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use of model/view programming



  • Hi all -

    I started to discuss this in another thread, but it got hijacked (by me), so I thought I'd start fresh.

    I'm writing an app using model/view for the first time. I think I get the concepts (at least most of them), but there are a couple of implementation details I'm not sure about.

    My program has:

    • a worker object, which contains a Devices object that is derived from QAbstractTableModel.
    • a widget object, which contains a QTableView.

    I understand that when I construct the widget, I need to do this:

        ui->tableView->setModel(???);
        tableView.show()
    

    But how do I point to the table model that is part of the worker object?

    I could have the worker object return the address of the table model to main(), who could then pass it to the widget c'tor. But this seems cumbersome and prone to failure. Can someone advise on a preferred method for accomplishing this?

    Thanks.


  • Lifetime Qt Champion

    Hi,

    As suggested in your other thread, your Worker class should rather be a member of your custom model. That way there's no need for your GUI to know anything about your worker.

    Unless you keep your worker and model separated and then you add specific APIs to your model to communicate with your Worker object.



  • Hi SGaist -

    Yes, I do remember you saying this, but...I don't see how this helps. As long as the view (ie, the widget) is logically separate from the model (the worker and its table), I still need some way of conveying the model information to the widget, don't I?

    Or, are you suggesting that my custom model contain my worker and my widget?


  • Lifetime Qt Champion

    That's the idea: you instantiate the model like any other Qt model and then set it on the widget(s) that will use it.



  • What does it mean to "set it on" the widget(s) that will use it?



  • @mzimmers
    Use the view-widget's method to "attach" it to the model. For example, a QTableView has setModel().



  • How do you make the view aware of the model, though? Is it something straightforward such as an additional argument on the c'tor?

    I get that you use setModel(), but where do you get the argument to put into the setModel() call?



  • @mzimmers
    It's your model. That's all. Like you wrote:

    a Devices object that is derived from QAbstractTableModel.

    http://doc.qt.io/qt-5/qabstractitemview.html#setModel

    You instantiate some QAbstractItemModel. That's all there is to it.



  • I can tell that this is another one of those times where I'm being extra-dense. Let me try to pose my question like this -- in the tutorial, there's this example:

    int main(int argc, char *argv[])
    {
        QApplication a(argc, argv);
        QTableView tableView;
        MyModel myModel(0);
        tableView.setModel( &myModel );
        tableView.show();
        return a.exec();
    }
    

    This is straightforward enough, because tableView is a top-level object. In my case, however, it's a part of ui, a (private) member of my widget object. I can't access it from main().

    class Devices : public QAbstractTableModel
    {
    private:
        ModelData devices;
    ...
    }
    class Worker : public QObject
    {
        Q_OBJECT
    private:
        Devices devices;
    ...
    }
    int main(int argc, char *argv[])
    {
        int rc;
        QApplication a(argc, argv);
        Widget widget;
    
        QThread* thread = new QThread;
        Worker* worker = new Worker();
    ...
    }
    

    So...am I doing it wrong? Or, should I just obtain the address to devices from the worker object, and pass it to the widget object at construction? Or, something else?



  • @mzimmers
    Now I'm really jumping in here, because I don't know what's going on. At the risk of sticking my neck out....

    Since @SGaist wrote:

    As suggested in your other thread, your Worker class should rather be a member of your custom model. That way there's no need for your GUI to know anything about your worker.

    So your Worker would be a member of your Devices. In your main, you don't directly create a Worker, you create a Devices (which creates its won Worker), and you create your view-widget. Then you set the view's model to your Devices instance.



  • Hi Jon -

    No worries about jumping in; I'm quite new to many of these concepts and appreciate all feedback.

    I'm still confused about the design that SGaist has proposed. In my (admittedly limited) Qt experience, I'm accustomed to the main() routine creating the worker and the widget objects, and using the signal/slot mechanism to connect them.

