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Thread: Imperative workflow in C++/Qml application

  1. #1
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    Sep 2013
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    Default Imperative workflow in C++/Qml application


    I'm working on a desktop application written in C++ with UI code in Qml. It's a typical 3d (engineering) application (think something like autocad, 3dsmax,..., but smaller). There is one thing which often comes back and to which I didn't find a satisfying solution yet and that's handling "a workflow with questions to the user". To give an example: Let's say the user wants to start a new project when another one is open - if there are changes, ask whether to save them or not - if no, quit, start a new project and ask properties for this project to open, if yes, check if there is already a file/folder to save - if no, ask a file to save, if yes check the types of items in the project - Depending on the type of items, ask the user if they are needed to be saved in the project or not - if not, go to next type, if yes, check if this item type needs certain "save settings" (for instance compression factor of a picture, whether to save a 3d model binary or ascii,...), and so on and so on... .
    Every time, i try to invent a new approach to solve such problems, but I never found a good solution. The reason is there is a lot of (already working) imperative code (and my mindset is probably to imperative :-)). Is there some sort of design pattern to follow to keep this clean? Or how do you solve such problems?
    Thanks for the ideas,


  2. #2
    Join Date
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    Graz, Austria
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    Default Re: Imperative workflow in C++/Qml application

    As you've said yourself there are quite a number of ways to do that.

    One approach that i find rather elegant myself is the request handler pattern used for example by the QML API of WebEngineView.

    The C++ side emits a signal that transports a request context, which is then used by the QML side to communicate the result back.

    See for example

    If you know or require that the QML side uses a modal dialog and thus a nested event loop, then you can retrieve the user's input right after the emit returns.

    Qt Code:
    1. RequestContext context;
    3. emit requestDialog(&context);
    5. // continue in "linear" workflow
    To copy to clipboard, switch view to plain text mode 

    If you want to handle the nested event loop on the C++ side, it will look more like this:
    Qt Code:
    1. RequestContext context;
    3. connect(&context, &RequestContext::done, &loop, &QEventLoop::quit);
    5. emit requestDialog(&context);
    7. // check if dialog has not already been
    8. if (!context.isDone() {
    9. loop.exec();
    10. }
    12. // continue in "linear" workflow
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