How to make Python wait while QGIS renders a shapefile

Question

I have written code for my QGIS 2.8.1 which can successfully take screenshot of one shape file, but unfortunately it doesn't work when I try with multiple shapefiles.

So basically if I replace allFiles = ["C:/Shapefiles/Map_00721.shp"] in the code below with allFiles = ["C:/Shapefiles/Map_00721.shp", "C:/Shapefiles/Map_00711.shp", "C:/Shapefiles/Map_00731.shp", "C:/Shapefiles/Map_00791.shp", "C:/Shapefiles/Map_00221.shp"], the loop iterates over the array without waiting for the rendering and snapshot process to happen.

I have tried using time.sleep in the code below, but it stops the rendering of shapefiles too and the result doesn't came as expected.

import ogr,os
from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.core import *
import qgis.utils
import glob
from time import sleep
import math
import processing
from processing.core.Processing import Processing
from PyQt4.QtCore import QTimer

Processing.initialize()
Processing.updateAlgsList()

OutputFileName = "ABC"    # Temprory global placeholder for filename
canvas = iface.mapCanvas()


def startstuffs():
    qgis.utils.iface.zoomToActiveLayer()    # Zoom to Layer
    scale=canvas.scale()    # Get current Scale
    scale =  scale * 1.5
    canvas.zoomScale(scale)   # Zoomout a bit
    QTimer.singleShot(2000,saveImg)   # Jump to save img

def saveImg():
    qgis.utils.iface.mapCanvas().saveAsImage(OutputFileName)
    QgsMapLayerRegistry.instance().removeAllMapLayers()


# Add array of address below
allFiles = ["C:/Shapefiles/Map_00721.shp"]
filesLen = len(allFiles)

TexLayer = "C:/US_County_NAD27.shp"

for lop in range(filesLen):

    currentShpFile = allFiles[lop]
    currentShpFileName = currentShpFile.strip("C:/Shapefiles/")
    OutputFileName = "C:/ImageOut/" + currentShpFileName + ".png"
    wb = QgsVectorLayer(currentShpFile, currentShpFileName, 'ogr')
    wbTex = QgsVectorLayer(TexLayer, 'CountyGrid', 'ogr')
    QgsMapLayerRegistry.instance().addMapLayer(wb)    # Add the shapefile
    QgsMapLayerRegistry.instance().addMapLayer(wbTex)    # Add the county shapefile
    qgis.utils.iface.setActiveLayer(wb)   # Makes wb as active shapefile

    QTimer.singleShot(3000, startstuffs)    # This start stuffs

print "Done!"

Answer 1

Avoid using time.sleep() since that will completely stall your entire program. Instead, use processEvents() which allows your program to render in the background.

import time

def spin(seconds):
    """Pause for set amount of seconds, replaces time.sleep so program doesn't stall"""

    time_end = time.time() + seconds
    while time.time() < time_end:
        QtGui.QApplication.processEvents()

This method should work fine for a quick fix, but in the long term it may generate difficult problems to track. It is better to use a QTimer with a event loop for a permanent solution.