Add setSubsetString into layerutils

[woutergd]

For a plugin in QField we are creating, we want to apply a filter dynamically to a layer. We were however unable to access the setSubsetString method on a vectorlayer from the qml files, so hereby a PR which add this method to the LayerUtils so it becomes accessible through plugins.

[nirvn]

@woutergd , greatings -- actually , the setSubsetString function is exposed through a Q_PROPERTY on the vector layer (which is more QMLesque :) ). To retrieve or set the current subset string, you'd go through the property.

E.g. to get the current subset string:

let layer = qgisProject.mapLayersByName('my_vector_layer')[0]
console.log(layer.subsetString)

To set a subset string, simply do:

let layer = qgisProject.mapLayersByName('my_vector_layer')[0]
layer.subsetString = '"TYPE" = 1'

Voila :)

That said, thanks for opening a pull request with new code, while this specific PR isn't going to be merged, I certainly hope this will not be the last time we see you guys around :)

Out of curiosity, what functionalities are you adding through your plugin?

[woutergd]

@nirvn Thanks for the feedback, you are right. We tried it in (almost) that way, but probably used just setSubstring instead of the subsetString method, which caused errors in our plugin. We checked both the C++ docs and Python docs of QGIS and both mentioned setSubstring so that caused the confusion. Are there any resources online that have documented the syntax you mention?

About the plugin: I can't disclose that publicly at the moment, but if you sent me a direct mail I am happy to share it with you.

[nirvn]

@woutergd , if you check the QGIS API documentation , you'll notice each class can feature a Properties section which lists all available Q_PROPERTY within a given class flagged as Q_OBJECT or Q_GADGET.

E.g., the QgsVectorLayer's subsetString : https://api.qgis.org/api/classQgsVectorLayer.html#a82752c93ea29f4789e2973ef2090b9e7

You can also spot invokable functions by looking for the Q_INKABLE tag next to individual functions (not great, but better than nothing).

I'm always delighted to hear about plugin use cases, you can reach out via [email protected] (since I do not have your email at hand :) ).