Allocation hooks and headless backends

Allocation hooks

• Allocation hooks are your own memory functions, which the library then uses instead of its default ones.
• Set them before Initialize:

#include <gui_core/memory/gui_allocation_hooks.h>
 
tachyon::AllocationHooks hooks;
hooks.allocate   = &MyAllocate;     // void* (size_t size, size_t alignment, void* user)
hooks.reallocate = &MyReallocate;   // void* (void* ptr, size_t size, size_t alignment, void* user)
hooks.free       = &MyFree;         // void  (void* ptr, void* user)
hooks.user       = &myHeap;
app.SetAllocationHooks(hooks);

• The hooks cover:
  • Vulkan objects
  • VMA
  • vk-bootstrap
  • SDL
• SDL's memory functions are shared by the whole process. SDL uses the hooks only if:
  • the GUI starts SDL
  • and SDL hasn't allocated anything yet
• The hooks must be thread-safe, because drivers call them from their own threads.
• They must outlive the GUI.
  • SDL keeps using them for a few allocations after SDL_Quit, so keep them alive for the whole process.
• tachyon::TrackingAllocator (gui_core/memory/gui_tracking_allocator.h) has ready-made hooks:
  • they count the allocations that are still live
  • they report leaks
  • Hooks() returns them

#include <gui_core/memory/gui_tracking_allocator.h>
 
static tachyon::TrackingAllocator tracker;   // lives as long as the process
app.SetAllocationHooks(tracker.Hooks());
// ...
tracker.ReportLeaks();   // logs each allocation still live

Headless backends

• Headless means running without a GPU or without a display.
• NullGraphicsContext (gui_core/graphics/null/gui_null_graphics_context.h) is a graphics backend without a GPU:
  • it draws nothing
  • the UI is still built, laid out and recorded
  • windows stay empty
• NullWindowBackend is a window backend without an OS:
  • input arrives only through the Inject* calls
• Together they need no GPU and no display, for example in tests.

app.SetGraphicsBackend(std::make_unique<tachyon::NullGraphicsContext>());
app.SetWindowBackend(std::make_unique<tachyon::NullWindowBackend>());

• They are also the smallest backends to start from when writing your own:
  • a graphics backend derives from tachyon::GraphicsContext
  • a window backend derives from tachyon::IWindowBackend

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