WEBVTT captioned by sachac
NOTE What is D-Bus?
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Welcome to my EmacsConf 2022 talk, The Wheels on D-Bus.
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In this talk, we'll cover what D-Bus is,
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why you might want to use it, and how to use it with Emacs.
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D-Bus is fundamentally based on passing messages
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in between processes, using the bus as a mediator.
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On top of this is built an RPC system with method invocation
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that has argument lists and return values,
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like you might find in any programming language.
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These are commonly used for verb-type actions
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like "restart my computer."
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You can also associate a collection of attributes
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with objects on the bus, and these are called properties.
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The properties can be read-only, write-only, or read-write.
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Signals are a way of notifying participants on the bus
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of updated state, and are the basis
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for building dynamic user interfaces
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that react to changes in the system.
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It has a static and strong type system,
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so if you send a message with the wrong type signature,
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it simply gets rejected instead of going through
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to the remote service.
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It also manages service life cycles,
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so you're not running services at all times.
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They can be started and stopped by D-Bus on demand.
NOTE Why D-Bus?
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D-Bus has two major use cases.
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The first is acting as a lower-level substrate
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for higher-level programs,
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like a graphical desktop environment.
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For example, if you want to manage your network connectivity
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from your graphical environment,
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instead of having to build all of that from the ground up,
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you can rely on the D-Bus service to do that
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and only build the graphical component of it.
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This gives you consistency between desktop environments
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and reduces code duplication.
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Another application is automating desktop programs.
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If your program offers a D-Bus service,
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then it can be remote-controlled,
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and if all of your programs offer D-Bus,
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you can control your entire desktop.
NOTE The D-Bus Model
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Let's look at the abstractions that D-Bus provides.
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The top level object is called a bus,
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and it's like a partition
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that messages get exchanged inside of.
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Messages don't cross buses.
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Inside of a bus are services.
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Services are normally identified in reverse FQDN order,
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so org.foobar.FooService.
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Each service provides some set of features
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related to a particular area of functionality.
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Inside of each service are objects.
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Objects use a path notation,
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and usually follow the same reverse FQDN format
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as the service identifier.
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Each object has one or more interfaces.
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An interface is like a facet that you can use
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to interact with an object,
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and inside of the interface are properties, methods,
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and signals, which we covered before.
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Properties are attributes that can be read or written.
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Methods are verbs that you can call to invoke an action,
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and a signal is something that's used to move state
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in between a service and another participant on the bus.
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There can be any number of interfaces on an object,
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any number of objects in a service,
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and any number of services on a bus,
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and any number of buses on a system.
NOTE Well-known Busses
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There are two well-known busses,
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and these roughly map to those two use cases
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I mentioned before.
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The system bus is for interfacing with hardware
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and operating-system-level concerns
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like disks, networks, and so forth.
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The session bus is tied to a user login,
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and is more in the desktop automation use case.
NOTE Common interfaces
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There are some common interfaces you'll find
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if you go exploring D-Bus.
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The Introspectable interface is the basis
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of a lot of the reflection features.
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It has a single method called introspect
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that returns the XML interface description
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of whatever you call it on.
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Peer is used for lower level connectivity,
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for example, pinging a service to see if it's running.
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And the Properties interface is the basis
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of the read-write properties,
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which are secretly method calls under the cover.
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Just about every object you interact with on D-Bus
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will support all three of these interfaces.
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Additionally, ObjectManager is used for services
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that manage collections of objects.
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For example, the disk service has an object
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for each disk that's attached,
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and the object manager allows you
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to enumerate all of those.
NOTE Emacs Native D-Bus
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Emacs supports D-Bus natively since version 23.1.
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It's a combination of native bindings
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with a C library and dbus.el.
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While there are some ports of D-Bus
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to non-Linux operating systems,
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it's probably only available on Linux
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and almost certainly only usable on Linux.
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If you want to interact with D-Bus from Emacs,
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it's fairly straightforward.
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There's a collection of functions like dbus-get-property
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or dbus-call-method, et cetera,
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and they almost all take this same set
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of four arguments at the beginning:
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bus, service, path, and interface.
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In this case, it takes a single additional property,
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which is the one to read.
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And what we're calling is the hostname1 service,
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which gives you just a little bit of information
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about the system, like its hostname or its chassis.
