我们westonsimple_shm案例为例

1. 连接server：

wl_display_connect 接口通过socket建立与Server端的连接返回wl_display，display->registry = wl_display_get_registry(display->display) // 申请创建registry，得到代理对象wl_registry。这个个对象相当于Client在Server端放的一个用于嗅探资源的Observer。Client通过它得到Server端有哪些Global对象的信息。Server端有一系列的Global对象，如wl_compositor, wl_shm等

wl_registry_add_listener(display->registry, &registry_listener,...) // 让Client监听刚才创建的wl_registry代理对象。这样， 当Client调用wl_display_get_registry()函数或者有新的Global对象加入到Server端时，Client就会收到event通知。

wl_display_roundtrip() // 等待前面的请求全被Server端处理完，它同步了Client和Server端。这意味着到这个函数返回时，Server端有几个Global对象，回调处理函数registry_handle_global()应该就已经被调用过几次了。 registry_handle_global()中会判断是当前这次event代表何种Global对象，然后调用wl_registry_bind()进行绑定，得到远程服务对象的本地代理对象。这些代理对象类型可以是wl_shm, wl_compositor等，但本质上都是wl_proxy类型

static void
registry_handle_global(void *data, struct wl_registry *registry,
		       uint32_t id, const char *interface, uint32_t version)
{
	struct display *d = data;

	if (strcmp(interface, "wl_compositor") == 0) {
		d->compositor =
			wl_registry_bind(registry,
					 id, &wl_compositor_interface, 1);
	} else if (strcmp(interface, "xdg_wm_base") == 0) {
		d->wm_base = wl_registry_bind(registry,
					      id, &xdg_wm_base_interface, 1);
		xdg_wm_base_add_listener(d->wm_base, &xdg_wm_base_listener, d);
	} else if (strcmp(interface, "zwp_fullscreen_shell_v1") == 0) {
		d->fshell = wl_registry_bind(registry,
					     id, &zwp_fullscreen_shell_v1_interface, 1);
	} else if (strcmp(interface, "wl_shm") == 0) {
		d->shm = wl_registry_bind(registry,
					  id, &wl_shm_interface, 1);
		wl_shm_add_listener(d->shm, &shm_listener, d);
	}
}

static void
registry_handle_global_remove(void *data, struct wl_registry *registry,
			      uint32_t name)
{
}

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

static struct display *
create_display(void)
{
	struct display *display;

	display = malloc(sizeof *display);
	if (display == NULL) {
		fprintf(stderr, "out of memory\n");
		exit(1);
	}
	display->display = wl_display_connect(NULL);
	assert(display->display);

	display->has_xrgb = false;
	display->registry = wl_display_get_registry(display->display);
	wl_registry_add_listener(display->registry,
				 &registry_listener, display);
	wl_display_roundtrip(display->display);
	if (display->shm == NULL) {
		fprintf(stderr, "No wl_shm global\n");
		exit(1);
	}

	wl_display_roundtrip(display->display);
	if (!display->has_xrgb) {
		fprintf(stderr, "WL_SHM_FORMAT_XRGB32 not available\n");
		exit(1);
	}

	return display;
}

2. 创建窗口：

window->surface = wl_compositor_create_surface() // 通过刚才绑定的wl_compositor服务创建Server端的weston_surface，返回代理对象 wl_surface。

xdg_shell_get_xdg_surface(..., window->surface, ...) // 通过刚才绑定的xdg_shell服务创建Server端的shell_surface，返回代理对象 xdg_surface。有些例子中用的是wl_shell_surface，它和xdg_surface的作用是一样的。xdg_surface是作为wl_shell_surface将来的替代品，但还没进Wayland核心协议。

为什么一个窗口要创建两个surface呢？因为Wayland协议假设Server端对Surface的管理分两个层次。以Weston为例，Compositor只负责合成（代码主要在compositor.c），它相当于Android中的SurfaceFligner，它所看到的主要是weston_surface。而Weston在启动时会加载shell模块（如desktop-shell.so，代码主要在desktop-shell/shell.c），它相当于Android中的WindowManagerService，它所看到的主要是shell_surface。shell_surface在结构上是weston_surface的进一步封装，为了做窗口管理。这样，合成渲染和窗口管理的模块既可以方便地相互访问又保证了较低的耦合度

static struct window *
create_window(struct display *display, int width, int height)
{
	struct window *window;