    I understand that SGaist's proposal entails main creating objects from Devices and Widget, and Devices would contain Worker. (I don't yet understand the rationale for this, but this hopefully will become clear as I go along.) What I don't understand, is how this solves the issue of conveying the model from Devices to Widget. Specifically, in Widget::Widget(), I know I should have a line like:

        ui->tableView->setModel(???);
    

    My question is WHAT should replace the "???" above? How do I tell my widget what/where the model is?

    This seems like a really simple question, so I'm probably not asking it well. Hopefully someone will get what I'm trying to say here.



  • @mzimmers said in use of model/view programming:

    ui->tableView->setModel(???);
    

    My question is WHAT should replace the "???" above? How do I tell my widget what/where the model is?

    Hello mzimmers ,

    Actually your first question almost answers the second one. I mean, you literally tell to your "Item view", or in other words, any widget inheriting from QAbstractItemView, like QTableView, that the model is the one given with setModel().

    ui->tableView.setModel( &myModel );
    

    From this point, the table view will access data from the model, by calling QAbstractItemModel::rowCount(), QAbstractItemModel::columnCount() and QAbstractItemModel::data(), which you must override when inheriting from QAbstractTableModel. These methods are called in standard C++ way, not using signal/slot mechanism, because the view needs the value at the instant is asks for it.

    You can also "connect" (using setModel(), not in a signal/slot way) multiple views to a single model. So the view(s) and the model must be in the same thread, otherwise, you may have, and you will certainly have, memory access errors.

    This doesn't prevent usage of a worker, inside the model, to handle heavy tasks, like populating or sorting your model and sending results using signals, but to provide the data to the view, you must be in the same thread.

    The only way I can see to achieve this design, is to make your own table view and you own model, by reimplementing QAbstractItemView and QAbstractItemModel, with your own mechanism supporting a model in a different thread.



  • @Gojir4 said in use of model/view programming:

    Actually your first question almost answers the second one. I mean, you literally tell to your "Item view", or in other words, any widget inheriting from QAbstractItemView, like QTableView, that the model is the one given with setModel().
    ui->tableView.setModel( &myModel );

    OK, let me ask the question this way: this would be my Widget c'tor:

    Widget::Widget(QWidget *parent) :
        QWidget(parent),
        ui(new Ui::Widget)
    {
        ui->tableView->setModel(&myModel)
        ui->setupUi(this);
    }
    

    This of course won't compile, because myModel is undefined within the Widget class. Where do I get it from?


  • Lifetime Qt Champion

    Make it a member of your class.

    Or add a method to Widget that takes a model in parameter and sets it on the view.



  • OK, now we're getting somewhere. Can I simply pass in a pointer to it in the c'tor, then?


  • Lifetime Qt Champion

    That's another possibility yes.



  • @mzimmers
    Yes, if you'd like to do it there, as per @SGaist 's example.

    Is your confusion that you don't know that you can, for example, make your example Widget constructor take extra parameters beyond what the base QWidget() takes? You can pass whatever additional stuff you like to your constructor if you wish to write it like that.



  • Thanks, guys. No, my confusion wasn't using the c'tor, it was whether manually passing a pointer to the model, into the display widget, was the right way to do this. I'm going to go implement this, and will return with my next confusion in a bit. Thanks again...

    UPDATE:

    Believe it or not, I have it working. (Still haven't implemented SGaist's design suggestion but I will.) So, I think the next thing to do is to implement my override of insertRows()/beginInsertRows().

    So, is the correct sequence in insertRows() to:

    1. call beginInsertRows() and add a row
    2. add the data to be inserted to my private copy of the data
    3. call endInsertRows()

    In other words, will the insertRows() function I write will modify my copy of the data (the model)?

    Also, beginInsertRows() has an argument const QModelIndex &parent. What is this, and where do I get it from?

    Thanks...



  • I've used the address book example as a guide, and it's sort of working. I can successfully add a row, and populate its contents. But when I try to update a row (actually just one column in the row), the update doesn't show in the widget.

    Here's my update code...am I forgetting a window refresh or something? I don't see anything like that in the address book example.