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And in this case, you can see I'm running
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this presentation off my laptop.
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The problem with this and what I don't like about it
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is that all of these identifiers are very verbose
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and very repetitive.
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And if you end up calling these a lot,
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it gets old really quickly.
NOTE Debase
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So I wrote a wrapper called Debase,
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which is convenience on top of the built-in functions.
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Most of the stock functions have Debase versions
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just by replacing "dbus" with "debase".
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And let's look how that works.
NOTE Debase: Objects
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The fundamental idea of Debase is that you can bind together
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all of those arguments into a single object
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that represents the endpoint.
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This is an EIEIO class, and it takes keyword arguments,
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so there's never any chance
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of mixing up which thing is what.
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So this sets the endpoint to that object,
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calls debase-get-property on it,
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and you can see it works exactly the same.
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The thing that's really nice about this, though,
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is it knows that so many of these arguments
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are very similar that it can compute most of them
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if you don't provide them all.
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So if you just say service, it will assume
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that you want the same object that matches
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and the same interface that matches,
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and it works just the same.
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I find this very, very convenient.
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You can also reuse the object
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instead of having to repeat every argument
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with every function call,
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which is a really great improvement in ergonomics.
NOTE Debase: Retarget objects
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Because so many objects have multiple interfaces,
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you often find yourself needing to look
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at a different aspect of that object.
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This is supported with the built-in EIEIO clone method,
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which takes an object
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and a set of keyword arguments to replace.
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So in this case, we can see we're calling
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the Properties method,
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but everything else on that endpoint is the same.
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And then we're gonna call the method GetAll
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on that Properties interface,
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and it's going to return all the properties
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of the org.freedesktop.hostname1 interface
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inside of that object.
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And if we run that, we can see there's the hostname
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and some other information about the laptop
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that I'm running this on.
NOTE Debase: Object binding
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Debase also supports object binding.
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This creates a lexical context in which the Debase object
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is the implicit target of any D-Bus function.
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This is really convenient if you need
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to fetch multiple properties or otherwise interact
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with the same endpoint in multiple different ways.
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And you can see I'm still on a laptop
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and it's still named meson.
NOTE Debase: Raw binding
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You can also, if you don't want to use the object,
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you can provide the raw argument list.
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Under the covers, this is basically an flet
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where you're currying all of these functions
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so they start with those argument lists.
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And you can see I'm running on a Linux machine,
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which should not be surprising.
NOTE Debase: Codegen
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Debase also has an experimental code generation feature.
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It outputs EIEIO code with one class per D-Bus interface.
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This includes accessors for all of its properties
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with an in-process cache, so if you read one property,
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you don't have to go back to the bus to read it again.
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It also outputs generic functions and method implementations
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for the D-Bus interface methods.
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It includes name-mangling options,
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so you can control how everything is named.
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And you can generate the code either
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via introspecting a live system
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or providing an XML interface description,
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which is handy if you want to use it
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as part of a non-interactive build.
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I think this has a lot of promise,
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but it doesn't feel quite right yet,
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so any feedback or contributions are very welcome.
NOTE Debase: Codegen example
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Let's generate some Elisp code
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for that hostname1 service we were interacting with before.
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debase-gen-class is the generation class,
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and it says to create a class that matches this interface,
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named "hostname1", and then the rest of these arguments
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are the same ones to target the endpoint,
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just like with debase-object,
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because it extends debase-object.
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debase-gen-code is a generic function
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that takes any debase-gen class.
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There are different classes for functions,
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properties, et cetera,
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and it creates all of the code for it.
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If we evaluate it, we can see the results
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look about like we would expect:
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creates a defclass named "hostname1",
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which extends debase-object,
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has all of the slots and accessors defined,
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and then methods that define everything
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that you might want to do with it, including documentation.
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This is based on introspecting a running system,
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but as I mentioned,
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you can provide an XML interface description instead,
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if you like.
NOTE Debase: ObjectManager
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Debase also comes with "debase-objectmanager",
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which is convenience for the D-Bus ObjectManager interface.
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This is used in a lot of places in D-Bus,
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where an object manages other objects.
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For example, the NetworkManager object
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manages network hardware objects,
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and using the ObjectManager interface,
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you can enumerate all of the network hardware,
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and by subscribing to the signals,
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you can be notified when they change.
