====== Callback system ======

New to version 8.5.0 is a centralized callback system. In prior versions, each system had a different callback interface. In this new approach, the callback registration system is housed in ''Include/SpeedTree/Core/Callbacks.h''. There are nine callbacks that can be registered, though each drops back to a default definition if not set (the callbacks are located in ''Source/Core/Source/Callbacks.cpp''):

^ **Callback function** ^ **Description** ^  
| **''SpeedTree::Callbacks::Message()''** | The SDK calls this function when it wants to communicate a non-error message to the client application. |
| **''SpeedTree::Callbacks::Error()''** | The SDK calls this function when an error message needs to be communicated to the client. |
| **''SpeedTree::Callbacks::HeapAlloc()''** | All SDK-internal heap allocations are routed through this function. |
| **''SpeedTree::Callbacks::HeapFree()''** | All SDK-internal heap free calls are routed here. |
| **''SpeedTree::Callbacks::GetFileSize()''** | Given a string filename, this function will return the size of that file in bytes. |
| **''SpeedTree::Callbacks::LoadBinaryFile()''** | Given a string filename and a buffer, this function will read a binary file and fill the provided buffer with its contents. |
| **''SpeedTree::Callbacks::UserRenderPipeline()''** | During graphics initialization, this function helps determine which shaders and render states should be applied based on various parameters. This is described in more detail in [[user-render-configuration|User Render Configuration]]. |
| **''SpeedTree::Callbacks::DirectMemAlloc()''** | Used by some non-desktop platforms for GPU memory allocation. |
| **''SpeedTree::Callbacks::DirectMemFree()''** | Used by some non-desktop platforms for GPU memory freeing. |

Note that due to DLL interface constraints, these functions cannot be set directly but are set through access functions. An example of setting a callback appears in the listing below.

<code cpp-qt>
#include <cstdio>
#include "Core/Callbacks.h"

// example callback functions to show expected function parameters

// message callback
void MyMessageCallback(const char* pMsg) { printf("SpeedTree SDK Message: [%s]\n", pMsg); }

// error callback
void MyErrorCallback(const char* pError) { fprintf(stderr, "SpeedTree SDK Error: [%s]\n", pError); }

// heap allocation callback
void* MyHeapAllocCallback(size_t siSizeInBytes, size_t siAlignment) { return malloc(siSizeInBytes); }

// heap free callback
void MyHeapFreeCallback(void* pBlock) { if (pBlock) free(pBlock); }

// file size callback prototype
size_t MyFileSizeCallback(const char* pFilename);

// load binary file callback prototype (returns true on success)
bool MyLoadFileCallback(const char* pFilename, void* pBuffer);


// example use
void main(void)
{
    // assign example callbacks
    SpeedTree::Callbacks::Message() = MyMessageCallback;
    SpeedTree::Callbacks::Error() = MyErrorCallback;
    SpeedTree::Callbacks::HeapAlloc() = MyHeapAllocCallback;
    SpeedTree::Callbacks::HeapFree() = MyHeapFreeCallback;
    SpeedTree::Callbacks::GetFileSize() = MyFileSizeCallback;
    SpeedTree::Callbacks::LoadBinaryFile() = MyLoadFileCallback;
                    
    // to test one, we can manually invoke an error callback with
    SpeedTree::CCore::Error("an error... we've just experienced an error");
}
</code>