displaying only part of model

mzimmers

Hi all -

I have a C++ model that is exposed to QML. I access the model in what I believe is in typical fashion:

GridView {
    model: zoneModel
    delegate: ZoneTile {
        titleText: model.name
        ...

There are places in my application where I'd like to exclude the first element in the list that the model returns.

First question: would it be preferable to accomplish this in the class that supports the model (say with a special function that slices the list), or in the QML? My sense is that the latter is preferable, but I can't figure out how to do it.

Making it invisible isn't enough, because it still takes up space. I could set its height and width to 0, and null out all its contents, but that seems kind of like a hack. Is there a better way to do this?

Thanks...

JonB

@mzimmers
I know nothing about QML. But if you have a source model and you want to alter it by, say, omitting the first row (element in list) it is trivially easy to do that by interposing a QAbstractProxyModel between the source model and your QML/view. Subclass QIdentityProxyModel (which passes through its source model unaltered) and have it (a) return base.rowCount() - 1 for its rowCount() override and (b) override mapToSource()/mapFromSource() to add/subtract 1 to/from the row number.

mzimmers

@JonB hey Jon - thanks for the suggestion. I've never really used proxy models before, but your suggestion prompted me to read a little more about them, and I can definitely see how they could also be useful in this situation.

I also decided, however, that what I'm trying to do is dumb, so I'm going to take a different approach. Basically I need to use the zones in the list as a filter for related items. I was trying to display them in a tab bar, but that's looking like a lousy idea, so I'm going to find a different way of displaying them.

Thanks for the suggestion, though; I'll be using it in the future.

mzimmers

And...the future is here!

class ZoneProxyModel : public QSortFilterProxyModel {
    Q_OBJECT
public:
    explicit ZoneProxyModel(QObject *parent = nullptr);
protected:
    bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const override {
        bool rc = true;
        if (sourceRow == 0)
            rc = false;
        return rc;
    }
};

Works perfectly. Thanks again, Jon.

SGaist

@mzimmers hi,

You could even make it simpler:

class ZoneProxyModel : public QSortFilterProxyModel {
    Q_OBJECT
public:
    explicit ZoneProxyModel(QObject *parent = nullptr);
protected:
    bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const override {
        Q_UNUSED(sourceParent);
        return sourceRow != 0;
    }
};

mzimmers

@SGaist maybe it's because I'm old school, or maybe it's because I have enough trouble reading other people's code, but...whatever the reason, I prefer to explicitly derive a function's return value, assign it to a variable, and return that variable. I realize it accomplishes nothing, and in the absence of a good optimizing compiler, is actually fractionally slower, but...it's just how I roll.