	window = zalloc(sizeof *window);
	if (!window)
		return NULL;

	window->callback = NULL;
	window->display = display;
	window->width = width;
	window->height = height;
	window->surface = wl_compositor_create_surface(display->compositor);

	if (display->wm_base) {
		window->xdg_surface =
			xdg_wm_base_get_xdg_surface(display->wm_base,
						    window->surface);
		assert(window->xdg_surface);
		xdg_surface_add_listener(window->xdg_surface,
					 &xdg_surface_listener, window);

		window->xdg_toplevel =
			xdg_surface_get_toplevel(window->xdg_surface);
		assert(window->xdg_toplevel);
		xdg_toplevel_add_listener(window->xdg_toplevel,
					  &xdg_toplevel_listener, window);

		xdg_toplevel_set_title(window->xdg_toplevel, "simple-shm");
		wl_surface_commit(window->surface);
		window->wait_for_configure = true;
	} else if (display->fshell) {
		zwp_fullscreen_shell_v1_present_surface(display->fshell,
							window->surface,
							ZWP_FULLSCREEN_SHELL_V1_PRESENT_METHOD_DEFAULT,
							NULL);
	} else {
		assert(0);
	}

	return window;

3. 分配buffer与绘制

wl_surface_damage() // 告诉Compositor该surface哪块区域是脏的，需要重绘。一开始是整个窗口区域。

redraw() //  接下来调用redraw()开始绘制的循环，这里是双buffer的软件渲染。

    window_next_buffer() // 取一个buffer，用作绘制。

        create_shm_buffer() // 如果该buffer尚未分配则用之前绑定的wl_shm服务分配一块共享内存。

            fd = os_create_anonymous_file() // 为了创建共享内存，先创建一个临时文件作为内存映射的backing file。

            mmap(..., fd,...) // 将该文件先映射到Client端的内存空间。

            pool = wl_shm_create_pool(..., fd,...) // 通过wl_shm服务创建共享内存池。将刚才的fd作为参数传过去，这样Server端就可以和Client通过这个fd映射到同一块物理内存。

            buffer->buffer = wl_shm_pool_create_buffer(pool, ...) // 通过这个共享内存池在Server端分配buffer，返回wl_buffer为其本地代理对象。

            wl_buffer_add_listener(buffer->buffer, &buffer_listener,...) // 监听这个buffer代理对象，当Server端不再用这个buffer时，会发送release事件。这样，Client就可以重用这个buffer作下一帧的绘制。

    paint_pixels() // Client在buffer上绘制自己的内容。

    wl_surface_attach()// 将绘制好的buffer attach到surface上。作用上类似于Android中的updateTexImage()，即把某一个buffer与surface绑定。

    wl_surface_damage()// 告诉Server端的Compositor这个surface哪块区域是脏区域，需要重新绘制。

    window->callback = wl_surface_frame() // 在Server端创建Frame callback，它会被放在该surface下的frame_callback_list列表中。返回它的代理对象wl_callback。

    wl_callback_add_listener(window->callback, &frame_listener, ...) // 监听前面得到的callback代理对象。在Server端Compositor在完成一帧的合成渲染后，会往这些callback对象发done的事件（参考weston_output_repaint()）。Client收到后会调用参数中wl_callback_listener中的done事件对应的方法，这里是redraw()函数。这样，就形成了一个循环。

    wl_surface_commit() // 在0.99版本后，为了保证原子性及使surface属性的改动顺序无关，Server端对于surface的属性（damage region, input region, opaque region, etc.）都是双buffer的(weston_surface_state)。所以commit前的改动都存在backing buffer中。只有当Client调用wl_surface_commit()时，这些改动才生效。

#include "config.h"

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <assert.h>
#include <unistd.h>
#include <sys/mman.h>
#include <signal.h>
#include <errno.h>

