ImplementingGameplayElementsUsingGamebryo
Chapter3:ScriptingSystemIntegration3
in a Python script, the variable name “X” and its value are stored in a dictionary as a key‐value
pair.Dictionariesalsostorefunctionandclassdefinitions.Ourscriptingsystemobjectcreatesits
dictionary after Python is
initialized. The scripting system will use the dictionary to store
variablesandsomeofthePythoncodethatissharedbetweenC++andPython.
InPythonScriptSystem::Init,youwillseeacalltoPy_Initialize
,afunctionprovided
by the Python API that initializes the Python runtime. Init also creates the dictionary object,
thencallsanotherPythonAPIfunction,PyDict_SetItemString,tosetthevalueofaPython
variable named “__builtins__” on the new dictionary. Once set, this variable references
another Python dictionary, owned
by the Python runtime’s “__builtin__” module, which
containsmanyofthecoredatastructures,classes,types,andfunctionsthatmakeupthePython
language. The reference to the built‐in dictionary is necessary, allowin g Python code that we
execute using our custom dictionary to use basic Python language features. Finally,
PythonScriptSystem::Init calls a secondary function to run a startup script from the file
assets/scripts/startup.py.ThisstartupscriptwillbecomeimportantinPart2.
Next,takealookatPythonScriptSystem::Terminate.Thisfunctiondoesthereverseofthe
initfunction.However,thecall to the Py_XDECREFfunctionmayrequiresomeexplaining. This
function decrements the reference count of the passed‐in object (in this case a pointer to our
dictionary). Python, like Gamebryo, uses reference coun ting to implem ent a form of garbage
collection.Thesurprisingthinghereisthatwehaveareferencecountdecrement,butthereisno
increment on the
dictionary object in the script system class. We did not need the increment
because the function PyDict_New (called in Init to create the dictionary) happens to return
whatPythoncallsanewreference,areferencethathasalreadybeenincrementedforus.Theonly
waytoknowthisisto
readthePythonAPIdocumentation.
Another place in the code where a new reference is returned is in
PythonScriptSystem::RunScriptFile. In this case, we call Py_DECREF, not
Py_XDECREF. The difference between these two functions is that Py_XDECREF checks the
incomingpointertoensureitisnotNULLbeforeproceedingto
reducethereferencecount.Inthe
RunScriptFile member function, we explicitly test the pointer, so there is no need for the
separatetestofPy_XDECREF.
SomePythonAPIfunctionsreturnanobjectreferencewithoutincrementingthereference count.
Python calls these borrowed references. You must not decrement the reference count
of a
borrowed reference, since this may cause the object to be destroyed prematurely, while the
Python runtime expects it to remain valid. If you need to use a Python object with a borrowed
referencebeyondthescopeinwhichyouretrievedthereference,simplycallPy_XINCREFtoturn
it
intoanewreference.
InitializingandTerminatingPython
Our basic scriptingsystem simply provides a singleton object which contains member functions
forinitialization,termination,andrunningsimplePythonscriptsfromafile.ToaccessPython,all
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d3dxvector32014-01-02是有点简单 不过还是谢谢了!