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"debase-objectmanager" keeps a local cache,
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and will fire a callback on any change.
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So it's the building block for that dynamic user interface,
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like you would see in a desktop system,
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but inside of Emacs.
NOTE Demo: Discomfort
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Let's do some demos.
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Discomfort is an interface I wrote for UDisks2,
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which is what manages all of the block device hardware.
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And again, it has that dynamic desktop-like interactivity,
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and mostly will just do what you mean.
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This is definitely alpha state.
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It doesn't have all the features,
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but it's good enough that I use it daily.
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So here's Discomfort,
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and you can see it has a list of all your hardware,
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what type it is, and where it's mounted.
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I have a little USB extension cable here,
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and I'm gonna plug in a disc,
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just to show you how this works.
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You can see when I plug it in,
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just a moment later,
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it shows up in that list, automatically.
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I don't have to press any key,
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I don't have to refresh it, it's just there.
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If I unplug it, it's gone.
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Plug it back in,
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and there it is.
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And you can see it's an encrypted volume.
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So in order to do anything with this,
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I'm going to have to supply a password.
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Just pressing Enter goes into the "do what I mean" mode,
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and it asks for the password.
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In this case, I've chosen the very secure password
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of "password".
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I hit Enter, and it unlocks it, and it mounts it,
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and it opens "dired" looking at it.
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And here's a little README.
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Let's see what it says.
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"Hello, EmacsConf."
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So that's my demo of discomfort.
NOTE Demo: Remote eval
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In addition to acting as a client for D-Bus,
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Emacs can also offer services to other D-Bus clients.
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This is a really interesting opportunity
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because it allows many different programs
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to integrate with Emacs
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in ways that were previously very difficult.
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You can use this as an alternative to Emacs.
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The difference is D-Bus provides a full API,
00:11:23.200 --> 00:11:24.999
so instead of emacsclient being
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a sort of fire-and-forget system,
00:11:26.680 --> 00:11:30.119
you can actually get results back from the remote operation.
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So here's some code.
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Here's a dbus-eval function, which takes a string,
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reads it, and evaluates it,
00:11:37.360 --> 00:11:39.359
and returns whatever that value is.
00:11:39.360 --> 00:11:41.839
Then we have a debase-bind block
00:11:41.840 --> 00:11:44.799
that sets up an object on the session bus.
00:11:44.800 --> 00:11:46.839
Again, that's my user login bus.
00:11:46.840 --> 00:11:49.559
It offers this D-Bus service Emacs.
00:11:49.560 --> 00:11:53.399
This is a constant inside of the dbus.el package.
00:11:53.400 --> 00:11:55.439
And again, the path is a constant in there.
00:11:55.440 --> 00:11:57.159
And we're gonna create this interface,
00:11:57.160 --> 00:12:02.519
org.gnu.Emacs.Eval, and then register a method called Eval
00:12:02.520 --> 00:12:04.759
that calls that dbus-eval function.
00:12:04.760 --> 00:12:08.119
Pretty straightforward, only a handful of lines of code.
00:12:08.120 --> 00:12:12.399
To test this out, we're going to use the dbus-send utility.
00:12:12.400 --> 00:12:15.399
This is a command line program that interacts with D-Bus.
00:12:15.400 --> 00:12:18.079
We're going to tell it to wait for and print the reply,
00:12:18.080 --> 00:12:20.999
that the message should be sent to the session bus,
00:12:21.000 --> 00:12:22.599
that we're going to talk
00:12:22.600 --> 00:12:25.639
to the org.gnu.Emacs service on that bus,
00:12:25.640 --> 00:12:30.879
and the /org/gnu/Emacs object inside that service.
00:12:30.880 --> 00:12:33.039
On that object, we're gonna interact
00:12:33.040 --> 00:12:35.999
with the org.gnu.Emacs.Eval interface
00:12:36.000 --> 00:12:37.639
and call its Eval method.
00:12:37.640 --> 00:12:40.639
We're gonna call that method with a single string argument,
00:12:40.640 --> 00:12:42.639
which is indicated by the string prefix,
00:12:42.640 --> 00:12:44.999
and then a form to evaluate.
00:12:45.000 --> 00:12:46.999
I actually have to run this from a shell,
00:12:47.000 --> 00:12:49.399
because if I try using it in Org, it wedges.
