From b301a7193a2a53c57d70a4b20ee9cf9d0ebd4e80 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=A4=8F=E6=A5=9A?= <771730766@qq.com>
Date: Wed, 8 Jan 2020 09:59:07 +0800
Subject: [PATCH] =?UTF-8?q?Created=20=E4=BB=A3=E7=A0=81=E7=AF=87=E4=B9=8Bo?=
 =?UTF-8?q?nceToken=20(markdown)?=
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+ZLMediaKit里面大量用到了一个名叫`onceToken`对象, 很多小伙伴对这个工具类不明就里，下面我在此解释下其作用：
+- 1、作为全局变量用，在程序加载时执行特定代码，例如生成默认配置文件：
+```C++
+////////////HLS相关配置///////////
+namespace Hls {
+#define HLS_FIELD "hls."
+//HLS切片时长,单位秒
+const string kSegmentDuration = HLS_FIELD"segDur";
+//HLS切片个数
+const string kSegmentNum = HLS_FIELD"segNum";
+//HLS切片从m3u8文件中移除后，继续保留在磁盘上的个数
+const string kSegmentRetain = HLS_FIELD"segRetain";
+//HLS文件写缓存大小
+const string kFileBufSize = HLS_FIELD"fileBufSize";
+//录制文件路径
+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、作为static变量，确保代码只执行一次：
+```C++
+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);
+
+	//后续代码省略
+}
+```
+
+- 3、作为局部变量，确保函数退出前做一些清理工作，例如释放锁：
+```C++
+    template<typename ...ArgsType>
+    bool emitEvent(const string &strEvent,ArgsType &&...args){
+		onceToken token([&] {
+			//上锁，记录锁定线程id
+			_mtxListener.lock();
+			if(_lock_depth++ == 0){
+				_lock_thread = this_thread::get_id();
+			}
+		}, [&]() {
+			//释放锁，取消锁定线程id
+			if(--_lock_depth == 0){
+				_lock_thread = thread::id();
+				if(_map_moved){
+					//还原_mapListener
+					_map_moved = false;
+					_mapListener = std::move(_mapListenerTemp);
+				}
+			}
+			_mtxListener.unlock();
+		});
+
+		//后续代码省略
+    }
+
+```
+
+- 4、这个对象取名源自pthread_once以及ios下的dispatch_once。