calamares/src/modules/README.md

Calamares modules

Calamares modules are plugins that provide features like installer pages, batch jobs, etc. An installer page (visible to the user) is called a "view", while other modules are "jobs".

Each Calamares module lives in its own directory.

All modules are installed in $DESTDIR/lib/calamares/modules.

There are two types of Calamares module:

• viewmodule, for user-visible modules. These use C++ and QWidgets or QML
• jobmodule, for not-user-visible modules. These may be done in C++, Python, or as external processes.

A viewmodule exposes a UI to the user.

There are three interfaces for Calamares modules:

• qtplugin (viewmodules, jobmodules),
• python (jobmodules only),
• process (jobmodules only, not recommended).

Module directory

Each Calamares module lives in its own directory. The contents of the directory depend on the interface and type of the module.

Module descriptor

A Calamares module must have a module descriptor file, named module.desc. For C++ (qtplugin) modules using CMake as a build- system and using the calamares_add_plugin() function -- this is the recommended way to create such modules -- the module descriptor file is optional, since it can be generated by the build system. For other module interfaces, the module descriptor file is required.

The module descriptor file must be placed in the module's directory. The module descriptor file is a YAML 1.2 document which defines the module's name, type, interface and possibly other properties. The name of the module as defined in module.desc must be the same as the name of the module's directory.

Module descriptors must have the following keys:

• name (an identifier; must be the same as the directory name)
• type ("job" or "view")
• interface (see below for the different interfaces; generally we refer to the kinds of modules by their interface)

Module descriptors for Python modules must have the following key:

• script (the name of the Python script to load, nearly always main.py)

Module descriptors may have the following keys:

• emergency (a boolean value, set to true to mark the module as an emergency module)
• noconfig (a boolean value, set to true to state that the module has no configuration file; defaults to false)
• requiredModules (a list of modules which are required for this module to operate properly)

Required Modules

A module may list zero (if it has no requirements) or more modules by name. As modules are loaded from the global sequence in settings.conf, each module is checked that all of the modules it requires are already loaded before it. This ensures that if a module needs another one to fill in globalstorage keys, that happens before it needs those keys.

Emergency Modules

Only C++ modules and job modules may be emergency modules. If, during an exec step in the sequence, a module fails, installation as a whole fails and the install is aborted. If there are emergency modules in the same exec block, those will be executed before the installation is aborted. Non-emergency modules are not executed.

If an emergency-module fails while processing emergency-modules for another failed module, that failure is ignored and emergency-module processing continues.

Use the EMERGENCY keyword in the CMake description of a C++ module to generate a suitable module.desc.

A module that is marked as an emergency module in its module.desc must also set the emergency key to true in its configuration file (see below). If it does not, the module is not considered to be an emergency module after all (this is so that you can have modules that have several instances, only some of which are actually needed for emergencies).

Module-specific configuration

A Calamares module may read a module configuration file, named <modulename>.conf. If such a file is present in the module's directory, it can be shipped as a default configuration file. This only happens if the CMake-time option INSTALL_CONFIG is on.

Modules that have noconfig set to true will not attempt to read a configuration file, and will not warn that one is missing; conversely if noconfig is set to false (or is missing, since the default value is false) if there is no configuration file, a warning is printed during Calamares start-up.

The sample configuration files may work and may be suitable for your distribution, but no guarantee is given about their stability beyond syntactic correctness.

The module configuration file, if it exists, is a YAML 1.2 document which contains a YAML map of anything.

All sample module configuration files are installed in $DESTDIR/share/calamares/modules but can be overridden by files with the same name placed manually (or by the packager) in /etc/calamares/modules.

C++ modules

Type: viewmodule, jobmodule Interface: qtplugin

Currently the recommended way to write a module which exposes one or more installer pages (viewmodule) is through a C++ and Qt plugin. Viewmodules must implement Calamares::ViewStep. They can also implement Calamares::Job to provide jobs.

To add a Qt plugin module, put it in a subdirectory and make sure it has a CMakeLists.txt with a calamares_add_plugin call. It will be picked up automatically by our CMake magic. The module.desc file is not recommended: nearly all cases can be described in CMake.

C++ Jobmodule

TODO: this needs documentation

C++ Widgets Viewmodule

TODO: this needs documentation

C++ QML Viewmodule

A QML Viewmodule (or view step) puts much of the UI work in one or more QML files; the files may be loaded from the branding directory or compiled into the module. Which QML is used depends on the deployment and the configuration files for Calamares.

Explicit properties

The QML can access data from the C++ framework though properties exposed to QML. There are two libraries that need to be imported explicitly:

import io.calamares.core 1.0
import io.calamares.ui 1.0

The ui library contains the Branding object, which corresponds to the branding information set through branding.desc. The Branding class (in src/libcalamaresui/Branding.h offers a QObject-property based API, where the most important functions are string() and the convenience functions versionedName() and similar.

The core library contains both ViewManager, which handles overall progress through the application, and Global, which holds global storage information. Both objects have an extensive API. The ViewManager can behave as a model for list views and the like.

These explicit properties from libraries are shared across all the QML modules (for global storage that goes without saying: it is the mechanism to share information with other modules).

Implicit properties

Each module also has an implicit context property available to it. No import is needed. The context property config (note lower case) holds the Config object for the module.

The Config object is the bridge between C++ and QML.

A Config object must inherit QObject and should expose, as Q_PROPERTY, all of the relevant configuration information for the module instance. The general description how to do that is available in the Qt documentation.

Python modules

Modules may use one of the python interfaces, which may be present in a Calamares installation (but also may not be). These modules must have a module.desc file. The Python script must implement the Python jobmodule interface.

To add a Python or process jobmodule, put it in a subdirectory and make sure it has a module.desc. It will be picked up automatically by our CMake magic. For all kinds of Python jobs, the key script must be set to the name of the main python file for the job. This is almost universally main.py.

CMakeLists.txt is not used for Python and process jobmodules.

Calamares offers a Python API for module developers, the core Calamares functionality is exposed as libcalamares.job for job data, libcalamares.globalstorage for shared data and libcalamares.utils for generic utility functions. Documentation is inline.

All code in Python job modules must obey PEP8, the only exception are libcalamares.globalstorage keys, which should always be camelCaseWithLowerCaseInitial to match the C++ identifier convention.

For testing and debugging we provide the testmodule.py script which fakes a limited Calamares Python environment for running a single jobmodule.

Python Jobmodule

Type: jobmodule Interface: python

A Python jobmodule is a Python program which imports libcalamares and has a function run() as entry point. The function run() must return None if everything went well, or a tuple (str,str) with an error message and description if something went wrong.

Python API

TODO: this needs documentation

PythonQt modules (deprecated)

Type: viewmodule, jobmodule Interface: pythonqt

The PythonQt modules are deprecated and will be removed in Calamares 3.3. Their documentation is also almost completely lacking.

Process modules

Use of this kind of module is not recommended.

Type: jobmodule Interface: process

A process jobmodule runs a (single) command. The interface is process, while the module type must be job or jobmodule.

The module-descriptor key command should have a string as value, which is passed to the shell -- remember to quote it properly. It is generally recommended to use a shellprocess job module instead (less configuration, easier to have multiple instances).