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UXCairoGraphics

UXCairoGraphics is the GTK4 realization of UXGraphics.

#use <UXKit>            // or #import "UXCairoGraphics.xc"

The cairo_t of the draw in flight

Section titled “The cairo_t of the draw in flight”

The drawing ops act on the cairo_t that GTK passed to the drawing area for the current draw. The shim (libUXGtk.c) sets it before the callback, and it is not valid outside the callback.

The NSGraphicsContext on macOS and the CGContext on iOS work the same way: the surface belongs to the draw in progress and is not an object you hold. For that reason the vocabulary has no context field for you to keep, and a draw request outside a draw callback has nothing to draw onto.

Coordinates passed in are bounds-relative. The bound origin makes them absolute, as on every backend.

No flipping needed

Section titled “No flipping needed”

Cairo’s space grows y down, which is the protocol’s direction. Unlike UXCocoaGraphics, where the shim declares a flipped view, nothing here has to be inverted.

Six of the seven backends draw y-down without a flip: cairo, Canvas2D, UIKit and Android Canvas, plus VDI and GDI. Only Cocoa needs one.

Clipping is a stack, not an object

Section titled “Clipping is a stack, not an object”
void ux_gtk_clip(i32 x, i32 y, i32 w, i32 h);
void ux_gtk_clip_end(void);

Clipping is push and pop, with no clip object. A clip object would be a cairo_rectangle_t, which is a struct, and structs do not travel into xtc.

The whole shim surface uses primitives only: integers, and u8* with an explicit capacity. Both sides of the foreign-function boundary can agree on that without knowing each other’s layout rules.

It strokes natively

Section titled “It strokes natively”
strokesNatively()      // true

Cairo is a good stroker, so a path goes to it directly with its own joins and sub-pixel precision. Arrowheads stay neutral, as on every backend.

See also

Section titled “See also”