onceToken

Within ZLMediaKit, there is extensive use of an object called onceToken. Many developers are unfamiliar with this utility class, so I will explain its purpose below: The onceToken primarily applies the RAII (Resource Acquisition Is Initialization) concept from C/C++, ensuring the execution of custom code during variable construction and destruction. It is mainly used in the following scenarios:

1. Used as a global variable to execute specific code during program loading, such as generating default configuration files:

   ////////////HLS-related configurations///////////
   namespace Hls {
   #define HLS_FIELD "hls."
   //HLS segment duration in seconds
   const string kSegmentDuration = HLS_FIELD"segDur";
   //Number of HLS segments
   const string kSegmentNum = HLS_FIELD"segNum";
   //Number of HLS segments retained on disk after removal from m3u8 file
   const string kSegmentRetain = HLS_FIELD"segRetain";
   //HLS file write buffer size
   const string kFileBufSize = HLS_FIELD"fileBufSize";
   //Recording file path
   const string kFilePath = HLS_FIELD"filePath";
   
   onceToken token([](){
       mINI::Instance()[kSegmentDuration] = 2;
       mINI::Instance()[kSegmentNum] = 3;
       mINI::Instance()[kSegmentRetain] = 5;
       mINI::Instance()[kFileBufSize] = 64 * 1024;
       mINI::Instance()[kFilePath] = "./www";
   },nullptr);
   } //namespace Hls
   

2. Used as a static variable to ensure code execution only once:

   int64_t HttpSession::onRecvHeader(const char *header,uint64_t len) {
       typedef void (HttpSession::*HttpCMDHandle)(int64_t &);
       static unordered_map<string, HttpCMDHandle> s_func_map;
       static onceToken token([]() {
           s_func_map.emplace("GET",&HttpSession::Handle_Req_GET);
           s_func_map.emplace("POST",&HttpSession::Handle_Req_POST);
       }, nullptr);
   
       //Omitted subsequent code
   }
   

3. Used as a local variable to ensure some cleanup work before a function exits, such as releasing a lock:

       template<typename ...ArgsType>
       bool emitEvent(const string &strEvent,ArgsType &&...args){
           onceToken token([&] {
               //Lock and record the locked thread ID
               _mtxListener.lock();
               if(_lock_depth++ == 0){
                   _lock_thread = this_thread::get_id();
               }
           }, [&]() {
               //Release the lock and clear the locked thread ID
               if(--_lock_depth == 0){
                   _lock_thread = thread::id();
                   if(_map_moved){
                       //Restore _mapListener
                       _map_moved = false;
                       _mapListener = std::move(_mapListenerTemp);
                   }
               }
               _mtxListener.unlock();
           });
   
           //Omitted subsequent code
       }
   

4. The name of this object is derived from pthread_once and dispatch_once in iOS.