D-Bus学习（六）：Method的收发小例子

　　我们继续学习D-Bus，参考http://dbus.freedesktop.org/doc/dbus/libdbus-tutorial.html ，相关的API接口reference参考http://dbus.freedesktop.org/doc/dbus/api/html/modules.html 。从底层，即libdbus学习如何发送Method以及如何等待应答，在上上次学习中，给出了同步的方式，这是更为高层的处理方式，建议使用。监听method和监听signal的方式非常相似。在给出例子之前，我希望和上次学习一样给出一个示意图，更好地了解D-Bus的各个概念。

监听Method call消息，并返回Method reply消息

　　Method的监听和signal的监听的处理时一样，但是信号是不需要答复，而Method需要。在下面的例子中，我们将学习如何在消息中加入多个参数（在D-Bus学习（四）中，我们加入了一个参数）的情况。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dbus/dbus-glib.h>
#include <dbus/dbus.h>
#include <unistd.h>

/*读取消息的参数，并且返回两个参数，一个是bool值stat，一个是整数level*/
void reply_to_method_call(DBusMessage * msg, DBusConnection * conn){
    DBusMessage * reply;
    DBusMessageIter arg;
    char * param = NULL;
    dbus_bool_t stat = TRUE;
    dbus_uint32_t level = 2010;
    dbus_uint32_t serial = 0;

    //从msg中读取参数，这个在上一次学习中学过
    if(!dbus_message_iter_init(msg,&arg))
        printf("Message has no args/n");
    else if(dbus_message_iter_get_arg_type(&arg) != DBUS_TYPE_STRING)
        printf("Arg is not string!/n");
    else
        dbus_message_iter_get_basic(&arg,& param);
    if(param == NULL) return;

    //创建返回消息reply
    reply = dbus_message_new_method_return(msg);
    //在返回消息中填入两个参数，和信号加入参数的方式是一样的。这次我们将加入两个参数。
    dbus_message_iter_init_append(reply,&arg);
    if(!dbus_message_iter_append_basic (&arg,DBUS_TYPE_BOOLEAN,&stat)){
        printf("Out of Memory!/n");
        exit(1);
    }
    if(!dbus_message_iter_append_basic (&arg,DBUS_TYPE_UINT32,&level)){
        printf("Out of Memory!/n");
        exit(1);
    }
//发送返回消息
    if( !dbus_connection_send (conn, reply, &serial)){
        printf("Out of Memory/n");
        exit(1);
    }
    dbus_connection_flush (conn);
    dbus_message_unref (reply);
}

/* 监听D-Bus消息，我们在上次的例子中进行修改 */
void listen_dbus()
{
    DBusMessage * msg;
    DBusMessageIter arg;
    DBusConnection * connection;
    DBusError err;
    int ret;
    char * sigvalue;

    dbus_error_init(&err);
    //创建于session D-Bus的连接
    connection = dbus_bus_get(DBUS_BUS_SESSION, &err);
    if(dbus_error_is_set(&err)){
        fprintf(stderr,"Connection Error %s/n",err.message);
        dbus_error_free(&err);
    }
    if(connection == NULL)
        return;
    //设置一个BUS name：test.wei.dest
    ret = dbus_bus_request_name(connection,"test.wei.dest",DBUS_NAME_FLAG_REPLACE_EXISTING,&err);
    if(dbus_error_is_set(&err)){
        fprintf(stderr,"Name Error %s/n",err.message);
        dbus_error_free(&err);
    }
    if(ret != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
        return;

    //要求监听某个singal：来自接口test.signal.Type的信号
    dbus_bus_add_match(connection,"type='signal',interface='test.signal.Type'",&err);
    dbus_connection_flush(connection);
    if(dbus_error_is_set(&err)){
        fprintf(stderr,"Match Error %s/n",err.message);
        dbus_error_free(&err);
    }

    while(true){
        dbus_connection_read_write (connection,0);
        msg = dbus_connection_pop_message (connection);

        if(msg == NULL){
            sleep(1);
            continue;
        }

        if(dbus_message_is_signal(msg,"test.signal.Type","Test")){
            if(!dbus_message_iter_init(msg,&arg))
                fprintf(stderr,"Message Has no Param");
            else if(dbus_message_iter_get_arg_type(&arg) != DBUS_TYPE_STRING)
                g_printerr("Param is not string");
            else
                dbus_message_iter_get_basic(&arg,&sigvalue);
        }else if(dbus_message_is_method_call(msg,"test.method.Type","Method")){
            //我们这里面先比较了接口名字和方法名字，实际上应当现比较路径
            if(strcmp(dbus_message_get_path (msg),"/test/method/Object") == NULL)
               reply_to_method_call(msg, connection);
        }
        dbus_message_unref(msg);
    }


