再探DBUS-GLIB Binding，GLIB事件与DBUS事件是如何关联的

DBus有两种API接口，一种是直接使用DBUS的 low-level API，一种是使用Binding，Bindings有不同的类型，有PERL Binding、PYTHON Binding、GLIB Binding等。这里主要关注使用GLIB binding。和low-level API不同的是，GLIB binding则能够完成GLIB OBJECT的本地事件（native signal）与DBus事件的绑定，下面描述使用DBUS，Signal事件发送和接收的基本过程。

1、Signal被发送到DBus Daemon，这个过程如果使用low-level API，这个过程需要程序直接完成；如果使用GLIB Binding，则GLIB OBJECT发送本地的Signal时自动完成。这个本地的Signal，就是g_signal_emit方法发出的GLIB OBJECT的Signal。
2、Signal包含接口的标识、Signal的标识、以及发送者的标识、其他参数。
3、DBus上的任何进程可以提交Signal的“过滤规则”。
4、DBus Daemon根据Signal过滤规则，将Signal发送到各个进程。
5、Signal接收进程接到Signal进行处理，如果使用low-level API，则直接处理Signal；如果使用的是GLIB Binding，GLIB Binding将在其代理对象上触发一个本地事件（emit a native signal ）

下面回到DBus-GLIB的例子程序，详细说明这个过程。DBus-GLIB Binding的例子运行过程如《初探DBUS(1)》一文所示(如果有疑问需要获取源码请参见初探DBUS(1))，在例子主干逻辑如下：
1、example-signal-recipient程序负责定期向发起example-signal-emitter程序emitHelloSignal远程方法调用
2、example-signal-emitter程序为emitHelloSignal的服务器端，接收到调用后，向example-signal-recipient发送HELLO_SIGNAL的SIGNAL事件
3、example-signal-recipient接收到事件并打印。

example-signal-emitter程序的分析如下：
（1）从example-signal-emitter.xml文件生成对应的example-signal-emitter-glue.h文件

example-signal-emitter.xml文件如下：

<?xml version="1.0" encoding="UTF-8" ?>

<node name="/">
  <interface name="org.designfu.TestService">

    <method name="emitHelloSignal">
    </method>
   
    <!-- Mark the signal as exported -->
    <signal name="HelloSignal"/>

  </interface>
</node>

命令行如下：
dbus-binding-tool --prefix=test_object --mode=glib-server --output=example-signal-emitter-glue.h ./example-signal-emitter.xml

example-signal-emitter-glue.h定义了本地的对象中哪个Signal与DBus的Signal绑定。example-signal-emitter-glue.h原码中定义了HelloSignal的DBus 的Signal与本地定义Signal绑定
const DBusGObjectInfo dbus_glib_test_object_object_info = {
  0,
  dbus_glib_test_object_methods,
  1,
"org.designfu.TestService\0emitHelloSignal\0S\0\0\0",
"org.designfu.TestService\0HelloSignal\0\0",
"\0"
};

(2)example-signal-emitter.c中定义了一个GOBJECT风格的“对象” TestObject。TestObject的class_init方法定义了hello_signal 的本地Signal。关于GOBJECT的对象模型请查看GOBJECT的相关文档。
static void test_object_class_init (TestObjectClass *klass)
{
signals[HELLO_SIGNAL] =
    g_signal_new ("hello_signal",
          G_OBJECT_CLASS_TYPE (klass),
                  G_SIGNAL_RUN_LAST | G_SIGNAL_DETAILED,
                  0,
                  NULL, NULL,
                  g_cclosure_marshal_VOID__STRING,
                  G_TYPE_NONE, 1, G_TYPE_STRING);
}

这里有一个问题，名为“HelloSignal”的DBus事件如何同名为“hello_signal”关联的？这个问题可以从./dbus-gobject.c的export_signals 函数得到解答，该函数调用了s = _dbus_gutils_wincaps_to_uscore (signame);_dbus_gutils_wincaps_to_uscore这个函数将HelloSignal翻译成了hello_signal

(3)当example-signal-emitter在接收到emitHelloSignal远程方法调用中发出"hello_signal"的本地Signal
g_signal_emit (obj, signals[HELLO_SIGNAL], 0, "Hello");

本地的Signal如何触发DBus的Signal呢？到回example-signal-emitter的主函数继续分析
  （3.1）初始化

  DBusGConnection *bus;
  DBusGProxy *bus_proxy;
  GError *error = NULL;
  TestObject *obj;
  GMainLoop *mainloop;
  guint request_name_result;

  g_type_init ();

  dbus_g_object_type_install_info (TEST_TYPE_OBJECT, &dbus_glib_test_object_object_info);

dbus_glib_test_object_object_info见第（1）步的的说明

（3.2）设置本地Signal与DBus Signal的绑定

  mainloop = g_main_loop_new (NULL, FALSE);

  bus = dbus_g_bus_get (DBUS_BUS_SESSION, &error);
  if (!bus)
    lose_gerror ("Couldn't connect to session bus", error);

  bus_proxy = dbus_g_proxy_new_for_name (bus, "org.freedesktop.DBus",
                     "/org/freedesktop/DBus",
                     "org.freedesktop.DBus");

  if (!dbus_g_proxy_call (bus_proxy, "RequestName", &error,
              G_TYPE_STRING, "org.designfu.TestService",
              G_TYPE_UINT, 0,
              G_TYPE_INVALID,
              G_TYPE_UINT, &request_name_result,
              G_TYPE_INVALID))
    lose_gerror ("Failed to acquire org.designfu.TestService", error);

  obj = g_object_new (TEST_TYPE_OBJECT, NULL);


  dbus_g_connection_register_g_object (bus, "/org/designfu/TestService/object", G_OBJECT (obj));

