mjhjmtu

I would like to dynamically draw the shortest line to the next object (Geometry Generator?).

Example: I have a network-layer (streets) and a point-layer. I know the distance from every point to the closes street (static, by using v.distance or one of the plugins NNjoin or refFunctions) but I would like to have a dynamically drawn line to the next street with the distance as label. It's important to have it dynamic, so if I move a point, the line and distance should update.
Since I use GeoPackage a SQL-Solution would also be nice.

In the screenshot below you can see my static solution:

You can do this with geometry generator in QGIS 3.x using the collect aggregate within the aggregate() function to extract the collected geometry of another layer and use it in an expression.

Here are the expressions I used to generate this example - the line layer was called Line; you just need to replace that with the name of your line layer.

Geometry generator (on point layer):

shortest_line($geometry,aggregate('Line','collect',$geometry))

Label:

round(length(shortest_line($geometry,aggregate('Line','collect',$geometry))),2)

Label placement (Label -> Placement -> Data defined):

Coordinate X - x(line_interpolate_point(shortest_line($geometry,aggregate('Line','collect',$geometry)),round(length(shortest_line($geometry,aggregate('Line','collect',$geometry))),2)/2))

Coordinate Y -
y(line_interpolate_point(shortest_line($geometry,aggregate('Line','collect',$geometry)),round(length(shortest_line($geometry,aggregate('Line','collect',$geometry))),2)/2))

Rotation -
line_interpolate_angle(shortest_line($geometry,aggregate('Line','collect',$geometry)),round(length(shortest_line($geometry,aggregate('Line','collect',$geometry))),2)/2)+90

Label alignment horizontal -
'Center'

Here are two Python solution:

Rubber Band approach:

1. searches for the nearest point on the road


2. creates a rubber band


3. 
   

   labels the rubber band

   
   
   

   from qgis.gui import *
   from qgis.utils import *
   from qgis.core import *
   from PyQt4.QtGui import *
   from PyQt4.QtCore import *
   
   p_lyr = QgsMapLayerRegistry.instance().mapLayersByName('points')[0]
   l_lyr = QgsMapLayerRegistry.instance().mapLayersByName('roads')[0]
   lines = [feature for feature in l_lyr.getFeatures()]
   
   def removeCanvasItems():
    canvas_items = [ i for i in iface.mapCanvas().scene().items() if issubclass(type(i), QgsRubberBand) or issubclass(type(i), QgsTextAnnotationItem) or issubclass(type(i), QgsVertexMarker)]
    if canvas_items:
    for item in canvas_items:
    if item in iface.mapCanvas().scene().items():
    iface.mapCanvas().scene().removeItem(item)
   
   def nearest_road():
    #removeCanvasItems() #removes the old rubberbands, calling this causes a mini dump
    for point in p_lyr.getFeatures():
    minDistPoint = min([l.geometry().closestSegmentWithContext(QgsPoint(point.geometry().asPoint())) for l in lines])[1]
    points = [QgsPoint(point.geometry().asPoint()), QgsPoint(minDistPoint[0], minDistPoint[1])]
    r_polyline = QgsRubberBand(iface.mapCanvas(), False)
    r_polyline.setToGeometry(QgsGeometry.fromPolyline(points), None)
    r_polyline.setWidth(2)
    r_polyline.setColor(QColor(0,255,0,255))
    seg = QgsFeature()
    seg.setGeometry(QgsGeometry.fromPolyline(points))
    geom = seg.geometry()
    length = seg.geometry().length()
    symbol = QgsMarkerSymbolV2()
    symbol.setSize(0)
    lbltext = QTextDocument(str(round(length,2)) + 'm')
    label = QgsTextAnnotationItem(iface.mapCanvas())
    label.setMapPosition(seg.geometry().interpolate(length/2.0).asPoint())
    label.setDocument(lbltext)
    label.setFrameSize(QSizeF(lbltext.size().width(),lbltext.size().height()))
    label.setFrameBorderWidth(0)
    label.setFrameColor(QColor("#ff4b00"))
    label.setFrameBackgroundColor(QColor("#ff4b00"))
    label.setMarkerSymbol(symbol)
   
   nearest_road() #calling the function the first time
   p_lyr.geometryChanged.connect(nearest_road) # creates new rubberband when point is moved
   p_lyr.featureAdded.connect(nearest_road) # creates new rubberband when a new point is created
   

   
   

When I want to remove the old rubber bands while adding a new feature or move an existing feature, I get a mini dump and QGIS crashes.

Vector Layer approach:

Instead of rubber bands you could use a layer, which stores the lines and the distance. In my case I have a shapefile with two attributes: id (int), and distance(double). With this layer you can better label and style your features.
The function removeFeatures(), removes all features from the distance layer. Better way is to get the ID of the feature that is moved and deletes only this line from the provider. I think I will update this soon.

from qgis.gui import *
from qgis.utils import *
from qgis.core import *
from PyQt4.QtGui import *
from PyQt4.QtCore import *

p_lyr = QgsMapLayerRegistry.instance().mapLayersByName('points')[0]
l_lyr = QgsMapLayerRegistry.instance().mapLayersByName('roads')[0]
lines = [feature for feature in l_lyr.getFeatures()]
d_lyr = QgsMapLayerRegistry.instance().mapLayersByName('distance')[0]
prov = d_lyr.dataProvider()

def removeFeatures():
 with edit(d_lyr):
 listOfIds = [feat.id() for feat in d_lyr.getFeatures()]
 d_lyr.deleteFeatures( listOfIds )

def nearest_road():
 removeFeatures()
 for point in p_lyr.getFeatures():
 minDistPoint = min([l.geometry().closestSegmentWithContext(QgsPoint(point.geometry().asPoint())) for l in lines])[1]
 points = [QgsPoint(point.geometry().asPoint()), QgsPoint(minDistPoint[0], minDistPoint[1])]
 seg = QgsFeature()
 seg.setGeometry(QgsGeometry.fromPolyline(points))
 geom = seg.geometry()
 length = seg.geometry().length()
 seg.setAttributes([1,seg.geometry().length()])
 prov.addFeatures([seg])
 d_lyr.updateExtents()
 d_lyr.triggerRepaint()
 d_lyr.updateFields()

nearest_road()
p_lyr.geometryChanged.connect(nearest_road)
p_lyr.featureAdded.connect(nearest_road)