Abstract
前人已有A Simple C++ Plugin System Summary，本文将处理，如何实现插件与主程序之间的接口
Introduction
支持任意的命令：插件的基本类往往只提供少数接口，因此有必要将一个接口转换为多个接口。下面的字符串接口即实现了这一功能。
数据共享，包括以下
1. 主程序中访问，修改插件参数，参见下面的指针类接口。
2. 插件访问，修改主程序参数，需要在插件的初始化函数中提供主程序引用参数，实测可行。
3. 插件A访问，修改插件B，插件C……中的参数（对插件A的开发者要求很高，插件B，插件C要提供接口供插件A调用，如插件A直接调用插件B，C，但这有时不太现实，如果插件B，C只能有一个实例。这样的话，要求主程序提供一套插件回调机制，插件A通过主程序回调其它插件）。
Related Work
字符串接口
The method we’ve been using recently is actually quite simple. The plugin implements a onCommand method which accepts arbitrary commands from the application. The advantage of using this type of interface is that you’re able to implement almost any kind of functionality without having to add new methods and break the API each time you roll out a new feature. Obviously it doesn’t have it be this simple, but you get the idea!
Command nodes (see code below) are namespaced using a REST style interface like so resource:action, and the data buffer contains either a JSON encoded message which for representing and converting to an STL container such a std::vector or std::map to pass to the internal API, or it may just be a raw data buffer that can be used directly.
bool onCommand(const char* node, const char* data, unsigned int size)
{
try {
// Handle a JSON encoded options hash
if (strcmp(node, "options:set") == 0) {
json::Value root;
json::Reader reader;
if (!reader.parse(data, size, root))
throw std::runtime_error("Invalid JSON format: " + reader.getFormatedErrorMessages());
// Do something with JSON data here...
}
// Handle raw file data
else if (strcmp(node, "file:write") == 0) {
std::string path("test.bin");
std::ofstream ofs(path, std::ios::out|std::ios::binary);
if (!ofs.is_open())
throw std::runtime_error("Cannot write to output file: " + path);
ofs.write(data, size);
ofs.close();
}
// Handle unknown commands
else throw std::runtime_error("Unknown command");
}
catch (std::exception& exc) {
// Catch exceptions here and return false.
// You could set a lastError string here which is exposed to
// the application that returns the error message as a char*.
// See the full example for details.
std::cerr << "Command error: " << exc.what() << std::endl;
return false;
}
return true;
}指针类接口
通过指针返回符号（函数或者成员变量），注意typedef void* gpointer; 这样，通过void * 可以返回任意数据类型，无论是函数还是成员变量。
/* the function signature for 'say_hello' */
typedef void (* SayHelloFunc) (const char *message);
gboolean
just_say_hello (const char *filename, GError **error)
{
SayHelloFunc say_hello;
GModule *module;
module = g_module_open (filename, G_MODULE_BIND_LAZY);
if (!module)
{
g_set_error (error, FOO_ERROR, FOO_ERROR_BLAH,
"%s", g_module_error ());
return FALSE;
}
if (!g_module_symbol (module, "say_hello", (gpointer *)&say_hello))
{
g_set_error (error, SAY_ERROR, SAY_ERROR_OPEN,
"%s: %s", filename, g_module_error ());
if (!g_module_close (module))
g_warning ("%s: %s", filename, g_module_error ());
return FALSE;
}
if (say_hello == NULL)
{
g_set_error (error, SAY_ERROR, SAY_ERROR_OPEN,
"symbol say_hello is NULL");
if (!g_module_close (module))
g_warning ("%s: %s", filename, g_module_error ());
return FALSE;
}
/* call our function in the module */
say_hello ("Hello world!");
if (!g_module_close (module))
g_warning ("%s: %s", filename, g_module_error ());
return TRUE;
}[快捷键接口]
类似Atom，在插件中加入信号处理，这样可以通过快捷键调用插件中的函数。
[流水线接口]
所有插件仅处理一种类型的数据，如矩阵。参见orange 实现机制 不过这是python，但原则上c++也可以这么做。但势必会麻烦一些，毕竟python是脚本语言。
[文件接口]
一个很直接的想法，将数据输出到系统文件中，然后让别的程序处理，最后同步即可。
Implement
实现插件访问，修改主程序中的数据DataDriveMain，使其成员变量frameNum增加1.
test4_main.cpp
#include "DataDriveFunctions.h"
#include "DataDriveMain.h"
#include "yzbx_config.h"
#include <dlfcn.h>
using namespace DataDrive ;
typedef Base* create_t(std::shared_ptr<DataDriveMain> data);
typedef void destroy_t(Base*);
int main(){
using std::cout;
using std::cerr;
// load the dll library
void* dll = dlopen("./test4.so", RTLD_LAZY);
if (!dll) {
cerr << "Cannot load library: " << dlerror() << '\n';
return 1;
}
// reset errors
dlerror();
// load the symbols
create_t* create_dll = (create_t*) dlsym(dll, "create");
const char* dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol create: " << dlsym_error << '\n';
return 1;
}
destroy_t* destroy_dll = (destroy_t*) dlsym(dll, "destroy");
dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol destroy: " << dlsym_error << '\n';
return 1;
}
// create an instance of the class
std::shared_ptr<DataDriveMain> data=std::make_shared<DataDriveMain>(DataDriveMain("/home/yzbx/config/surveillance-video-system.ini"));
Base* base = create_dll(data);
// use the class
cout << "The frameNum is: " << data->frameNum << '\n';
base->run();
cout << "The frameNum is: " << data->frameNum << '\n';
// destroy the class
destroy_dll(base);
// unload the dll library
dlclose(dll);
}test4_dll.cpp
#include "DataDriveFunctions.h"
#include "DataDriveMain.h"
#include "yzbx_config.h"
using namespace DataDrive ;
class DllTest:public Base{
public:
GET_CLASS_NAME
DllTest(std::shared_ptr<DataDriveMain> data):Base(data){}
bool run();
};
bool DllTest::run(){
data->frameNum++;
return true;
};
extern "C" Base * create(std::shared_ptr<DataDriveMain> data){
return new DllTest(data);
};
extern "C" void destroy(Base * p){
delete p;
};
build.sh
g++ -std=c++14 -fPIC -I/home/yzbx/git/surveillance-video-system/objectTracking/src/objectTracking/yzbxLib `pkg-config --cflags Qt5Core` test4_dll.cpp -shared -o test4.so `pkg-config --libs Qt5Core` -L/home/yzbx/build/build-objectTracking-Desktop_Qt_5_6_0_GCC_64bit-Debug/yzbxLib -lyzbxLib
g++ -std=c++14 -fPIC -I/home/yzbx/git/surveillance-video-system/objectTracking/src/objectTracking/yzbxLib `pkg-config --cflags Qt5Core` test4_main.cpp -o test4 -L/home/yzbx/build/build-objectTracking-Desktop_Qt_5_6_0_GCC_64bit-Debug/yzbxLib -lyzbxLib `pkg-config --libs Qt5Core` `pkg-config --libs bgslibrary` `pkg-config --libs opencv` -lboost_filesystem -lboost_program_options -lboost_system -ldloutput
(env2) $: cd -
/home/yzbx/test/plugin-framework/dlopen
(env2) $: ls
a.out hello.so test1-libm-so test3_triangle.cpp triangle.so
bin Jamroot test2_hello.cpp test4
build.sh polygon.hpp test2_main.cpp test4_dll.cpp
config test1 test3_main test4_main.cpp
error.log test1.c test3_main.cpp test4.so
(env2) $: ./test4
The frameNum is: 0
The frameNum is: 1
(env2) $: