Last year (2017) I worked on the QGIS Grass Processing
time, I finally found how it worked. It is not so complicated but it
is not so easy to understand because there is a "trick" on what is
ext mechanism. Sometimes, developpers ask me for help on
Here is my last email answer about this subject. I also put it on my blog for people who want to understand or work on this part of QGIS.
Hello, it's been a little bit more than one year since I have "retired" from the QGIS project. It's just because I do have other projects that I am working for and I had to cut down from some projects. Well, I can only tell you about the GRASS Processing from QGIS 2.18 (but I bet that this is still the same in QGIS 3). If you want to use the QGIS GRASS plugin, it is completely different and I can't help you on this part. All you have to know is that: * The GRASS provider for QGIS Processing is just a graphical interface in QGIS towards GRASS. * There is an on-the-fly creation of a GRASS database in a temporary directory. Then all the input layers/rasters are imported (or referenced) into this GRASS database. * When you launch an algorithm from the GRASS provider, QGIS launch (via the shell or a system call) GRASS command line with the right parameters onto the temporary GRASS database. You can read this command line in the log. * If the algorithm returns some results, those are extracted from the GRASS temporary database and converted into layers/rasters (and directly opened into QGIS if option has been enabled). Then the GRASS temporary database is deleted. To add your add-on in the GRASS provider, you need to: * Build a description file in python/plugins/processing/algs/grass7/description/. Those files just describe the different parameters of your algorithm (add-on). You can read some of those files to understand the different options. The description file filename must respect the algorithm name. * If your algorithm doesn't require something else than: import input data/launch the algorithm/extract data, then you are done. There is nothing more to do than the description file. * If your algorithm need more actions like a preliminary check on data, more than one GRASS command involved or a transformation on output data before extracting them, then you need to use the ext mechanism. The 'ext' mechanism is a (some say kind of dirty) way to add per algorithm additional logic. There are 4 different levels where you can add logic: - Checking the input parameters, if you want to verify that two excluding options have been both enabled. - Processing inputs import: if you need to do more than importing input layers. - Processing the command by itself: if you need to chain more than one GRASS command for your algorithm. - Processing the outputs export: if you need to do special things before exporting layers or if you need special export methods. Whenever you want to add some logic on one (or more) level(s), you have to create a .py file named accordingly to the algorithm name (modify '.' with '_'), into python/plugins/processing/algs/grass7/ext. Then you need to create methods that will respect names: - Input parameters: checkParameterValuesBeforeExecuting - Inputs import: processInputs - Command: processCommand - Outputs: processOutputs If there is a Python file with the algorithm name in the ext directory, methods will be imported from the file and run instead of the common methods (there are "standard" processCommand/processInputs/processOutputs/checkParameterValuesBeforeExecuting in the code of the GRASS provider for QGIS Processing, in python/plugins/processing/algs/grass7/Grass7Algorithm.py). If we take the example of v.what.rast, there is an ext file: ext/v_what_rast.py. In this file there is a processCommand method. It just launch the standard processCommand but with the delOutputs option set to True (we do not want to have standard outputs). Then there is also a customized processOutputs which export the input vector as an output for QGIS. We need to do this because v.what.rast modify values directly into the input vector layer. It doesn't generate dedicated output so we have to build this output ourself. Clearly, if you want to do special things in the ext mechanism, you will need to read (and understand) the GRASS provider code standard methods in Grass7Algorithm.py. I hope that these explanations will ease your work…