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[[cha:qtvcp:devel]]
= QtVCP Development

== Overview

The intention of QtVCP is to *supply an infrastructure to support screen and VCP panel building for LinuxCNC*.

By providing a _diverse widget_ set and supporting _custom coding_,
QtVCP hopes that development energy will be expended in _one toolkit_
rather than continuous re-invention.

By using the same toolkit across many screens/panels,
users should have an easier time customizing/creating these,
and developers should find it easier to help trouble shoot with less effort.

QtVCP uses a *Qt Designer built `.ui` file* and a *Python handler file*

* to *load and control a screen/panel that displays Qt widgets* and
* to *control LinuxCNC's motion controller or HAL pins*.

There are _builtin screens and panels_, easily loaded by a user, or users can build/modify one of their own.

QtVCP uses *libraries and custom widgets* to hide some of the complexity of interfacing to LinuxCNC.
By using QtVCP's library rather than LinuxCNC's, we can mitigate minor LinuxCNC code changes.

== Builtin Locations

Builtin screens and panels are stored in separate folders:

* _Screens_ in `share/qtvcp/screens`
* _Panels_ in `share/qtvcp/panels`
* _Stock images_ in `share/qtvcp/images`

Screens and panels are sorted by their folder name, which is also the name used to load them.

Inside the folder would be:

* the `.ui` _file_,
* the _handler file_, and
* possibly the `.qss` _theme file_.

== QtVCP Startup To Shutdown

*QtVCP source* is located in `+src/emc/usr_intf/qtvcp+` folder of LinuxCNC source tree.

=== QtVCP Startup

When QtVCP first starts:

. It must decide if this object is a screen or a panel.
. It searches for and collects information about paths of required files
  and useful folders.
. It then:
.. Builds the HAL component,
.. Loads the window instance,
.. Adds handler extensions,
.. Installs an event filter.

Now the window/widgets are instantiated, the HAL pins are built.
This also initiates the `+_init_hal()+` function of the widgets.
. The `+initialized__()+` handler function is called
. The `STATUS` library is forced to update.
. HAL component is set ready at this point.
. A variety of optional switch arguments are set, including calling a
  `POSTGUI` HAL file (if a screen).
. Terminate signals are trapped and QtVCP now polls for events.

=== QtVCP Shutdown

Finally when QtVCP is asked to shutdown:

. It calls shutdown functions in the handler file,
. `STATUS` monitoring is shut down
. HAL component gets killed

== Path Information

When QtVCP loads it collects paths information.

This is available in the handler file's `+__init__()+` function as *`path`*:

*`IMAGEDIR`*:: Path of builtin images
*`SCREENDIR`*:: Path of builtin motion controller screens
*`PANELDIR`*:: Path of builtin accessory panels
*`WORKINGDIR`*:: Path of where QtVCP was launched from
*`CONFIGPATH`*:: Path of the launched configuration
*`BASEDIR`*:: General path, used to derive all paths
*`BASENAME`*:: Generic name used to derive all paths
*`LIBDIR`*:: Path of QtVCP's Python library
*`HANDLER`*:: Path of handler file
*`XML`*:: Path of .ui file
*`DOMAIN`*:: Path of translation
*`IS_SCREEN`*:: Screen/panel switch

== Idiosyncrasies

These try to cover non-obvious situations.

=== Error Code Collecting

*LinuxCNC's error code collecting can only be read from one place*.
//FIXME which place ?

When read, it is *'consumed'*, i.e. _no other object can read it_.

In QtVCP screens, it is recommended to _use the_ `ScreenOptions` _widget to set up error reading_.

_Errors are then *sent to other objects* via_ *`STATUS`* _signals_.

=== Jog Rate

*LinuxCNC has no internal record of jog rate*: _you must specify it at
the time of jogging_.

QtVCP uses the `STATUS` library to _keep track of the latest linear and
angular jog rates_.

It is *always specified in machine units per minute* and _must be
converted when in non-machine units mode_. +
So, if your machine is imperial based but you are in metric mode, changes
to jog rate sent to `ACTION` functions must be converted to imperial. +
In the same manner, if the machine is metric based and you are in
imperial mode, changes to jog rate must be sent to `ACTION` functions in
metric units. +
_For angular jog rates the units don't change in metric/imperial mode_
so you can send them to `ACTION` functions without conversion.

While you are free to ignore this jogging record while building screens,
anyone modifying your screen and using the builtin jog rate widgets
would not get the desired results as the `ACTION` library's *`DO_JOG`*
function gets it's jog rate from the `STATUS` library.

=== Keybinding

WARNING: Keybinding is always a _difficult-to-get-right-in-all-cases_
  affair.

Custom keybinding functions are to be _defined in the handler file_.

Most importantly widgets that require regular key input and not jogging,
should be checked for in the `processed_key_event__` function.
//TODO Elaborate ?

=== Preference File

Some QtVCP widgets use the preference file to record important
information.

This _requires the preference file to be set up early_ in the widget
initialization process. +
The easiest way to do this is to *use the `ScreenOptions` widget*.

=== Widget Special Setup Functions

QtVCP looks for and calls the `+_hal_init()+` function _when the widget
is first loaded_.

It is not called when using Qt Designer editor.

After this function is called the widget has access to some special
variables:

*`self.HAL_GCOMP`*:: The _HAL component_ instance
*`self.HAL_NAME`*:: This _widget's name_ as a string
*`self.QT_OBJECT_`*:: This _widget's PyQt object instance_
*`self.QTVCP_INSTANCE_`*:: The _very top level parent_ of the screen
*`self.PATHS_`*:: The _instance of QtVCP's path_ library
*`self.PREFS_`*:: The _instance of an optional preference file_
*`self.SETTINGS_`*:: The `Qsettings` _object_

When making a custom widget, _import and sub class _the
*`+_HalWidgetBase+`* class for this behavior.

=== Dialogs

Dialogs (AKA "pop up windows") are _best loaded with the_ `ScreenOptions`
_widget_, but they can be placed on the screen in Qt Designer.

It doesn't matter where on the layout but _to make them hidden_, cycle
the `state` property to true then false.

By default, if there is a preference file, the dialogs will remember
their last size/placement. +
It is possible to override this so they open in the same location each
time.
//TODO How ?

=== Styles (Themes)

While it is possible to set styles _in Qt Designer_, it is more
convenient to change them later if they are all set in a
*_separate_ `.qss` _file_*.

The file should be put in the _same location as the handler file_.

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