本篇是創建游戲內核（20）的續篇，有關DirectAudio和DirectShow的基礎知識請參閱用DirectX
Audio和DirectShow播放聲音和音樂。
為了增強MUSIC_CHANNEL類的音樂回放特性，可以使用DLS類。
來看看類DLS的定義:
//======================================================================================
// This class encapsulates for downloadable sound.
//======================================================================================
typedef class DLS
{
public:
DLS();
~DLS();
IDirectMusicCollection8* get_dm_colletion();
BOOL create(SOUND_PTR sound);
BOOL load(const char* filename = NULL);
BOOL free();
long get_num_patches();
long get_patch(long index);
BOOL exists(long patch);
private:
SOUND_PTR _sound;
IDirectMusicCollection* _dm_collection;
} *DLS_PTR;
以及類DLS的實現：
//------------------------------------------------------------------------------
// Constructor, zero member data.
//------------------------------------------------------------------------------
DLS::DLS()
{
memset(this, 0, sizeof(*this));
}
//------------------------------------------------------------------------------
// Destructor, release DirectMusic resource.
//------------------------------------------------------------------------------
DLS::~DLS()
{
free();
}
//------------------------------------------------------------------------------
// Release DirectMusic collection object.
//------------------------------------------------------------------------------
BOOL DLS::free()
{
if(_sound == NULL || _sound->get_dm_loader() == NULL)
return FALSE;
if(_dm_collection)
{
if(FAILED(_sound->get_dm_loader()->ReleaseObjectByUnknown(_dm_collection)))
return FALSE;
}
release_com(_dm_collection);
return TRUE;
}
//------------------------------------------------------------------------------
// Create DLS object.
//------------------------------------------------------------------------------
BOOL DLS::create(SOUND_PTR sound)
{
free();
_sound = sound;
if(_sound == NULL || _sound->get_dm_loader() == NULL)
return FALSE;
return TRUE;
}
//------------------------------------------------------------------------------
// Load DirectMusic collection object from specified filename.
//------------------------------------------------------------------------------
BOOL DLS::load(const char* filename)
{
free();
if(_sound == NULL || _sound->get_dm_loader() == NULL)
return FALSE;
DMUS_OBJECTDESC object_desc;
ZeroMemory(&object_desc, sizeof(DMUS_OBJECTDESC));
object_desc.dwSize = sizeof(DMUS_OBJECTDESC);
object_desc.guidClass = CLSID_DirectMusicCollection;
if(filename == NULL)
{
// get the default collection
object_desc.guidObject = GUID_DefaultGMCollection;
object_desc.dwValidData = DMUS_OBJ_CLASS | DMUS_OBJ_OBJECT;
}
else
{
// get the colletion object
object_desc.dwValidData = DMUS_OBJ_CLASS | DMUS_OBJ_FILENAME | DMUS_OBJ_FULLPATH;
mbstowcs(object_desc.wszName, filename, MAX_PATH);
}
HRESULT rv = _sound->get_dm_loader()->GetObject(&object_desc,
IID_IDirectMusicCollection8, (LPVOID*)&_dm_collection);
if(rv != S_OK)
{
if(rv == DMUS_E_LOADER_FAILEDCREATE)
err_msg_box("The object could not be found or created.");
else if(rv == DMUS_E_LOADER_FAILEDOPEN)
err_msg_box("File open failed because the file does not exist or is locked.");
else if(rv == DMUS_E_LOADER_FORMATNOTSUPPORTED)
err_msg_box("The object cannot be loaded because the data format is not supported.");
else if(rv == DMUS_E_LOADER_NOCLASSID)
err_msg_box("No class identifier was supplied in the object description.");
else if(rv == E_FAIL)
err_msg_box("The method did not succeed.");
else if(rv == E_INVALIDARG)
err_msg_box("Invalid argument. Often, this error results from failing to initialize the dwSize member"
" of a structure before passing it to the method.");
else if(rv == E_OUTOFMEMORY)
err_msg_box("Insufficient memory to complete the task.");
else if(rv == E_POINTER)
err_msg_box("An invalid pointer, usually NULL, was passed as a parameter.");
else if(rv == REGDB_E_CLASSNOTREG)
err_msg_box("The object class is not registered.");
return FALSE;
}
return TRUE;
}
//------------------------------------------------------------------------------
// Return DirectMusic collection object.
//------------------------------------------------------------------------------
IDirectMusicCollection8* DLS::get_dm_colletion()
{
return _dm_collection;
}
//------------------------------------------------------------------------------
// Return number of patches in DirectMusic collection.
//------------------------------------------------------------------------------
long DLS::get_num_patches()
{
DWORD patch;
long index = 0;
// retrieves the patch number and name of an instrument by its index in the collection
while(_dm_collection->EnumInstrument(index, &patch, NULL, 0))
++index;
return index;
}
//------------------------------------------------------------------------------
// Return patch number by index of the instrument.
//------------------------------------------------------------------------------
long DLS::get_patch(long index)
{
DWORD patch;
if(_dm_collection == NULL)
return -1;
if(FAILED(_dm_collection->EnumInstrument(index, &patch, NULL, 0)))
return -1;
return (long) patch;
}
//------------------------------------------------------------------------------
// Judge whether specified patch exists.
//------------------------------------------------------------------------------
BOOL DLS::exists(long patch)
{
IDirectMusicInstrument8* dm_instrument;
if(_dm_collection == NULL)
return FALSE;
// retrieve an instrument by its patch number
if(FAILED(_dm_collection->GetInstrument(patch, &dm_instrument)))
return FALSE;
dm_instrument->Release();
return TRUE;
}
DLS類的惟一用途是包含單個DLS集。像MUSIC_CHANNEL類一樣，也只能調用一次DLS::create函數，因為DLS::free函數只能釋放一個已加載的DLS集。注意在DLS::load函數中，filename參數的缺省值為NULL，此缺省值指定了默認的DLS集。
調用DLS::get_num_patches函數可以得到類中所包含的樂器的數目。使用DLS::get_patch函數可以遍歷每種樂器，得到它的音色（patch）數。使用DLS::exists函數可以檢查在DLS集中是否存在某種特定的音色，如果此函數返回值為TRUE，就表示存在這種音色；否則表示不存在這種音色。
該類的實現可能并不完整，暫不提供測試代碼。