00:12:49.400 --> 00:12:51.959
org-babel blocks waiting on completion,
00:12:51.960 --> 00:12:54.519
which blocks the D-Bus service from responding.
00:12:54.520 --> 00:12:57.399
I really wish Emacs was multi-threaded.
00:12:57.400 --> 00:12:59.919
But let's try it out.
00:12:59.920 --> 00:13:02.719
So if we run this, we can see that we get a return,
00:13:02.720 --> 00:13:05.239
and that's an unsigned integer of 32 bits
00:13:05.240 --> 00:13:06.439
with a value of 3.
00:13:06.440 --> 00:13:09.959
So like I was saying, this is really a two-way API
00:13:09.960 --> 00:13:11.719
where you can communicate back and forth
00:13:11.720 --> 00:13:13.399
between Emacs and another program.
00:13:13.400 --> 00:13:14.959
It's not just fire-and-forget.
00:13:14.960 --> 00:13:16.279
I think that's really cool.
NOTE Demo: Remote Org capture
00:13:16.280 --> 00:13:18.519
Let's try another demo.
00:13:18.520 --> 00:13:20.599
What about a remote org-capture?
00:13:20.600 --> 00:13:22.999
What if you could trigger an org-capture
00:13:23.000 --> 00:13:24.679
from any program on your desktop?
00:13:24.680 --> 00:13:26.239
I think that would be pretty cool.
00:13:26.240 --> 00:13:30.239
And we can see, there it is.
00:13:30.240 --> 00:13:38.919
All right, I think I've got that one covered.
00:13:38.920 --> 00:13:42.319
So I do want to say that remote eval is probably a bad idea
00:13:42.320 --> 00:13:43.479
from a security perspective,
00:13:43.480 --> 00:13:46.759
but the point of this is some quick and dirty demonstrations
00:13:46.760 --> 00:13:49.799
of what can happen and to get people's imaginations flowing,
00:13:49.800 --> 00:13:51.719
because I think this is something
00:13:51.720 --> 00:13:54.079
that offers a lot of promise for Emacs.
00:13:54.080 --> 00:13:57.799
I think having a full-blown Emacs desktop environment
00:13:57.800 --> 00:13:59.879
where it can do all the things that a GNOME
00:13:59.880 --> 00:14:02.759
or a KDE environment can do is very exciting.
00:14:02.760 --> 00:14:06.439
And if you want to have a traditional GUI with Emacs
00:14:06.440 --> 00:14:08.679
as a more integrated participant of it,
00:14:08.680 --> 00:14:11.879
its service mechanism offers a lot of ability to do that.
NOTE Future directions
00:14:11.880 --> 00:14:15.999
In the micro sense, I think there's a lot of improvements
00:14:16.000 --> 00:14:19.279
that can be made to either dbus.el or to dbase.
00:14:19.280 --> 00:14:21.639
The main one is handling of the type system.
00:14:21.640 --> 00:14:25.839
Lisp's dynamic type system doesn't mesh particularly well
00:14:25.840 --> 00:14:28.799
with the static strong type system that D-bus offers,
00:14:28.800 --> 00:14:31.359
and having some convenience to assist that
00:14:31.360 --> 00:14:32.639
would be very helpful.
00:14:32.640 --> 00:14:35.319
There's also some weird interfaces.
00:14:35.320 --> 00:14:38.119
For example, some things return identifiers
00:14:38.120 --> 00:14:40.919
as an array of integer code points instead of a string,
00:14:40.920 --> 00:14:43.719
and there should be a common way of handling that.
00:14:43.720 --> 00:14:46.159
I also think that the service support could be improved.
00:14:46.160 --> 00:14:48.039
Even though I gave the demo service,
00:14:48.040 --> 00:14:50.479
it's not really a great D-bus citizen
00:14:50.480 --> 00:14:53.079
because it doesn't offer that introspection mechanism,
00:14:53.080 --> 00:14:55.919
and so the actual methods are pretty much invisible
00:14:55.920 --> 00:14:56.919
to other participants,
00:14:56.920 --> 00:15:00.079
unless they already know that you're using Emacs.
00:15:00.080 --> 00:15:01.799
That's my talk.
00:15:01.800 --> 00:15:02.559
Thank you.
00:15:02.560 --> 00:15:07.320
You can find me on mastodon.social or on libera.chat.
