本节体验下多线程。
python示例
在src/demo_python_pkg/demo_python_pkg/下新建文件，learn_thread.py
import threading
import requestsclass Download:def download(self,url,callback):print(f'线程：{threading.get_ident()} 开始下载：{url}')reponse = requests.get(url)reponse.encoding = 'utf-8'callback(url,reponse.text)def start_download(self,url,callback):thread = threading.Thread(target=self.download,args=(url,callback))thread.start() def world_cout(url,result):print(f"{url}:{len(result)}->{result[:30]}")def main():download = Download()download.start_download('https://fishros.com/d2lros2/#/humble/chapt1/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB',world_cout) download.start_download('https://fishros.com/d2lros2/#/humble/chapt2/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB',world_cout) download.start_download('https://fishros.com/d2lros2/#/humble/chapt3/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB',world_cout) python的线程库是threading，http请求库是requests.
Download 类定义了两个方法download、start_download。其中download 是真正的下载，start_download启动thread来运行目标函数download。
回调函数world_cout，用于处理下载完成的数据。main函数是入口，实例化一个Download类型的对象download,分别去下载，url 测试下。书上例子是自己造的TXT。
在setup.py添加learn_thread节点，编译后运行。
bohu@bohu-TM1701:~/2/2_ws$ colcon build
Starting >>> demo_cpp_pkg
Finished <<< demo_cpp_pkg [0.16s]
Starting >>> demo_python_pkg
Finished <<< demo_python_pkg [1.24s] Summary: 2 packages finished [1.75s]
bohu@bohu-TM1701:~/2/2_ws$ ros2 run demo_python_pkg learn_thread
线程：126326222620224 开始下载：https://fishros.com/d2lros2/#/humble/chapt1/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB
线程：126326212134464 开始下载：https://fishros.com/d2lros2/#/humble/chapt2/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB
线程：126326126151232 开始下载：https://fishros.com/d2lros2/#/humble/chapt3/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB
https://fishros.com/d2lros2/#/humble/chapt1/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB:5794-><!DOCTYPE html>
<html lang="en
https://fishros.com/d2lros2/#/humble/chapt3/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB:5794-><!DOCTYPE html>
<html lang="en
https://fishros.com/d2lros2/#/humble/chapt2/%E7%AB%A0%E8%8A%82%E5%AF%BC%E8%AF%BB:5794-><!DOCTYPE html>
<html lang="enC++示例
先下载依赖库
bohu@bohu-TM1701:~/2/2_ws/src/demo_cpp_pkg/include$ git clone https://gitee.com/ohhuo/cpp-httplib.git
正克隆到 'cpp-httplib'...
remote: Enumerating objects: 4527, done.
remote: Total 4527 (delta 0), reused 0 (delta 0), pack-reused 4527
接收对象中: 100% (4527/4527), 2.27 MiB | 2.55 MiB/s, 完成.
处理 delta 中: 100% (3057/3057), 完成.下载完成后，还要在CMakeLists.txt 添加目录
include_directories(include) #包含include头文件目录
在2_ws/src/demo_cpp_pkg/src下新建learn_thread.cpp文件。
#include <iostream>
#include <thread>
#include <chrono> //时间相关
#include <functional>
#include <cpp-httplib/httplib.h>
using namespace std;class Download
{public:void download(const string &host,const string &path,const function<void(const string &,const string &)> &callback){cout<<" 线程ID： "<< this_thread::get_id() << endl;httplib::Client client(host);auto response = client.Get(path);if(response && response->status==200){callback(path,response->body);} };void start_download(const string &host,const string &path,const function<void(const string &,const string &)> &callback){auto download_fun = bind(&Download::download,this,placeholders::_1,placeholders::_2,placeholders::_3);thread thread(download_fun,host,path,callback);thread.detach(); }; };int main()
{auto d= Download();auto word_count = [](const string &path,const string &result) -> void{cout << "下载完成" << path <<""<<result.length()<<"->"<<result.substr(0,100)<< endl;};d.start_download("http://0.0.0.0:8000","/novel1.txt",word_count);d.start_download("http://0.0.0.0:8000","/novel2.txt",word_count);d.start_download("http://0.0.0.0:8000","/novel3.txt",word_count);this_thread::sleep_for(chrono::seconds(10));return 0;
}开头还是引用头文件。然后声明了Download类，添加了download函数和start_download函数。
download函数使用了httplib下载，start_download函数里面thread.detach(); 将线程与当前进程分离，使得线程可以后台运行。
main函数是入口，通过lambda创建了回调函数。并三次调用start_download 下载。最后延迟了10秒，防止程序直接退出。
在CMakeLists.txt 添加节点，编译运行。运行结果：
bohu@bohu-TM1701:~/2/2_ws$ ros2 run demo_cpp_pkg learn_thread
线程ID： 138510887552576
线程ID： 138510877066816
线程ID： 138510747043392
下载完成/novel2.txt85-> BH8VYW，你好，这里是BH8ZZZ，你的信号是59，能否抄收我的信号？
下载完成/novel3.txt85-> BH8ZZZ，你好，这里是BH8VYW，你的信号是59，能够抄收你的信号。
下载完成/novel1.txt70->CQ，CQ，CQ，这里是BH8VYW，这里是BH8VYW，收到请回答。
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