How do I implement PyQGIS multithreading to update a progress bar

Question

I am developing a PyQGIS plugin using python. I facing a problem during the implementation of threading in my plugin. I have written a Python script which contains a class wrapping all geo-processing methods in use. The problem I am facing is processing the methods in threads. To give a better picture of my problem, I have a UI which calls the method calcNDVI() as:

self.lst.calcNDVI(self.RedBand, self.NIRBand, None, self.outputRaster, self.sensorType, self.dataType, self.addToQGIS)

This function call, calls a calcNDVI() method which does all the magic and adds the result to a QGIS project. The method call is also associated with passing some arguments.

I have gone through many sources but I can't get to understand how I can implement my algorithms in a threaded way. The code I have come through implements everything in the run method which makes it difficult for me as I have more than one method to be processed in my class, and I would also like the user to be able to choose which method to be run through the UI.

I am processing Landsat datasets which take too much time to process.

Answer 1

First, implement all your heavy calculations in a QRunnable subclass. Take a look here to get more details (http://pyqt.sourceforge.net/Docs/PyQt4/qrunnable.html). After that in your ok button (or in the method that will start the process) you can do something like this:

    # Initiating processing
    task = ThreadTask(init parameters...)
    task.setParameters( your parameters here...)
    # Connect your signal here (somethig like this)
    task.signals.processFinished.connect(self.processFinished)
    task.signals.errorOccurred.connect(self.errorOccurred)
    task.signals.userCanceled.connect(self.userCanceled)
    # Setting the progress bar
    self.progressMessageBar = self.iface.messageBar().createMessage('Processing landsat image...')
    self.progressBar = QtGui.QProgressBar()
    self.progressBar.setAlignment(Qt.AlignLeft|Qt.AlignVCenter)
    self.progressMessageBar.layout().addWidget(self.progressBar)
    self.iface.messageBar().pushWidget(self.progressMessageBar, self.iface.messageBar().INFO)
    self.progressBar.setRange(0, 0)
    self.progressMessageBar.destroyed.connect(task.signals.cancel)
    # Starting process
    self.threadpool.start(task)

Remember to create a QThreadPool object to start your thread:

self.threadpool = QThreadPool()