    Thanks...

        // update the row. Easiest to just do all fields.
        deviceTable[row.Srow] = device;
        QModelIndex id = index(row.Srow, 0, QModelIndex());
        setData(id, device.macAddr, Qt::EditRole);
        id = index(row.Srow, 1, QModelIndex());
        setData(id, device.devName, Qt::EditRole);
        id = index(row.Srow, 2, QModelIndex());
        setData(id, device.latestHB, Qt::EditRole);
    
        //emit a signal to make the view re-read identified data.
        QModelIndex topLeft = createIndex(0, 0);
        row.Urow = deviceTable.size();
        QModelIndex bottomRight = createIndex(row.Srow, NBR_COLS_IN_TABLE);
        emit dataChanged(topLeft, bottomRight);
    

    Edit: it was pointed out that my row and column values were off by 1 each; I've corrected that (not reflected in the code above), and the behavior is unchanged.

    The field should update about once a second, but it updates whenever it loses or gains focus. Any suggestions are appreciated.


  • Qt Champions 2017

    If I recall correctly, dataChanged should be enough to trigger the view to update. @VRonin can you work your magic here, what are we missing?



    • createIndex(); should be called only from index()
    • You never mentioned where you are potting the code above. In what method is it?

    But most importantly:
    model and view can not live on 2 different threads. The view will call methods from the model directly and that's a race condition



  • @VRonin

    1. noted (and changed) about createIndex().
    2. the code in I posted is part of an update routine. When this program receives a message from the target device, it updates the UI to reflect the information in the message. So, this code is in a method in my subclass of QAbstractTableModel.
    3. I wasn't aware that model/view had to be in the same thread. This will require some attention on my part.

    Thanks...



  • @mzimmers said in use of model/view programming:

    the code in I posted is part of an update routine. When this program receives a message from the target device, it updates the UI to reflect the information in the message. So, this code is in a method in my subclass of QAbstractTableModel.

    If that's how you update your model, why are going through the hell that is subclassing a model rather than just using QStandardItemModel?



  • @VRonin: I'm new to model/view concepts, and needed an example. I used the address book example, which may not have been the best, but it was all I could find. And it uses the technique I tried to copy. I agree that it's been more trouble than I expected.



  • If you really want to gown that route I recommend chapter 3 of Advanced Qt Programming. There are traps everywhere when you subclass QAbstractItemModel



  • What I really want is to keep this as simple as possible. Somehow, I get the impression that I've been less than fully successful so far in this endeavor.

    The requirements of this project are modest enough: My UI must maintain a small table with information on an handful of devices that are wirelessly connected. Originally it seemed logical to contain the data model within the worker object, but SGaist and other Qt mavens have informed me this would be Doing It Wrong, so I'm looking at changing that. SGaist suggested my worker be part of my model class, but I don't yet understand the benefit of that. Would it be wrong to make the worker, widget and model classes all peers (and created in main())?

    At this point, I'm not even sure I need threads, as my socket communications can be made entirely non-blocking.


  • Qt Champions 2017

    @mzimmers said in use of model/view programming:

    SGaist suggested my worker be part of my model class, but I don't yet understand the benefit of that.

    Only convenience. The idea is that you could, if needed, connect the worker's signals to object(s) internal to the model and vice versa.

    Would it be wrong to make the worker, widget and model classes all peers (and created in main())?

    Not at all, if you can cleanly do the connects between the components, there should be no compelling reason to have one of the objects be inside the other. Plus as your worker is the root object in a separate thread then it's free-floating, in the sense it can't have a parent, thus you don't really have anything to gain to have it anywhere but at the point where you create it and initialize the connects. After that the signal-slot mechanism, provided it was set up correctly, should do everything for you.

    At this point, I'm not even sure I need threads, as my socket communications can be made entirely non-blocking.

    If you are not sure then with 99% probability you don't. As you pointed out yourself most of the things can be used in a non-blocking manner, which lifts the need to have threading in most of the cases.


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