How can I insert the result of the dijkstra algorithm as a feature in a table with PyQGIS?

Question

I ask for your help with the PyQGIS API. Being a new user of GIS I would like to carry out an operation.

It is for me to look for the shortest path in a graph using the DIJKSTRA algorithm.

The code of the QGIS doc after some small adaptations works well except that I do not recover anything at the end if it is not a graphic plot. After the execution I would like to retrieve the shortest path found (the result) as an entity that I insert in a table and so on for possibly other shorter paths that I will calculate.

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

vl = qgis.utils.iface.mapCanvas().currentLayer()
director = QgsLineVectorLayerDirector(vl, -1, '', '', '', 3)
properter = QgsDistanceArcProperter()
director.addProperter(properter)
crs = qgis.utils.iface.mapCanvas().mapRenderer().destinationCrs()
builder = QgsGraphBuilder(crs)

pStart = QgsPoint(-0.835953, 0.15679)
pStop = QgsPoint(-1.1027, 0.699986)
tiedPoints = director.makeGraph(builder, [pStart, pStop])
graph = builder.graph()
tStart = tiedPoints[0]
tStop = tiedPoints[1]
idStart = graph.findVertex(tStart)
idStop = graph.findVertex(tStop)
(tree, cost) = QgsGraphAnalyzer.dijkstra(graph, idStart, 0)

if tree[idStop] == -1:
  print "Path not found"
else:
  p = []
  curPos = idStop
  while curPos != idStart:
    p.append(graph.vertex(graph.arc(tree[curPos]).inVertex()).point())
    curPos = graph.arc(tree[curPos]).outVertex();
  p.append(tStart)
  rb = QgsRubberBand(qgis.utils.iface.mapCanvas())
  rb.setColor(Qt.red)
  for pnt in p:
    rb.addPoint(pnt)

May somebody help?

Answer 1

This applies for QGIS 2.18: You may first create a (memory) layer (if you don't already have one) and a data provider and add it to the registry and the layer list using the following code:

my_lyr = QgsVectorLayer('Linestring?crs=epsg:31468', 'network result', 'memory')
# 31468 is only an example, set the epsg code you require
my_lyr_dp = my_lyr.dataProvider()

root = QgsProject.instance().layerTreeRoot()
network_analysis_group = root.addGroup('Network results')

QgsMapLayerRegistry.instance().addMapLayer(my_lyr, False)
network_analysis_group.addLayer(my_lyr)

You need the data provider to add new features, so if you already have a layer you wish to add features to, you can e.g. select it in the legend and create a data provider by

selected_lyr = iface.activeLayer()
my_lyr_dp = selected_lyr.dataProvider() #cp. above

Then from the p in your code above (a list made of QgsPoint() entities) you can create a feature in your layer like

f = QgsFeature()
geom = QgsGeometry().fromPolyline(p) #this is your p
f.setGeometry(geom)
my_lyr_dp.addFeatures([f])
my_lyr.updateExtents()
iface.mapCanvas().refreshAllLayers()

If you want to 'clear' the layer (delete all previous network analysis results) you can do this by

fids = [f.id() for f in my_lyr.getFeatures()]
my_lyr_dp.deleteFeatures(fids)
my_lyr.updateExtents()
iface.mapCanvas().refreshAllLayers()