Graphics Reference
In-Depth Information
• A plugin is simply a directory that contains a Python package initialization file
(
__init__.py
), some Python modules, and other files created using Qt
Designer.
• The
__init__.py
module must define a top-level function named
ClassFactory
that accepts an
iface
parameter and returns an object that
represents the plugin.
• The plugin object must define an
initGui()
method, which is called to initial-
ize the plugin's user interface, and an
unload()
method, which is called to re-
move the plugin from the QGIS application.
• The plugin can interact with and manipulate the QGIS application via the
iface
object passed to the class factory.
• The
resources.qrc
file lists various resources such as images, which are
used by the plugin.
• The
resources.qrc
file is compiled into a
resources.py
file using the
PyQt command-line tools.
• Dialog boxes and other windows are created using a Qt Designer template, which
are typically stored in a file with a name of the form
ui_Foo.ui
.
• The UI template files are then compiled into Python code using the PyQt
command-line tools. If the template is named
ui_foo.ui
, then the associated
Python module will be named
ui_foo.py
.
• Once the user interface for a dialog box has been defined, you create a subclass of
QtGui.QDialog
, and load that user interface module into it. This defines the
contents of the dialog box based on your template.
• Your plugin can then display the dialog box as required, extracting the entered
values and using the results to interact with QGIS via the
iface
variable.
Plugins are a useful way of extending and customizing QGIS. We will return to the topic
of QGIS plugins in
Chapter 4
,
Creating QGIS Plugins
, where we will create our own plu-
gin from scratch.
