PyQt4，防止app.exec_（）挂起主线程

Question

I have some simple code that loads google.com with the PyQt4 library.
This is the code:

import sys
from PyQt4.QtGui import QApplication
from PyQt4.QtCore import QUrl
from PyQt4.QtWebKit import QWebView

class Browser(QWebView):
    def __init__(self):
        QWebView.__init__(self)
        self.loadFinished.connect(self._result_available)

    def _result_available(self, ok):
        frame = self.page().mainFrame()
        #print(frame.toHtml())
        app.exit()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    view = Browser()
    view.load(QUrl('http://www.google.com'))
    print('start')
    app.exec_()#hangs the main thread
    print('end')

My problem with this code is that app.exec_() hangs the main thread for a while, between the print start and print end.
Is there a way to scrape a website in PyQt4 without making the main thread hang for a while.
I would like to resume the main thread's normal execution of code after app.exec_() .

Answer 1

It's probably just taking time to download the content from the web. app.exec_() is what runs your application basically. So if anything is indeed hanging it's actually some other factor that influences the proper execution of your application. Any statements that are after the app.exec_() will be executed once you close the application. Usually app.exec_() is called at the end of the main (in C++ you do return app.exec_() which you can also do in PyQt of course).

When working with content that requires time to be downloaded and displayed in the UI, you have to add multithreading in order to allow the main thread to continue work properly (thus not creating the so called UI freeze ) but at the same time do some work in the background. The sole purpose of the main thread is to keep track of the UI and run it. Anything else that you add to the main thread will prevent the UI from working in a fluent manner. You have several options here - inheriting QThread (I wouldn't suggest doing that for this scenario in particular), using QThread + QObject in order to incorporate slots and signal in your application (for example: page downloaded? -> if yes, signal the UI to display the content), QRunnable etc.

So my advice is to download the content in a separate thread and once it's done you can add it to your UI as well as provide some other form of visual feedback (this also includes print statements :P).