ICall_fetchServiceMsg函数在什么情况下能取到消息

static void SimpleBLEPeripheral_taskFxn(UArg a0, UArg a1)
{
// 初始化
SimpleBLEPeripheral_init();

// 应用主循环
for (;;)
{
// 等待信号量
// 一个消息message入队到消息接收队列或者ICall_signal被调用
ICall_Errno errno = ICall_wait(ICALL_TIMEOUT_FOREVER);
//等待成功
if (errno == ICALL_ERRNO_SUCCESS)
{
ICall_EntityID dest;
ICall_ServiceEnum src;
ICall_HciExtEvt *pMsg = NULL;

if (ICall_fetchServiceMsg(&src, &dest,
(void **)&pMsg) == ICALL_ERRNO_SUCCESS)
{
uint8 safeToDealloc = TRUE;

if ((src == ICALL_SERVICE_CLASS_BLE) && (dest == selfEntity))
{
ICall_Event *pEvt = (ICall_Event *)pMsg;

// 查看是否是BLE Stack 事件
if (pEvt->signature == 0xffff)
{
if (pEvt->event_flag & SBP_CONN_EVT_END_EVT)
{
// 重新发送ATT响应
SimpleBLEPeripheral_sendAttRsp();
}
}
else
{
// 处理 inter-task 消息
safeToDealloc = SimpleBLEPeripheral_processStackMsg((ICall_Hdr *)pMsg);
}
}
//销毁
if (pMsg && safeToDealloc)
{
ICall_freeMsg(pMsg);
}
}

// 如果RTOS队列不是空的（有任务），处理APP消息
while (!Queue_empty(appMsgQueue))
{
sbpEvt_t *pMsg = (sbpEvt_t *)Util_dequeueMsg(appMsgQueue);
if (pMsg)
{
// 处理消息
SimpleBLEPeripheral_processAppMsg(pMsg);

// 销毁 消息
ICall_free(pMsg);
}
}
}

if (events & SBP_PERIODIC_EVT)
{
events &= ~SBP_PERIODIC_EVT;

Util_startClock(&periodicClock);

// 运行 periodic 应用任务
SimpleBLEPeripheral_performPeriodicTask();
}

... ...

}
}