#include <wayland-client.h>
#include "shared/os-compatibility.h"
#include <libweston/zalloc.h>
#include "xdg-shell-client-protocol.h"
#include "fullscreen-shell-unstable-v1-client-protocol.h"

struct display {
	struct wl_display *display;
	struct wl_registry *registry;
	struct wl_compositor *compositor;
	struct xdg_wm_base *wm_base;
	struct zwp_fullscreen_shell_v1 *fshell;
	struct wl_shm *shm;
	bool has_xrgb;
};

struct buffer {
	struct wl_buffer *buffer;
	void *shm_data;
	int busy;
};

struct window {
	struct display *display;
	int width, height;
	struct wl_surface *surface;
	struct xdg_surface *xdg_surface;
	struct xdg_toplevel *xdg_toplevel;
	struct buffer buffers[2];
	struct buffer *prev_buffer;
	struct wl_callback *callback;
	bool wait_for_configure;
};

static int running = 1;

static void
redraw(void *data, struct wl_callback *callback, uint32_t time);

static void
buffer_release(void *data, struct wl_buffer *buffer)
{
	struct buffer *mybuf = data;

	mybuf->busy = 0;
}

static const struct wl_buffer_listener buffer_listener = {
	buffer_release
};

static int
create_shm_buffer(struct display *display, struct buffer *buffer,
		  int width, int height, uint32_t format)
{
	struct wl_shm_pool *pool;
	int fd, size, stride;
	void *data;

	stride = width * 4;
	size = stride * height;

	fd = os_create_anonymous_file(size);
	if (fd < 0) {
		fprintf(stderr, "creating a buffer file for %d B failed: %s\n",
			size, strerror(errno));
		return -1;
	}

	data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (data == MAP_FAILED) {
		fprintf(stderr, "mmap failed: %s\n", strerror(errno));
		close(fd);
		return -1;
	}

	pool = wl_shm_create_pool(display->shm, fd, size);
	buffer->buffer = wl_shm_pool_create_buffer(pool, 0,
						   width, height,
						   stride, format);
	wl_buffer_add_listener(buffer->buffer, &buffer_listener, buffer);
	wl_shm_pool_destroy(pool);
	close(fd);

	buffer->shm_data = data;

	return 0;
}

static void
handle_xdg_surface_configure(void *data, struct xdg_surface *surface,
			     uint32_t serial)
{
	struct window *window = data;

	xdg_surface_ack_configure(surface, serial);

	if (window->wait_for_configure) {
		redraw(window, NULL, 0);
		window->wait_for_configure = false;
	}
}

static const struct xdg_surface_listener xdg_surface_listener = {
	handle_xdg_surface_configure,
};

static void
handle_xdg_toplevel_configure(void *data, struct xdg_toplevel *xdg_toplevel,
			      int32_t width, int32_t height,
			      struct wl_array *state)
{
}

static void
handle_xdg_toplevel_close(void *data, struct xdg_toplevel *xdg_toplevel)
{
	running = 0;
}

static const struct xdg_toplevel_listener xdg_toplevel_listener = {
	handle_xdg_toplevel_configure,
	handle_xdg_toplevel_close,
};

static struct window *
create_window(struct display *display, int width, int height)
{
	struct window *window;

	window = zalloc(sizeof *window);
	if (!window)
		return NULL;

	window->callback = NULL;
	window->display = display;
	window->width = width;
	window->height = height;
	window->surface = wl_compositor_create_surface(display->compositor);

	if (display->wm_base) {
		window->xdg_surface =
			xdg_wm_base_get_xdg_surface(display->wm_base,
						    window->surface);
		assert(window->xdg_surface);
		xdg_surface_add_listener(window->xdg_surface,
					 &xdg_surface_listener, window);

		window->xdg_toplevel =
			xdg_surface_get_toplevel(window->xdg_surface);
		assert(window->xdg_toplevel);
		xdg_toplevel_add_listener(window->xdg_toplevel,
					  &xdg_toplevel_listener, window);

		xdg_toplevel_set_title(window->xdg_toplevel, "simple-shm");
		wl_surface_commit(window->surface);
		window->wait_for_configure = true;
	} else if (display->fshell) {
		zwp_fullscreen_shell_v1_present_surface(display->fshell,
							window->surface,
							ZWP_FULLSCREEN_SHELL_V1_PRESENT_METHOD_DEFAULT,
							NULL);
	} else {
		assert(0);
	}

	return window;
}

static void
destroy_window(struct window *window)
{
	if (window->callback)
		wl_callback_destroy(window->callback);

	if (window->buffers[0].buffer)
		wl_buffer_destroy(window->buffers[0].buffer);
	if (window->buffers[1].buffer)
		wl_buffer_destroy(window->buffers[1].buffer);

	if (window->xdg_toplevel)
		xdg_toplevel_destroy(window->xdg_toplevel);
	if (window->xdg_surface)
		xdg_surface_destroy(window->xdg_surface);
	wl_surface_destroy(window->surface);
	free(window);
}

static struct buffer *
window_next_buffer(struct window *window)
{
	struct buffer *buffer;
	int ret = 0;

	if (!window->buffers[0].busy)
		buffer = &window->buffers[0];
	else if (!window->buffers[1].busy)
		buffer = &window->buffers[1];
	else
		return NULL;

	if (!buffer->buffer) {
		ret = create_shm_buffer(window->display, buffer,
					window->width, window->height,
					WL_SHM_FORMAT_XRGB8888);

		if (ret < 0)
			return NULL;