  printf ("test service running\n");

  g_main_loop_run (mainloop);

  exit (0);

dbus_g_connection_register_g_object方法中调用了上文提到的export_signals ，export_signals 对每个export的本地signal进行了如下的操作：

          closure = dbus_g_signal_closure_new (connection, object, signame, (char*) iface);
          g_closure_set_marshal (closure, signal_emitter_marshaller);

          g_signal_connect_closure_by_id (object,
                          id,
                          0,
                          closure,
                          FALSE);

          g_closure_add_finalize_notifier (closure, NULL,
                           dbus_g_signal_closure_finalize);

closure是什么呢？文档说“A GClosure represents a callback supplied by the programmer.” g_signal_connect_closure_by_id函数为每个本地的Signal（参数id，是本地Signal的ID）挂载一个处理回调的closure，closure中关键函数是signal_emitter_marshaller

static void
signal_emitter_marshaller (GClosure        *closure,
                           GValue          *retval,
                           guint            n_param_values,
                           const GValue    *param_values,
                           gpointer         invocation_hint,
                           gpointer         marshal_data)
{
  DBusGSignalClosure *sigclosure;
  DBusMessage *signal;
  DBusMessageIter iter;
  guint i;
  const char *path;

  sigclosure = (DBusGSignalClosure *) closure;

  g_assert (retval == NULL);

  path = _dbus_gobject_get_path (sigclosure->object);

  g_assert (path != NULL);

  signal = dbus_message_new_signal (path,
                                    sigclosure->sigiface,
                                    sigclosure->signame);
  if (!signal)
    {
      g_error ("out of memory");
      return;
    }

  dbus_message_iter_init_append (signal, &iter);

  /* First argument is the object itself, and we can't marshall that */
  for (i = 1; i < n_param_values; i++)
    {
      if (!_dbus_gvalue_marshal (&iter,
                                (GValue *) (&(param_values[i]))))
        {
          g_warning ("failed to marshal parameter %d for signal %s",
                     i, sigclosure->signame);
          goto out;
        }
    }
  dbus_connection_send (DBUS_CONNECTION_FROM_G_CONNECTION (sigclosure->connection),
                        signal, NULL);
out:
  dbus_message_unref (signal);
}

可以很清晰的看到，dbus_connection_send 的过程，于是，本地的signal触发--〉dbus_g_closure-->signal_emitter_marshaller，在 signal_emitter_marshaller将DBus的Signal发送了出去。



-----------------------------------------------------------------------------------------------------------------
example-signal-recipient的主干逻辑代码如下：

(1)初始化Session Bus连接，并获取远程对象。
  DBusGConnection *bus;
  DBusGProxy *remote_object;
  GError *error = NULL;
  GMainLoop *mainloop;

  g_type_init ();

  mainloop = g_main_loop_new (NULL, FALSE);

  bus = dbus_g_bus_get (DBUS_BUS_SESSION, &error);
  if (!bus)
    lose_gerror ("Couldn't connect to session bus", error);

  /* We use _for_name_owner in order to track this particular service
   * instance, which lets us receive signals.
   */
  remote_object = dbus_g_proxy_new_for_name (bus,
                         "org.designfu.TestService",
                         "/org/designfu/TestService/object",
                         "org.designfu.TestService");
  if (!remote_object)
    lose_gerror ("Failed to get name owner", error);

(2)被注释掉的重要步骤：注册Signal处理的marshaller。
由于例子使用的远程调用方法emitHelloSignal不带参数，因此可以使用系统内置的marshaller，因此例子程序中将 dbus_g_object_register_marshaller注释了。这里参照pidgin 项目（另一个使用DBus的著名开源项目）的DBus用法，将该步骤补上，在pidgin的例子中，Signal的interface名为"im.pidgin.purple.PurpleInterface"
(2.1)查看Signal的“函数原型”，使用dbus-monitor命令进行查找：

命令行：dbus-monitor type=signal interface="im.pidgin.purple.PurpleInterface"
结果：

signal sender=:1.21 -> dest=(null destination) path=/im/pidgin/purple/PurpleObject; interface=im.pidgin.purple.PurpleInterface; member=ReceivedImMsg
int32 1097
string "[e-mail]"
string "<FONT FACE="Times"><FONT COLOR="#000000">Hi!</FONT></FONT>"
int32 8728
uint32 0
(2.2)生成marshal.list文件

内容如下：

VOID:INT,STRING,STRING,INT,UINT

其含义是：返回为VOID，参数类型按次序为
INT,STRING,STRING,INT,UINT


(2.3)生成marshal.h和marshal.c

glib-genmarshal --header --prefix=marshal marshal.list > marshal.h
glib-genmarshal --body --prefix=marshal marshal.list > marshal.c

（2.4）注册marshaller


dbus_g_object_register_marshaller(marshal_VOID__INT_STRING_STRING_INT_UINT,
G_TYPE_NONE, G_TYPE_INT, G_TYPE_STRING,
G_TYPE_STRING, G_TYPE_INT, G_TYPE_UINT,
G_TYPE_INVALID);



(3)设置需处理的HelloSignal事件
dbus_g_proxy_add_signal (remote_object, "HelloSignal", G_TYPE_STRING, G_TYPE_INVALID);


dbus_g_proxy_connect_signal (remote_object, "HelloSignal", G_CALLBACK (hello_signal_handler),
                   NULL, NULL);



(4)设置一个定时器，定期发起emitHelloSignal调用。
定时器回调方法如下：

static gboolean emit_signal (gpointer arg)
{
  DBusGProxy *proxy = arg;

  dbus_g_proxy_call_no_reply (proxy, "emitHelloSignal", G_TYPE_INVALID);
  return TRUE;
}