How to display and select features within a radius in a QGIS form widget?

Question

Problem: I need a form widget that displays and allows the multi-selection of features from a layer ("POIs") within a defined radius (e.g., 3000m). Ideally, the features should be sorted by distance. The selected features should be stored in a list.

Background: I believe QGIS can handle this, but I haven't found a ready-made solution. I'm also willing to integrate a QML widget / Python into my forms. However, I don't want to use QT Designer for custom UIs, as I foresee other problems with that approach. My current forms are extremely complex but work smoothly, and I want to stick with drag-and-drop functionality.

Question: Is it possible to solve this problem using QGIS's built-in tools?

Additional Information: I'm using QGIS version 3.24.3. I'm familiar with Python and QML programming. I'm willing to learn new things if necessary.

Answer 1

You can make use of an Action on the attribute form.

Once the Action is configured, you can drag it into your form as usual.

The code

Here I subclass QgsMapToolEmitPoint and give it some custom functionality. Namely, leveraging QgsSpatialIndex to find the nearest features to the point clicked on the map canvas within a given distance. Conveniently, the list of feature IDs returned by the QgsSpatialIndex.nearestNeighbor() method are already sorted by distance.

I perform a selection of the found features to provide some visual feedback. The tool can be unset by pressing the X key.

I am sure there is a more elegant way of displaying the returned feature IDs, this was a quick hack. Suggestions welcome.

from qgis.PyQt.QtCore import Qt
from qgis.PyQt.QtWidgets import QLabel
from qgis.gui import QgsMapToolEmitPoint
from qgis.utils import iface

class GetFeaturesInRadius(QgsMapToolEmitPoint):
    def __init__(self, canvas):
        super().__init__(canvas)
        
        self.canvas = iface.mapCanvas()
        self.lyr = iface.activeLayer()
        
        self.num_feats = self.lyr.featureCount()
        self.features = self.lyr.getFeatures()
        self.index = QgsSpatialIndex(
                         self.features,
                         # full feature geometry is stored, 
                         # otherwise only the feature bounding box is used
                         flags=QgsSpatialIndex.FlagStoreFeatureGeometries
                         )
        
        # get the layour of the attribute form
        self.layout = form.children()[1]
        item_count = self.layout.count()

        # remove any QLabels already in the layout 
        # (otherwise they stack up each time the action is triggered)
        for i in range(item_count):
            item = self.layout.itemAt(i)
            if isinstance(item, QLabel):
                self.layout.removeWidget(item)
        
        # add a label to the attribute form to display the found feature IDs
        self.label = QLabel()
        self.layout.addWidget(self.label)

        # fire the handle_click method when the canvas is clicked
        self.canvasClicked.connect(self.handle_click)
        
    def handle_click(self, evt):
        self.lyr.removeSelection()
        # evt is a QgsPointXY of the click location
        # num_feats is the maximum number of features for nearestNeighbor to find
        # 200 is the distance which you will need to adjust (perhaps dynamically somehow)
        fids = self.index.nearestNeighbor(evt, self.num_feats, 200)
        
        # select the features for visual feedback and write the IDs to the label
        self.lyr.selectByIds(fids)        
        self.label.setText(', '.join([str(id) for id in fids]))
    
    # override the keyPressEvent of QgsMapToolEmitPoint 
    # to deactivate the tool when X is pressed
    def keyPressEvent(self, event):
        if event.key() == Qt.Key_X:
            self.lyr.removeSelection()
            self.canvas.unsetMapTool(self)
            self.layout.removeWidget(self.label)

# instantiate the tool and set it on the map canvas
canvas = iface.mapCanvas()           
tool = GetFeaturesInRadius(canvas)
canvas.setMapTool(tool)

Result