		/* paint the padding */
		memset(buffer->shm_data, 0xff,
		       window->width * window->height * 4);
	}

	return buffer;
}

static void
paint_pixels(void *image, int padding, int width, int height, uint32_t time)
{
	const int halfh = padding + (height - padding * 2) / 2;
	const int halfw = padding + (width  - padding * 2) / 2;
	int ir, or;
	uint32_t *pixel = image;
	int y;

	/* squared radii thresholds */
	or = (halfw < halfh ? halfw : halfh) - 8;
	ir = or - 32;
	or *= or;
	ir *= ir;

	pixel += padding * width;
	for (y = padding; y < height - padding; y++) {
		int x;
		int y2 = (y - halfh) * (y - halfh);

		pixel += padding;
		for (x = padding; x < width - padding; x++) {
			uint32_t v;

			/* squared distance from center */
			int r2 = (x - halfw) * (x - halfw) + y2;

			if (r2 < ir)
				v = (r2 / 32 + time / 64) * 0x0080401;
			else if (r2 < or)
				v = (y + time / 32) * 0x0080401;
			else
				v = (x + time / 16) * 0x0080401;
			v &= 0x00ffffff;

			/* cross if compositor uses X from XRGB as alpha */
			if (abs(x - y) > 6 && abs(x + y - height) > 6)
				v |= 0xff000000;

			*pixel++ = v;
		}

		pixel += padding;
	}
}

static const struct wl_callback_listener frame_listener;

static void
redraw(void *data, struct wl_callback *callback, uint32_t time)
{
	struct window *window = data;
	struct buffer *buffer;

	buffer = window_next_buffer(window);
	if (!buffer) {
		fprintf(stderr,
			!callback ? "Failed to create the first buffer.\n" :
			"Both buffers busy at redraw(). Server bug?\n");
		abort();
	}

	paint_pixels(buffer->shm_data, 20, window->width, window->height, time);

	wl_surface_attach(window->surface, buffer->buffer, 0, 0);
	wl_surface_damage(window->surface,
			  20, 20, window->width - 40, window->height - 40);

	if (callback)
		wl_callback_destroy(callback);

	window->callback = wl_surface_frame(window->surface);
	wl_callback_add_listener(window->callback, &frame_listener, window);
	wl_surface_commit(window->surface);
	buffer->busy = 1;
}

static const struct wl_callback_listener frame_listener = {
	redraw
};

static void
shm_format(void *data, struct wl_shm *wl_shm, uint32_t format)
{
	struct display *d = data;

	if (format == WL_SHM_FORMAT_XRGB8888)
		d->has_xrgb = true;
}

struct wl_shm_listener shm_listener = {
	shm_format
};

static void
xdg_wm_base_ping(void *data, struct xdg_wm_base *shell, uint32_t serial)
{
	xdg_wm_base_pong(shell, serial);
}

static const struct xdg_wm_base_listener xdg_wm_base_listener = {
	xdg_wm_base_ping,
};

static void
registry_handle_global(void *data, struct wl_registry *registry,
		       uint32_t id, const char *interface, uint32_t version)
{
	struct display *d = data;

	if (strcmp(interface, "wl_compositor") == 0) {
		d->compositor =
			wl_registry_bind(registry,
					 id, &wl_compositor_interface, 1);
	} else if (strcmp(interface, "xdg_wm_base") == 0) {
		d->wm_base = wl_registry_bind(registry,
					      id, &xdg_wm_base_interface, 1);
		xdg_wm_base_add_listener(d->wm_base, &xdg_wm_base_listener, d);
	} else if (strcmp(interface, "zwp_fullscreen_shell_v1") == 0) {
		d->fshell = wl_registry_bind(registry,
					     id, &zwp_fullscreen_shell_v1_interface, 1);
	} else if (strcmp(interface, "wl_shm") == 0) {
		d->shm = wl_registry_bind(registry,
					  id, &wl_shm_interface, 1);
		wl_shm_add_listener(d->shm, &shm_listener, d);
	}
}

static void
registry_handle_global_remove(void *data, struct wl_registry *registry,
			      uint32_t name)
{
}

