Command graph development

This page provides further detail on how Qtile's command graph works. If you just want to script your Qtile window manager the earlier information, in addition to the documentation on the available commands should be enough to get started.

To develop the Qtile manager itself, we can dig into how Qtile represents these objects, which will lead to the way the commands are dispatched.

Client-Server Scripting Model

Qtile has a client-server control model - the main Qtile instance listens on a named pipe, over which marshalled command calls and response data is passed. This allows Qtile to be controlled fully from external scripts. Remote interaction occurs through an instance of the libqtile.command.interface.IPCCommandInterface class. This class establishes a connection to the currently running instance of Qtile. A libqtile.command.client.InteractiveCommandClient can use this connection to dispatch commands to the running instance. Commands then appear as methods with the appropriate signature on the InteractiveCommandClient object. The object hierarchy is described in the Architecture section of this manual. Full command documentation is available through the Qtile Shell.

Digging Deeper: Command Objects

All of the configured objects setup by Qtile are CommandObject subclasses. These objects are so named because we can issue commands against them using the command scripting API. Looking through the code, the commands that are exposed are commands that are decorated with the @expose_command() decorator. When writing custom layouts, widgets, or any other object, you can add your own custom functions and, once you add the decorator, they will be callable using the standard command infrastructure. An available command can be extracted by calling .command() with the name of the command.

In addition to having a set of associated commands, each command object also has a collection of items associated with it. This is what forms the graph that is shown above. For a given object type, the items() method returns all of the names of the associated objects of that type and whether or not there is a defaultable value. For example, from the root, .items("group") returns the name of all of the groups and that there is a default value, the currently focused group.

To navigate from one command object to the next, the .select() method is used. This method resolves a requested object from the command graph by iteratively selecting objects. A selector like [("group", "b"), ("screen", None)] would be to first resolve group "b", then the screen associated to the group.

The Command Graph

In order to help in specifying command objects, there is the abstract command graph structure. The command graph structure allows us to address any valid command object and issue any command against it without needing to have any Qtile instance running or have anything to resolve the objects to. This is particularly useful when constructing lazy calls, where the Qtile instance does not exist to specify the path that will be resolved when the command is executed. The only limitation of traversing the command graph is that it must follow the allowed edges specified in the first section above.

Every object in the command graph is represented by a CommandGraphNode. Any call can be resolved from a given node. In addition, each node knows about all of the children objects that can be reached from it and have the ability to .navigate() to the other nodes in the command graph. Each of the object types are represented as CommandGraphObject types and the root node of the graph, the CommandGraphRoot represents the Qtile instance. When a call is performed on an object, it returns a CommandGraphCall. Each call will know its own name as well as be able to resolve the path through the command graph to be able to find itself.

Note that the command graph itself can standalone, there is no other functionality within Qtile that it relies on. While we could have started here and built up, it is helpful to understand the objects that the graph is meant to represent, as the graph is just a representation of a traversal of the real objects in a running Qtile window manager. In order to tie the running Qtile instance to the abstract command graph, we move on to the command interface.

Executing graph commands: Command Interface

The CommandInterface is what lets us take an abstract call on the command graph and resolve it against a running command object. Put another way, this is what takes the graph traversal .group["b"] and executes the info() command against the addressed screen object. Additional functionality can be used to check that a given traversal resolves to actual objcets and that the requested command actually exists. Note that by construction of the command graph, the traversals here must be feasible, even if they cannot be resolved for a given configuration state. For example, it is possible to check the screen assoctiated to a group, even though the group may not be on a screen, but it is not possible to check the widget associated to a group.

The simplest form of the command interface is the QtileCommandInterface, which can take an in-process Qtile instance as the root CommandObject and execute requested commands. This is typically how we run the unit tests for Qtile.

The other primary example of this is the IPCCommandInterface which is able to then route all calls through an IPC client connected to a running Qtile instance. In this case, the command graph call can be constructed on the client side without having to dispatch to Qtile and once the call is constructed and deemed valid, the call can be executed.

In both of these cases, executing a command on a command interface will return the result of executing the command on a running Qtile instance. To support lazy execution, the LazyCommandInterface instead returns a LazyCall which is able to be resolved later by the running Qtile instance when it is configured to fire.

Tying it together: Command Client

So far, we have our running Command Objects and the Command Interface to dispatch commands against these objects as well as the Command Graph structure itself which encodes how to traverse the connections between the objects. The final component which ties everything together is the Command Client, which allows us to navigate through the graph to resolve objects, find their associated commands, and execute the commands against the held command interface.

The idea of the command client is that it is created with a reference into the command graph and a command interface. All navigation can be done against the command graph, and traversal is done by creating a new command client starting from the new node. When a command is executed against a node, that command is dispatched to the held command interface. The key decision here is how to perform the traversal. The command client exists in two different flavors: the standard CommandClient which is useful for handling more programatic traversal of the graph, calling methods to traverse the graph, and the InteractiveCommandClient which behaves more like a standard Python object, traversing by accessing properties and performing key lookups.

Returning to our examples above, we now have the full context to see what is going on when we call:

from libqtile.command.client import CommandClient
c = CommandClient()
from libqtile.command.client import InteractiveCommandClient
c = InteractiveCommandClient()

In both cases, the command clients are constructed with the default command interface, which sets up an IPC connection to the running Qtile instance, and starts the client at the graph root. When we call"status") or c.status, we navigate the command client to the status command on the root graph object. When these are invoked, the commands graph calls are dispatched via the IPC command interface and the results then sent back and printed on the local command line.

The power that can be realized by separating out the traversal and resolution of objects in the command graph from actually invoking or looking up any objects within the graph can be seen in the lazy module. By creating a lazy evaluated command client, we can expose the graph traversal and object resolution functionality via the same InteractiveCommandClient that is used to perform live command execution in the Qtile prompt.