Filtered WFS-layers

[ZakarFin]

We are beginning to implement functionality that allows administrators to add WFS-based layers with preset filters. We also have future plans for allowing end-users to define filters for the layers. This is useful if a WFS has a lot of features but the administrator wants end-users to have dedicated layers for given subset of the features for simplicity.

Consider having a Web Feature Service listing different kind of service points all over the world. Using this service an admin could add the service as a layer in Oskari named "Gas stations in Finland" and define a filter where everything where country != 'FI' and type != 'gas station' would not be fetched (and shown) from this "wider datapool" for the end-user.

The filter syntax would be the same kind of JSON as for attribute based styling for vector features in the frontend so it's consistent between the functionalities.

Plans are to implement this in autumn 2020 by NLS Finland.

[ZakarFin]

Initial version is included in Oskari 2.1.0 version. This will allow admins to write a JSON-fragment on a textarea that defines the filter. This will probably be continued to have more admin/user-friendly UI for defining filters in the (near) future.

[ZakarFin]

For future reference:

• related PR: WFSFilter draft version oskari-server#643
• filter is defined in layer.attributes under the "filter" key
• docs: https://oskari.org/documentation/examples/oskari-filter