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I am curious why there is facility within QT Designer to create signals and slots as, to my surprise, I did not need to do this. All of the necessary coding was accomplished completely within the plugin, and without using QT Designer.

I installed QGIS 2.6.0, lately, and have successfully created a signal / slot in a test plugin (an effort beginning with PluginBuilder): getting a ListView widget to respond to a combobox change

Can someone explain to me a) what is the use of the slot/signal facility in QT Desginer, when using QGIS and python and b) how is that slot/signal facility in QT Designer used?

Maybe a pointer to a full web-explanation on this point would be helpful. The QT tutorials I've found, so far, don't help, or, at least, don't help in the context of a QGIS plugin.

  • Signals and slots are event driven, an action emits a signal and the slot responds. There may be many listeners to a given signal (or none) - to keep from emitting/responding to every little thing which would (potentially) slow down the system you need to customize which signals will be emitted and which you will listen to. I hope that helps. – Michael Stimson Nov 16 '14 at 23:47
  • Sorry, @Michael, your answer leans in the wrong direction...I've narrowed the question, rewording it clarified the confusion. This is not a general question about how/why slots/signals work, rather, why QT Designer has the facility to create them, in the context of making qgis plugins. – John Nov 17 '14 at 14:40
  • @John: Some find it easier to create the slot/signal connections using a graphical tool, rather than "hand-coding". You could ask the same question about Qt Designers facilities for creating GUIs: You could also just write the .ui file by hand, but the Qt Designer provides an interface to help you. – Jake Nov 17 '14 at 17:14
  • Thanks, @Jake. The code for the QT slot/signal will therefore end up in the ui_test.py file, as opposed to in the file containing most of my code, the test.py file. Correct? What is the practical consequence? Using QT, how would one connect to a specific python function? I guess I'm still missing the manual pieces for making the QT variant work. And: how and what code pieces it is adding, using which entries there in QT?. Is it telling that none of the instructions manuals for QGIS/plugins/etc which I've found use QT for the connections? – John Nov 17 '14 at 20:54
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    Oh I get it... in the same way that a form can be created in .net using the toolbox or specified textually. I know I find it easier to use the GUI and then modify the fiddly bits that don't do exactly what I want - remembering that once modified it can't/shouldn't be modified in designer again or the modifications will be lost. QT is not just QGIS, it's a generic library for creating forms that is valid across platforms and some languages (python, C) in the same way that python isn't just QGIS but a generic scripting language. To integrate it's a matter of making the function match. – Michael Stimson Nov 17 '14 at 21:36

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