping can constrain drawing to shapes or regions.

CGContext supports drawing images through functions like CGContextDrawImage, and can render gradients and patterns via additional Core Graphics facilities. Text rendering typically relies on higher-level text systems (such as Core Text or UIKit/AppKit) in conjunction with a context.

CGContext is a foundational element of Core Graphics, providing low-level, high-performance drawing capabilities that underpin many rendering tasks across Apple platforms.