Writing to QGIS Project files

[MattReimer]

QRave plugin uses the QGIS project file to store which projects are loaded and that information gets rehydrated when the project file loads so this lets users save the layer state of their riverscapes projects AND keep specific riverscapes projects loaded as well.

Here's the code that makes that happen:

    def set_project_settings(self, projects: List[str]):
      """Save the given project settings to the QGIS project file."""
      # Get the saved project path
      qgs_path = self.qproject.absoluteFilePath()
      if projects and os.path.isdir(os.path.dirname(qgs_path)):
          qgs_path_dir = os.path.dirname(qgs_path)
          # Swap all abspaths for relative ones. This is important for portability
          projects = [(name, basename, safe_make_relpath(xml_path, qgs_path_dir))
                      for name, basename, xml_path in projects]
      self.qproject.writeEntry(
          CONSTANTS['settingsCategory'], 'qrave_projects', json.dumps(projects))

    def remove_project_settings(self):
      """Remove the saved project settings from the QGIS project file."""
      self.qproject.removeEntry(
          CONSTANTS['settingsCategory'], 'qrave_projects')

    def get_project_settings(self):
        """Load the saved project settings from the QGIS project file."""
        qrave_projects = []
        try:
            qrave_projects_raw, type_conversion_ok = self.qproject.readEntry(
                CONSTANTS['settingsCategory'],
                'qrave_projects'
            )

            if type_conversion_ok is False:
                qrave_projects = []
            else:
                qrave_projects = json.loads(qrave_projects_raw)

                if qrave_projects is None or not isinstance(qrave_projects, list):
                    qrave_projects = []

        except Exception as e:
            self.settings.log(
                'Error loading project settings: {}'.format(e), Qgis.Warning)

        qgs_path = self.qproject.absoluteFilePath()
        if os.path.isfile(qgs_path):
            qgs_path_dir = os.path.dirname(qgs_path)
            # Change all relative paths back to absolute ones
            qrave_projects = [(name, basename, safe_make_abspath(
                xml_path, qgs_path_dir)) for name, basename, xml_path in qrave_projects]

        return qrave_projects

get_project_settings() gets called in 3 different places:

1. When Closing a project
2. When reloading the tree
3. WHen the path to the QGIS project file changes

Then you end up with something like this in the project.qgs file (take the project.qgz file and unzip it to see the .qgs xml content):

<RiverscapesViewer>
  <enabled type="int">1</enabled>
  <!-- Note how it's just stringified json -->
  <qrave_projects type="QString">[["BRAT for HUC 1704021804", "BRAT for HUC 1704021804", "../../../Volumes/SAMSUNG2TB/Riverscapes/cybercastor/brat/1704021804/project.rs.xml"]]</qrave_projects>
</RiverscapesViewer>