Recursive loop to modify vector layer and refresh mapcanvas

Question

I'm trying to plot one point "jumping" each second in the canvas (like a gps tracking). I need to take points from a .csv file.

This functionality is made in a QWidget of my application, so the code is the next:

class StartTrack(QWidget):
def __init__(self, MapExplorer):
 QWidget.__init__(self)
 self.MapExplorer = MapExplorer # MapExplorer is my principal class
def starting(self): # I call this method when push a button in the UI
 self.j = 0
 self.addTarget()
def addTarget(self):
 with open('C:\Users\Nacho\SkyLife\GIS4Ship\Archivos\Lex\data\Targets_com.csv') as f:
 self.j += 1
 self.lector = csv.reader(f)
 for i in range(0,self.j):
 row = self.lector.next()
 x = row[1]
 y = row[2]
 # I have this on MapExplorer:
 # self.target_layer = QgsVectorLayer('Point?crs=EPSG:4326', 'points' , 'memory')
 # self.trackprovider = self.target_layer.dataProvider()
 # QgsMapLayerRegistry.instance().addMapLayer(self.target_layer)
 trackprovider = self.MapExplorer.trackprovider
 target = self.MapExplorer.target_layer
 features = []
 feature = QgsFeature()
 feature.setGeometry(QgsGeometry.fromWkt("POINT ("+x+" "+y+")"))
 features.append(feature)
 trackprovider.addFeatures(features)
 target.updateExtents()
 target.triggerRepaint()
 print(self.j, row)
 QTimer.singleShot(1000, self.addTarget) # Recursive call

My problem is that I can't "delete" the previous point at canvas. I really delete each feature in each iteration, because I'm redefining everything: trackprovider (QgsVectorLayer.dataProvider), target (QgsVectorLayer), features (list of feature), and feature (point).

I want to see only one point each iteration (like moving through canvas) instead of this: enter image description here

I've already tried to delete feature from list and from layer too, but not results.

Am I missing something related to refreshing canvas, layer or provider?

• Germán Carrillo: You can find a similar example here: gis.stackexchange.com/questions/189735/… Instead of deleting features you could filter them out with a where clause (see setSubsetString).
• artwork21: I'm not familiar with the MapExplorer.trackprovider method, have you looked into creating a in-memory point layer as a QgsVectorLayer? You may have better control commuting and deleting features from this layer type.
• Nacho F.: Thank you for your comments and sorry for my late answer, I've been busy and also have tried more ways to get my goal. First, @artwork21, trackprovider is my assignment to the dataProvider method. And for the in-memory point layer you referred, the next is the way for that no? # self.target_layer = QgsVectorLayer('Point?crs=EPSG:4326', 'points' , 'memory') # self.trackprovider = self.target_layer.dataProvider()
• Nacho F.: @GermánCarrillo, I took this recursive idea from that example and I investigated about setSubsetString, but I don't know what is the syntax to use it, I'm not familiarised with SQL and can't find useful list or tables for that, so if could tell me some reference I will be grateful. I tried others ways to get the points appear and dissapear in each loop iteration (deleting or filtering), and I get that the only way is by using dataProvider, isn't it? Thank you again.
• Germán Carrillo: What do you intend to do for each feature. Do you intend to generate an image with the canvas content?

Answer 1

I wrote my own script running it in QGIS. And I use my postgresql database with postgis extension to store and load the layers.

First, I create a points layer via my add_layer function and give it to the QgsMapLayerRegistry. Afterwards I start recursive function calling itself every 2000 milliseconds to show next point. Therefore, I use the setSubsetString on the point_id of the layer.

from qgis.core import QgsDataSourceURI, QgsVectorLayer, QgsMapLayerRegistry
from PyQt4.QtCore import QTimer
def show_next_point():
 global current_point
 if current_point == 10:
 current_point = 0
 subset = '"point_id" = {}'.format(current_point)
 point_layer.setSubsetString(subset)
 current_point += 1
 QTimer.singleShot(2000, show_next_point)
def add_layer():
 uri = QgsDataSourceURI()
 uri.setConnection('localhost', '5432', 'postgres', 'postgres', password)
 uri.setDataSource('', '(SELECT * FROM points)', 'geom', '', 'point_id')
 return QgsVectorLayer(uri.uri(), 'point_layer', 'postgres')
map_registry = QgsMapLayerRegistry.instance()
point_layer = add_layer()
map_registry.addMapLayer(point_layer)
# Start recursive function to iterate trough points:
current_point = 0
show_next_point()

To create the point-table and fill it with 10 points I used:

import psycopg2
# Insert your password:
connection_string = (
 'dbname=postgres ' +
 'host=localhost ' +
 'port=5432 ' +
 "user='postgres' " +
 "password='" + password + "'"
)
connection = psycopg2.connect(connection_string)
cursor = connection.cursor()
def create_table():
 cursor.execute('CREATE EXTENSION postgis')
 create_sql = (
 'CREATE TABLE points ('
 'point_id integer NOT NULL, '
 'geom geometry(Point, 4326), '
 'CONSTRAINT points_pkey PRIMARY KEY (point_id)'
 ')'
 )
 cursor.execute(create_sql)
 for i in range(10):
 sql = (
 'INSERT INTO '
 'points '
 '(point_id, geom) '
 'VALUES '
 '(%(point_id)s, ST_SetSRID(ST_MakePoint( %(lon)s, %(lat)s), 4326)) '
 )
 params = {
 'lat': i*10,
 'lon': 90,
 'point_id': i
 }
 cursor.execute(sql, params)
 connection.commit()
create_table()

Hope this helps!