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List:       freedesktop-dbus
Subject:    D-Bus Introspection: pure data structure representation
From:       Felipe Gasper <felipe () felipegasper ! com>
Date:       2019-05-17 16:03:26
Message-ID: 41AFAE34-69D9-4D73-9EEE-8DCD99892699 () felipegasper ! com
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Hi all,

	Sometime back I came up with a schema for representing a D-Bus interface as a pure \
data structure rather than as an XML document. This would allow applications to \
interface with D-Bus without needing to load XML libraries, which seem to be in \
progressively less use nowadays as formats such as JSON have become predominant.

	Here is an example interface description, using JSON:

-----
{
   "name" : "MyInterface",
   "annotations" : {
      "if_foo" : "bar"
   },
   "methods" : {
      "MyMethod" : {
         "annotations" : {
            "meth_baz" : "bar",
            "meth_foo" : "bar"
         },
         "inputs" : [
            {
               "annotations" : {
                  "input_baz" : "bar",
                  "input_foo" : "bar"
               },
               "name" : "haha",
               "type" : "s"
            },
            {
               "annotations" : {},
               "name" : "hoho",
               "type" : "s"
            },
            {
               "name" : "hehe",
               "type" : "s"
            }
         ],
         "outputs" : [
            {
               "annotations" : {
                  "output_foo" : "bar"
               },
               "name" : "haha",
               "type" : "s"
            },
            {
               "annotations" : {},
               "name" : "hoho",
               "type" : "s"
            },
            {
               "name" : "hehe",
               "type" : "s"
            }
         ]
      }
   },
   "properties" : {
      "propOne" : {
         "access" : "read",
         "annotations" : {
            "prop_baz" : "bar",
            "prop_foo" : "bar"
         },
         "type" : "as"
      },
      "propThree" : {
         "access" : "read",
         "annotations" : {},
         "type" : "as"
      },
      "propTwo" : {
         "access" : "read",
         "type" : "as"
      }
   },
   "signals" : {
      "MySignal" : {
         "annotations" : {
            "sig_foo" : "bar"
         },
         "parameters" : [
            {
               "annotations" : {
                  "parm_foo" : "bar"
               },
               "name" : "haha",
               "type" : "s"
            }
         ]
      }
   }
}
-----

	Note that I've separated method inputs/outputs into separate lists rather than \
keeping them together as a single list. I'm not opposed to combining them as the \
standard XML representation does, but I just think it's much easier to understand \
when they're in separate lists.

	This would seem simple enough to implement by adding a new method, e.g., \
"IntrospectAsStruct", in the Introspectable interface, with a single parameter that \
gives the serialization format (e.g., JSON, YAML, CBOR, MessagePack, Sereal, …) to \
use. It would likely also be good to add a property, Serializations, that gives \
supported serializations, either as strings or as numerical constants.

	So, e.g.,:

dbus-send --dest my.service /my/service \
org.freedesktop.DBus.Introspectable.IntrospectAsStruct format:json

… would return a JSON string.

	What are folks' thoughts here? I feel like XML is an increasingly obscure and \
unnecessary stumbling block to use of D-Bus's introspection.

	Thank you for your time!

-Felipe Gasper
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