From 0f3a2c93e1d756921bc59d2faf3a1486db1e2b70 Mon Sep 17 00:00:00 2001
From: David Robillard <d@drobilla.net>
Date: Mon, 29 Jul 2019 00:24:09 +0200
Subject: X11: Implement double buffering for Cairo

Also save and restore cairo context state around callbacks, so applications
don't need to worry about smashing cairo state across exposures.
---
 pugl/detail/x11_cairo.c | 97 +++++++++++++++++++++++++++++++++----------------
 1 file changed, 66 insertions(+), 31 deletions(-)

diff --git a/pugl/detail/x11_cairo.c b/pugl/detail/x11_cairo.c
index 787c111..1867007 100644
--- a/pugl/detail/x11_cairo.c
+++ b/pugl/detail/x11_cairo.c
@@ -32,8 +32,10 @@
 #include <stdlib.h>
 
 typedef struct  {
-	cairo_surface_t* surface;
-	cairo_t*         cr;
+	cairo_surface_t* back;
+	cairo_t*         backCr;
+	cairo_surface_t* front;
+	cairo_t*         frontCr;
 } PuglX11CairoSurface;
 
 static int
@@ -52,31 +54,36 @@ puglX11CairoConfigure(PuglView* view)
 static int
 puglX11CairoCreate(PuglView* view)
 {
-	PuglInternals* const       impl = view->impl;
-	PuglX11CairoSurface* const surface =
-		(PuglX11CairoSurface*)calloc(1, sizeof(PuglX11CairoSurface));
-
-	impl->surface = surface;
-
-	surface->surface = cairo_xlib_surface_create(
-		impl->display, impl->win, impl->vi->visual, view->width, view->height);
-
-	if (!surface->surface) {
-		return 1;
+	PuglInternals* const impl    = view->impl;
+	const double         width   = view->width;
+	const double         height  = view->height;
+	PuglX11CairoSurface  surface = { 0 };
+
+	surface.back = cairo_xlib_surface_create(
+		impl->display, impl->win, impl->vi->visual, width, height);
+	surface.front = cairo_surface_create_similar(
+		surface.back, CAIRO_CONTENT_COLOR, width, height);
+	surface.backCr  = cairo_create(surface.back);
+	surface.frontCr = cairo_create(surface.front);
+
+	cairo_status_t st = CAIRO_STATUS_SUCCESS;
+	if (!surface.back || !surface.backCr ||
+	    !surface.front || !surface.frontCr ||
+	    (st = cairo_surface_status(surface.back)) ||
+	    (st = cairo_surface_status(surface.front)) ||
+	    (st = cairo_status(surface.backCr)) ||
+	    (st = cairo_status(surface.frontCr))) {
+		cairo_destroy(surface.frontCr);
+		cairo_destroy(surface.backCr);
+		cairo_surface_destroy(surface.front);
+		cairo_surface_destroy(surface.back);
+		return PUGL_ERR_CREATE_CONTEXT;
 	}
 
-	cairo_status_t st = cairo_surface_status(surface->surface);
-	if (st) {
-		fprintf(stderr, "error: failed to create cairo surface (%s)\n",
-		        cairo_status_to_string(st));
-	} else if (!(surface->cr = cairo_create(surface->surface))) {
-		fprintf(stderr, "error: failed to create cairo context\n");
-	} else if ((st = cairo_status(surface->cr))) {
-		cairo_surface_destroy(surface->surface);
-		fprintf(stderr, "error: cairo context is invalid (%s)\n",
-		        cairo_status_to_string(st));
-	}
-	return (int)st;
+	impl->surface = calloc(1, sizeof(PuglX11CairoSurface));
+	*(PuglX11CairoSurface*)impl->surface = surface;
+
+	return 0;
 }
 
 static int
@@ -85,22 +92,40 @@ puglX11CairoDestroy(PuglView* view)
 	PuglInternals* const       impl    = view->impl;
 	PuglX11CairoSurface* const surface = (PuglX11CairoSurface*)impl->surface;
 
-	cairo_destroy(surface->cr);
-	cairo_surface_destroy(surface->surface);
+	cairo_destroy(surface->frontCr);
+	cairo_destroy(surface->backCr);
+	cairo_surface_destroy(surface->front);
+	cairo_surface_destroy(surface->back);
 	free(surface);
 	impl->surface = NULL;
 	return 0;
 }
 
 static int
-puglX11CairoEnter(PuglView* PUGL_UNUSED(view), bool PUGL_UNUSED(drawing))
+puglX11CairoEnter(PuglView* view, bool drawing)
 {
+	PuglInternals* const       impl    = view->impl;
+	PuglX11CairoSurface* const surface = (PuglX11CairoSurface*)impl->surface;
+
+	if (drawing) {
+		cairo_save(surface->frontCr);
+	}
+
 	return 0;
 }
 
 static int
-puglX11CairoLeave(PuglView* PUGL_UNUSED(view), bool PUGL_UNUSED(drawing))
+puglX11CairoLeave(PuglView* view, bool drawing)
 {
+	PuglInternals* const       impl    = view->impl;
+	PuglX11CairoSurface* const surface = (PuglX11CairoSurface*)impl->surface;
+
+	if (drawing) {
+		cairo_set_source_surface(surface->backCr, surface->front, 0, 0);
+		cairo_paint(surface->backCr);
+		cairo_restore(surface->frontCr);
+	}
+
 	return 0;
 }
 
@@ -110,7 +135,17 @@ puglX11CairoResize(PuglView* view, int width, int height)
 	PuglInternals* const       impl    = view->impl;
 	PuglX11CairoSurface* const surface = (PuglX11CairoSurface*)impl->surface;
 
-	cairo_xlib_surface_set_size(surface->surface, width, height);
+	cairo_xlib_surface_set_size(surface->back, width, height);
+
+	cairo_destroy(surface->frontCr);
+	cairo_surface_destroy(surface->front);
+	if (!(surface->front = cairo_surface_create_similar(
+		      surface->back, CAIRO_CONTENT_COLOR, width, height))) {
+		return PUGL_ERR_CREATE_CONTEXT;
+	}
+
+	surface->frontCr = cairo_create(surface->front);
+	cairo_save(surface->frontCr);
 
 	return 0;
 }
@@ -121,7 +156,7 @@ puglX11CairoGetContext(PuglView* view)
 	PuglInternals* const       impl    = view->impl;
 	PuglX11CairoSurface* const surface = (PuglX11CairoSurface*)impl->surface;
 
-	return surface->cr;
+	return surface->frontCr;
 }
 
 const PuglBackend*
-- 
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