X11 剪切板

参考:

首先在 X11 上剪切板被称作 Selections，系统上可以有任意多个 Selections，并且有三个预定义的标准 Selections:

primary - 当前选中的文本，例如终端上点击鼠标中键可以立即粘贴选中的文本
secondary - 没有被使用
clipboard - 通常意义上的剪切板，不同进程间交换数据时使用它

只有 Selection 的拥有者（owner）可以修改它，其它进程只能读取。owner 需要负责保存 Selection 的内容，并在其它进程请求读取的时候返回给对方。

当用户使用 Ctrl + C 进行复制时，当前应用就会申请成为 clipboard 的拥有者，之前的拥有者需要让出。

如果应用程序只希望在内部复制粘贴数据，不希望与其它进程交换信息，可以创建一个任意命名的属于自己的剪切板。

以下代码可以查看 clipboard 的当前 owner:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <xcb/xcb.h>

int main(void)
{
    // 建立连接
    xcb_connection_t* conn = xcb_connect(NULL, NULL);

    // 获取 screen
    const xcb_setup_t* setup = xcb_get_setup(conn);
    xcb_screen_iterator_t iter = xcb_setup_roots_iterator(setup);
    xcb_screen_t* screen = iter.data;

    // 获取 root 窗口
    xcb_window_t root = screen->root;

    // 创建 ATOM
    xcb_atom_t nameAtom;
    {
        const char* name = "PRIMARY";
        xcb_intern_atom_cookie_t cookie = xcb_intern_atom(conn, 0, strlen(name), name);
        xcb_intern_atom_reply_t* reply = xcb_intern_atom_reply(conn, cookie, NULL);
        nameAtom = reply->atom;
        free(reply);
    }
    
    // 读取剪切板的 owner
    xcb_window_t owner;
    {
        xcb_get_selection_owner_cookie_t cookie =  xcb_get_selection_owner(conn, nameAtom);
        xcb_get_selection_owner_reply_t* reply = xcb_get_selection_owner_reply(conn, cookie, NULL);
        owner = reply->owner;
        free(reply);
    }

    // 读取窗口名称
    #define BUFFER_SIZE 64
    char name[BUFFER_SIZE] = {0};
    {
        xcb_get_property_cookie_t cookie = xcb_get_property(conn, 0, owner, XCB_ATOM_WM_NAME, XCB_ATOM_STRING, 0, BUFFER_SIZE/4);
        xcb_get_property_reply_t* reply = xcb_get_property_reply(conn, cookie, NULL);
        int len = xcb_get_property_value_length(reply);
        len = len > 64 ? 64 : len;
        strncpy(name, xcb_get_property_value(reply), len);
        free(reply);
    }
    #undef BUFFER_SIZE

    // 打印
    printf("Owner ID: 0x%x\n", owner);
    printf("Owner Name: %s\n", name);
}

以下代码可以将自己设为 clipboard 的 owner，并将所有的粘贴内容替换为当前的时间字符串:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <xcb/xcb.h>
#include <xcb/xfixes.h>

int main(void)
{
    // 建立连接
    xcb_connection_t* conn = xcb_connect(NULL, NULL);

    // 获取 screen
    const xcb_setup_t* setup = xcb_get_setup(conn);
    xcb_screen_iterator_t iter = xcb_setup_roots_iterator(setup);
    xcb_screen_t* screen = iter.data;

    // 获取 root 窗口
    xcb_window_t root = screen->root;

    // 创建一个窗口
    xcb_window_t window = xcb_generate_id(conn);
    {
        uint32_t mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK;
        uint32_t values[2];
        values[0] = screen->white_pixel;
        values[1] = XCB_EVENT_MASK_KEY_PRESS | XCB_EVENT_MASK_KEY_RELEASE;
        xcb_create_window(conn, XCB_COPY_FROM_PARENT, window, root, 0, 0, 300, 200, 50, XCB_WINDOW_CLASS_INPUT_OUTPUT, screen->root_visual, mask, values);
        xcb_map_window(conn, window);
    }

    // 创建 ATOM
    xcb_atom_t nameAtom;
    {
        const char* name = "CLIPBOARD";
        xcb_intern_atom_cookie_t cookie = xcb_intern_atom(conn, 0, strlen(name), name);
        xcb_intern_atom_reply_t* reply = xcb_intern_atom_reply(conn, cookie, NULL);
        nameAtom = reply->atom;
        free(reply);
    }

    // 创建 ATOM
    xcb_atom_t utf8StrAtom;
    {
        const char* name = "UTF8_STRING";
        xcb_intern_atom_cookie_t cookie = xcb_intern_atom(conn, 0, strlen(name), name);
        xcb_intern_atom_reply_t* reply = xcb_intern_atom_reply(conn, cookie, NULL);
        utf8StrAtom = reply->atom;
        free(reply);
    }
    
    // 创建一个 selection 并将自己设为的 owner
    xcb_set_selection_owner(conn, window, nameAtom, XCB_TIME_CURRENT_TIME);

    // 检查是否成功
    xcb_window_t owner;
    {
        xcb_get_selection_owner_cookie_t cookie = xcb_get_selection_owner(conn, nameAtom);
        xcb_get_selection_owner_reply_t* reply = xcb_get_selection_owner_reply(conn, cookie, NULL);
        owner = reply->owner;
    }

    if (owner != window)
    {
        printf("failed to set selection owner, owner id is 0x%x\n", owner);
        return 1;
    }

    // 读取事件
    xcb_generic_event_t* event;
    while ((event = xcb_wait_for_event(conn)))
    {
        printf("event type: %d\n", event->response_type);
        // 其它进程请求读取这个 selection
        if(event->response_type == XCB_SELECTION_REQUEST)
        {
            xcb_selection_request_event_t* request = (xcb_selection_request_event_t*)event;

            xcb_selection_notify_event_t response; // 通知对端的事件
            response.response_type =XCB_SELECTION_NOTIFY;
            response.requestor = request->requestor;
            response.selection = request->selection;
            response.target = request->target;
            response.property = request->property;
            response.time = request->time;

            // 只处理 UTF8_STRING
            if (request->property == XCB_ATOM_NONE || request->target != utf8StrAtom)
            {
                response.property = XCB_ATOM_NONE;
            }
            else
            {
                // 返回当前时间
                time_t nowTime = time(NULL);
                char* nowStr = ctime(&nowTime);
                xcb_change_property(conn, XCB_PROP_MODE_REPLACE, request->requestor, request->property, request->target, 8, strlen(nowStr), nowStr);
            }
            
            xcb_send_event(conn, 1, request->requestor, 0, (char*)&response);
            xcb_flush(conn);

            printf("done\n");
        }
    }

    xcb_disconnect(conn);
    return 0;
}

当一个应用进行复制时，实际上是将自己设为了 clipboard 的 owner，并在其它应用请求粘贴时返回数据。为了避免复制的数据丢失，成为 owner 时要将之前的数据都保存过来。

如果 owner 退出，selection 会丢失。为了避免这一情况，需要一个 clipboard manager 进程作为 clipboard 的 owner。当其它进程进行复制操作成为 owner 时，clipboard manager应当进行以下操作:

向新的 owner 询问所有数据，并将这些数据保存起来
重新将自己声明为 owner