}
int main( int argc , char ** argv){
    listen_dbus();
    return 0;
}

发送Method call消息，并等待Method reply消息

　　下面的例子在上次学习信号发送的例子上进行修改。如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dbus/dbus-glib.h>
#include <dbus/dbus.h>
#include <unistd.h>
//建立与session D-Bus daemo的连接，并设定连接的名字，相关的代码已经多次使用过了
DBusConnection * connect_dbus(){
    DBusError err;
    DBusConnection * connection;
    int ret;

    //Step 1: connecting session bus
    /* initialise the erroes */
    dbus_error_init(&err);
    /* Connect to Bus*/
    connection = dbus_bus_get(DBUS_BUS_SESSION, &err);
    if(dbus_error_is_set(&err)){
        fprintf(stderr,"Connection Err : %s/n",err.message);
        dbus_error_free(&err);
    }
    if(connection == NULL)
        return NULL;

    //step 2: 设置BUS name，也即连接的名字。
    ret = dbus_bus_request_name(connection,"test.wei.source",DBUS_NAME_FLAG_REPLACE_EXISTING,&err);
    if(dbus_error_is_set(&err)){
        fprintf(stderr,"Name Err : %s/n",err.message);
        dbus_error_free(&err);
    }
    if(ret != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
        return NULL;

    return connection;
}

void send_a_method_call(DBusConnection * connection,char * param)
{
    DBusError err;
    DBusMessage * msg;
    DBusMessageIter    arg;
    DBusPendingCall * pending;
    dbus_bool_t * stat;
    dbus_uint32_t * level;

    dbus_error_init(&err);

    //针对目的地地址，请参考图，创建一个method call消息。 Constructs a new message to invoke a method on a remote object.
    msg = dbus_message_new_method_call ("test.wei.dest","/test/method/Object","test.method.Type","Method");
    if(msg == NULL){
        g_printerr("Message NULL");
        return;
    }

    //为消息添加参数。Append arguments
    dbus_message_iter_init_append(msg, &arg);
    if(!dbus_message_iter_append_basic (&arg, DBUS_TYPE_STRING,&param)){
        g_printerr("Out of Memory!");
        exit(1);
    }

    //发送消息并获得reply的handle 。Queues a message to send, as with dbus_connection_send() , but also returns a DBusPendingCall used to receive a reply to the message.
    if(!dbus_connection_send_with_reply (connection, msg,&pending, -1)){
        g_printerr("Out of Memory!");
        exit(1);
    }

    if(pending == NULL){
        g_printerr("Pending Call NULL: connection is disconnected ");
    dbus_message_unref(msg);
        return;
    }

    dbus_connection_flush(connection);
    dbus_message_unref(msg);

    //waiting a reply，在发送的时候，已经获取了method reply的handle，类型为DBusPendingCall。
    // block until we recieve a reply， Block until the pending call is completed.
    dbus_pending_call_block (pending);
    // get the reply message，Gets the reply, or returns NULL if none has been received yet.
    msg = dbus_pending_call_steal_reply (pending);
    if (msg == NULL) {
        fprintf(stderr, "Reply Null/n");
         exit(1);
    }
     // free the pending message handle
     dbus_pending_call_unref(pending);
    // read the parameters
    if (!dbus_message_iter_init(msg, &arg))
        fprintf(stderr, "Message has no arguments!/n");
    else if ( db us_message_iter_get_arg_type (&arg) != DBUS_TYPE_BOOLEAN)
        fprintf(stderr, "Argument is not boolean!/n");
    else
        dbus_message_iter_get_basic (&arg, &stat);

    if (!dbus_message_iter_next(&arg))
        fprintf(stderr, "Message has too few arguments!/n");
    else if ( db us_message_iter_get_arg_type (&arg) != DBUS_TYPE_UINT32 )
        fprintf(stderr, "Argument is not int!/n");
    else
        dbus_message_iter_get_basic (&arg, &level);

    printf("Got Reply: %d, %d/n", stat, level);
    dbus_message_unref(msg);
}

int main( int argc , char ** argv){
    DBusConnection * connection;
    connection = connect_dbus();
    if(connection == NULL)
        return -1;

    send_a_method_call(connection,"Hello, D-Bus");
    return 0;
}