Binding to globals

Upon creating a registry object, the server will emit the global event for each global available on the server. You can then bind to the globals you require.

在我们创建好注册表对象（registry object）后，server 端会即刻为每个可用的全局对象发送 global event（到 client），这样（client）就可以绑定到对应的全局对象上。

This process of taking a known object and assigning it an ID is called binding the object. Once the client binds to the registry like this, the server emits the global event several times to advertise which interfaces it supports. Each of these globals is assigned a unique name, as an unsigned integer. The interface string maps to the name of the interface found in the protocol: wl_display from the XML above is an example of such a name. The version number is also defined here — for more information about interface versioning, see appendix C.

获取已知对象并为其分配 ID 的过程称为绑定 (binding)对象。一旦 client 像这样绑定到 register 对象， server 就会多次发出 global 事件来表明它支持哪些接口。其中的每一个全局对象都被分配了一个唯一的无符号整数 name ， interface 字符串对应了协议中接口名称：之前的 XML 文件中 wl_display 就是一个例子。还有版本号（version）也是在这里定义的 — 有关接口版本控制的更多信息，请参见附录 C。

To bind to any of these interfaces, we use the bind request, which works similarly to the magical process by which we bound to the wl_registry. For example, consider this wire protocol exchange:

我们使用 ‘bind’ request 来完成接口的绑定，它的工作原理和我们绑定 wl_registry 是一致的。举个例子，观察以下网络协议交互内容：

C->S    00000001 000C0001 00000002            .... .... ....

S->C    00000002 001C0000 00000001 00000007   .... .... .... ....
        776C5f73 686d0000 00000001            wl_s hm.. ....
        [...]

C->S    00000002 00100000 00000001 00000003   .... .... .... ....

The first message is identical to the one we’ve already dissected. The second one is an event from the server: object 2 (which the client assigned the wl_registry to in the first message) opcode 0 (“global”), with arguments 1, “wl_shm”, and 1 — respectively the name, interface, and version of this global. The client responds by calling opcode 0 on object ID 2 (wl_registry::bind) and assigns object ID 3 to global name 1 — binding to the wl_shm global. Future events and requests for this object are defined by the wl_shm protocol, which you can find in wayland.xml.

第一条消息我们前面已经分析过了。第二条消息是来自 server 端返回的事件：对象 2（也就是在第一条消息中由 client 分配的 wl_registry 接口所绑定的对象）、[001C是消息长度]操作码 0（即 global 事件），接下来是对应的整形参数 1、”wl_shm” 字符串以及整数 1 — 它们分别对应全局对象的 name、interface 和版本号。接着，client 端通过调用 object ID 为 2 的对象上的执行操作码 0（即 wl_registry::bind 请求），并使用 global name 1 来代指 object ID 3 这个对象，即绑定（binding）到 wl_shm 这个全局对象上。此后该对象所有的事件和请求均由 wl_shm 协议定义，你可以在 wayland.xml 中找到相关内容。

Once you’ve created this object, you can utilize its interface to accomplish various tasks — in the case of wl_shm, managing shared memory between the client and server. Most of the remainder of this book is devoted to explaining the usage of each of these globals.

Armed with this information, we can write our first useful Wayland client: one which simply prints all of the globals available on the server.

一旦你成功创建了这个对象，你就可以利用它的接口来完成各种任务 — 例如在 wl_shm 这个案例中，你可以用它来管理 client 和 server 端之间的共享内存。其实本书绝大部分内容都是在讲解这些全局对象的用法。

Armed with this information, we can write our first useful Wayland client: one which simply prints all of the globals available on the server.

有了这些信息，我们就可以写出第一个实用的 Wayland client 应用程序了：一个将 server 端所有可用的 global 都打印出来的应用程序。

#include <stdint.h>
#include <stdio.h>
#include <wayland-client.h>

static void
registry_handle_global(void *data, struct wl_registry *registry,
		uint32_t name, const char *interface, uint32_t version)
{
	printf("interface: '%s', version: %d, name: %d\n",
			interface, version, name);
}

static void
registry_handle_global_remove(void *data, struct wl_registry *registry,
		uint32_t name)
{
	// This space deliberately left blank
}

static const struct wl_registry_listener
registry_listener = {
	.global = registry_handle_global,
	.global_remove = registry_handle_global_remove,
};

int
main(int argc, char *argv[])
{
	struct wl_display *display = wl_display_connect(NULL);
	struct wl_registry *registry = wl_display_get_registry(display);
	wl_registry_add_listener(registry, &registry_listener, NULL);
	wl_display_roundtrip(display);
	return 0;
}

Feel free to reference previous chapters to interpret this program. We connect to the display (chapter 4.1), obtain the registry (this chapter), add a listener to it (chapter 3.4), then round-trip, handling the global event by printing the globals available on this compositor. Try it for yourself:

请基于前面章节所学的内容来尽量理解这段代码。首先我们建立起与 display 之间的连接通道（第 4.1 章节的内容），然后获取注册表对象（本章的内容），并为其添加一个监听器（第 3.4 章节的内容），最后使用 round-trip 来处理 global event，即打印出该 compositor 上所有可用的 global 对象。你可以自己试试这个例子：

$ cc -o globals -lwayland-client globals.c

Note: this chapter the last time we’re going to show wire protocol dumps in hexadecimal, and probably the last time you’ll ever see them in general. A better way to trace your Wayland client or server is to set the WAYLAND_DEBUG variable in your environment to 1 before running your program. Try it now with the “globals” program!

注意：这一章是我们最后一次展示十六进制的有线协议 dump，可能也是你最后一次看到它们的情况。跟踪 Wayland client 或 server 的一个更好的方法是在运行程序之前将环境中的 WAYLAND_DEBUG 变量设置为 1。现在试试 “globals” 程序吧!

原文链接:https://wayland-book.com/