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

static struct display *
create_display(void)
{
	struct display *display;

	display = malloc(sizeof *display);
	if (display == NULL) {
		fprintf(stderr, "out of memory\n");
		exit(1);
	}
	display->display = wl_display_connect(NULL);
	assert(display->display);

	display->has_xrgb = false;
	display->registry = wl_display_get_registry(display->display);
	wl_registry_add_listener(display->registry,
				 &registry_listener, display);
	wl_display_roundtrip(display->display);
	if (display->shm == NULL) {
		fprintf(stderr, "No wl_shm global\n");
		exit(1);
	}

	wl_display_roundtrip(display->display);

	/*
	 * Why do we need two roundtrips here?
	 *
	 * wl_display_get_registry() sends a request to the server, to which
	 * the server replies by emitting the wl_registry.global events.
	 * The first wl_display_roundtrip() sends wl_display.sync. The server
	 * first processes the wl_display.get_registry which includes sending
	 * the global events, and then processes the sync. Therefore when the
	 * sync (roundtrip) returns, we are guaranteed to have received and
	 * processed all the global events.
	 *
	 * While we are inside the first wl_display_roundtrip(), incoming
	 * events are dispatched, which causes registry_handle_global() to
	 * be called for each global. One of these globals is wl_shm.
	 * registry_handle_global() sends wl_registry.bind request for the
	 * wl_shm global. However, wl_registry.bind request is sent after
	 * the first wl_display.sync, so the reply to the sync comes before
	 * the initial events of the wl_shm object.
	 *
	 * The initial events that get sent as a reply to binding to wl_shm
	 * include wl_shm.format. These tell us which pixel formats are
	 * supported, and we need them before we can create buffers. They
	 * don't change at runtime, so we receive them as part of init.
	 *
	 * When the reply to the first sync comes, the server may or may not
	 * have sent the initial wl_shm events. Therefore we need the second
	 * wl_display_roundtrip() call here.
	 *
	 * The server processes the wl_registry.bind for wl_shm first, and
	 * the second wl_display.sync next. During our second call to
	 * wl_display_roundtrip() the initial wl_shm events are received and
	 * processed. Finally, when the reply to the second wl_display.sync
	 * arrives, it guarantees we have processed all wl_shm initial events.
	 *
	 * This sequence contains two examples on how wl_display_roundtrip()
	 * can be used to guarantee, that all reply events to a request
	 * have been received and processed. This is a general Wayland
	 * technique.
	 */

	if (!display->has_xrgb) {
		fprintf(stderr, "WL_SHM_FORMAT_XRGB32 not available\n");
		exit(1);
	}

	return display;
}

static void
destroy_display(struct display *display)
{
	if (display->shm)
		wl_shm_destroy(display->shm);

	if (display->wm_base)
		xdg_wm_base_destroy(display->wm_base);

	if (display->fshell)
		zwp_fullscreen_shell_v1_release(display->fshell);

	if (display->compositor)
		wl_compositor_destroy(display->compositor);

	wl_registry_destroy(display->registry);
	wl_display_flush(display->display);
	wl_display_disconnect(display->display);
	free(display);
}

static void
signal_int(int signum)
{
	running = 0;
}

int
main(int argc, char **argv)
{
	struct sigaction sigint;
	struct display *display;
	struct window *window;
	int ret = 0;

	display = create_display();
	window = create_window(display, 250, 250);
	if (!window)
		return 1;

	sigint.sa_handler = signal_int;
	sigemptyset(&sigint.sa_mask);
	sigint.sa_flags = SA_RESETHAND;
	sigaction(SIGINT, &sigint, NULL);

	/* Initialise damage to full surface, so the padding gets painted */
	wl_surface_damage(window->surface, 0, 0,
			  window->width, window->height);

	if (!window->wait_for_configure)
		redraw(window, NULL, 0);

	while (running && ret != -1)
		ret = wl_display_dispatch(display->display);

	fprintf(stderr, "simple-shm exiting\n");

	destroy_window(window);
	destroy_display(display);

	return 0;
}

上面：weston 测试案例：simple-shm.c  下面是rk3399 设备效果图：