[Qgis-developer] [Processing] Parameters and widgets refactoring

[Arnaud Morvan]

Here the idea to create some class to get processing UI more modular, 
flexible and more user friendly.

Actually in processing all parameter's widgets are hardcoded in 
ParametersPanel, depending on parameter types. This is not flexible to 
create custom parameters and related panels.
On top of this, we have a lot of code duplicated between :
     Standard algorithm dialog/panel
     Batch dialog/panel
     Modeler dialog/panel

A more modular vision could be to create a widget class for each 
parameter type.
ParametersPanel would only have to iterate over parameters and create 
appropriate widgets.

Using this, it could be easy to create new parameter types and related 
widgets without creating a custom dialog and panel class for each case.

Proposed design:
     Parameter class would have a new "widget" attribute (name as 
string) and optionally widget options.
     Existing parameters will have a default widget name already given.
     Processing UI will maintain a global a registry for widgets (name 
=> class)
     Each processing plugin could register it's own parameters types and 
related widgets.
     Each algorithm could override the default widget for a given parameter.

With this design, we have a dissociation between the parameters that are 
core related and widgets that are ui oriented. Widgets could be easily 
extended without altering the core.

For example, in postgis related algorithms, it could be possible to 
create custom widgets, capable of database introspection using existing 
GeoDB class to propose drop down lists of schemas and tables or primary 
keys and geometry column detection, without altering existing parameter 
types.

This is a draft, your comments are welcome.

Arnaud Morvan
Camptocamp