【MSCE C++ UP9】请教一下，回调函数SetUnloadProgramFunction传入的函数参数意思

Question

下面的例子中，回调函数SetUnloadProgramFunction传入的函数参数有么这一段注释： 
 /*------------------------------------------------------------------ | If the value of unloadType is negative, then MicroStation ignores | the return value of this function. In this case, we can assume | that MicroStation is shutting down or was terminated so this | function should not attempt to abort the unload. +------------------------------------------------------------------*/ 
 老师能帮忙解释下是什么意思不？例子中如果myapp_unloadFunction的返回值为true的话，发现应用程序就无法卸载。 
 Public bool myapp_unloadFunction(UnloadProgramReason unloadType)
{
	/*------------------------------------------------------------------
	| If the value of unloadType is negative, then MicroStation ignores
	| the return value of this function. In this case, we can assume
	| that MicroStation is shutting down or was terminated so this
	| function should not attempt to abort the unload.
	+------------------------------------------------------------------*/
	if (unloadType < 0)
	{
 return true;
	}

	OUTPUT("Unload......");
	return false;
}

extern "C" DLLEXPORT void MdlMain(int, WCharCP[])
{
	SystemCallback::SetUnloadProgramFunction(myapp_unloadFunction);
}

Yongan.Fu · Accepted Answer

unloadType是系统传递进来的值，表明你的MDL应用程序被卸载的原因，定义如下： 
 enum UnloadProgramReason { SYSTEM_TERMINATED_UNLOADING_APPDOMAIN = -6, SYSTEM_TERMINATED_ENTERING_RESTRICTED_RIGHTS_MODE = -5, SYSTEM_TERMINATED_UNLOAD_UI_SERVERS = -4, SYSTEM_TERMINATED_LIBRARY_UNLOAD = -3, SYSTEM_TERMINATED_SHUTDOWN = -2, SYSTEM_TERMINATED_FATAL = -1, SYSTEM_TERMINATED_COMMAND = 1, SYSTEM_TERMINATED_EXIT = 2, SYSTEM_TERMINATED_BY_APP = 3, SYSTEM_TERMINATED_EXCEPTION = 4 }; 
 而该函数的返回值决定了是否要阻止当前MDL应用程序的卸载，正如你观察到的，返回true表示阻止卸载（只有当退出整个MicroStation时才会被卸载），返回false允许卸载。 
 有些重要的程序（比如自定义元素Handler等）是不能被卸载的，一旦卸载将会导致异常出现（系统不认识自定义元素了）。对于这些程序，要在其unload回调函数中返回true。

【MSCE C++ UP9】请教一下，回调函数SetUnloadProgramFunction传入的函数参数意思

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