How to remove items of c++ model from within qml delegate?

[flashmozzg]

My model is subclass of QAbstractListModel
What methods should I implement? I guess it's deleteRows but how do I "call" them inside of qml code? Especially if I don't have access to my model (or it's methods because of some strange qt magic which turns my model to QQmlDMAbstractItemModelData which has all the rolls but no methods, and I don't want to store my model as property in all qml delegates)
I mean then I do something like role = data it implicitly calls setData, when I just use role it uses data(). But how to make it remove current item (at index)?

The only sort of broken solution I found is to have "somemethod" var property in all of my delegates and to set it to my model desired methods while I still have direct access to y model.

(btw, simply trying to iterate over all "model after binding" properties with for in leads to a crash via assert)

[wysota]

Can you prepare a minimal compilable example demonstrating that you don't have access to methods defined in the C++ model?

[flashmozzg]

Can you prepare a minimal compilable example demonstrating that you don't have access to methods defined in the C++ model?

Well, that's probably not exactly the case. I CAN access them while I have DIRECT access to my model. But I can't or don't know how I can access them inside of a delegate.
For example I do something like

engine.rootContext()->setContextProperty("testModel", &model);

engine.rootContext()->setContextProperty("testModel", &model);

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And then in qml I can call my "testMethod()" via "testModel.testMethod()" (if it's marked as Q_INVOKABLE of course).
But if I have something like

MyCustomDelegate {
    model: testModel
}

MyCustomDelegate {
    model: testModel
}

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and in MyCustomDelegate.qml I write model.testMethod(), testMethod or something I get that this property is not a function.
So right now I have something like this:

MyCustomDelegate {
    model: testModel
    testMethod: testModel.testMethod
}

MyCustomDelegate {
    model: testModel
    testMethod: testModel.testMethod
}

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where testMethod is var property of my delegate.

[wysota]

This is a matter of proper encapsulation. The delegate should not directly invoke methods on the model. Instead it should signal its actions to its environment where it can be intercepted by something which has direct access to the model.

ListView {
  model: someModel
 
  delegate: MyItemDelegate {
    onDeleteRequested: someModel.deleteRow(index)
  }
}

ListView {
  model: someModel

  delegate: MyItemDelegate {
    onDeleteRequested: someModel.deleteRow(index)
  }
}

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[flashmozzg]

This is a matter of proper encapsulation. The delegate should not directly invoke methods on the model. Instead it should signal its actions to its environment where it can be intercepted by something which has direct access to the model.

ListView {
  model: someModel
 
  delegate: MyItemDelegate {
    onDeleteRequested: someModel.deleteRow(index)
  }
}

ListView {
  model: someModel

  delegate: MyItemDelegate {
    onDeleteRequested: someModel.deleteRow(index)
  }
}

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Alright but is there some standard/built in way to remove (maybe add) elements to model from within the delegate? Just how there is way to read and change items.
(I mean since there already removeRows in the interface, isn't there a more convenient way to call/use it?)

The inconvenience with above method that my view is in separate qml. So I'd have to bind signal from delegate to signal from view.

[wysota]

Alright but is there some standard/built in way to remove (maybe add) elements to model from within the delegate?

No, there are different kinds of model. How would you delete a row from a model defined by an integer (e.g. "10")?

Just how there is way to read and change items.

There is no standard way to modify items if the model doesn't allow it.

(I mean since there already removeRows in the interface, isn't there a more convenient way to call/use it?)

No, there is no built-in method calling removeRows. You should equip your model with one but then you have to call the model directly and not through the data proxy which only gives you access to particular record in the model.

[flashmozzg]

There is no standard way to modify items if the model doesn't allow it.

But if you allow (via flag) - there is. You can simply do this:

roleName = newValue

roleName = newValue

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And it will call setData and such.
Also, standard qml ListModel has append and remove methods.
So if removeRows is already virtual and in interface why isn't it "translated" into remove method of the model?

Btw, I have another question related to removing and c++ <-> qml interop.
I've written removeRows like this:

bool MyModel::removeRows(int row, int count, const QModelIndex &parent)
{
	if (count == 0 || mItems.isEmpty())
		return true;
	beginRemoveRows(parent, row, row + count - 1);
	for (int i = row; i < row + count; ++i)
	{
		AbstractItem *item = mItems.takeAt(row);
		delete item;
	}
	endRemoveRows();
	return true;
}

bool MyModel::removeRows(int row, int count, const QModelIndex &parent)
{
	if (count == 0 || mItems.isEmpty())
		return true;
	beginRemoveRows(parent, row, row + count - 1);
	for (int i = row; i < row + count; ++i)
	{
		AbstractItem *item = mItems.takeAt(row);
		delete item;
	}
	endRemoveRows();
	return true;
}

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But when I delete item it doesn't update view of my model! How to do this? I thought begin/endRemoveRows should be enough.

[wysota]

But if you allow (via flag) - there is. You can simply do this:

roleName = newValue

roleName = newValue

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And it will call setData and such.

If there is a "setData" behind the model

Also, standard qml ListModel has append and remove methods.

Exactly. The methods are defined in a particular model, they are not some general functions that work on every model, just this one.

So if removeRows is already virtual and in interface why isn't it "translated" into remove method of the model?

It is not a slot so it is not visible from within the script. Besides the API would not make sense then (e.g. QtQuick doesn't use QModelIndex).

I thought begin/endRemoveRows should be enough.

It is enough. If it doesn't work then you probably didn't call it correctly. For instance your for loop completely ignores the "parent" but beginRemoveRows() doesn't.

[flashmozzg]

If there is a "setData" behind the model
Exactly. The methods are defined in a particular model, they are not some general functions that work on every model, just this one.
It is not a slot so it is not visible from within the script. Besides the API would not make sense then (e.g. QtQuick doesn't use QModelIndex).

But setData isn't a slot/visible either =)
And it uses QModelIndex too just fine. In ListView case it contains index of the row.

It is enough. If it doesn't work then you probably didn't call it correctly. For instance your for loop completely ignores the "parent" but beginRemoveRows() doesn't.

But parent is empty anyway. It is subclass of listmodel - there are only rows. There shouldn't be any parents (it's by default QModelIndex()).

I even found almost the same implementation in qt docs http://doc.qt.io/qt-5/model-view-programming.html:

bool StringListModel::removeRows(int position, int rows, const QModelIndex &parent)
{
    beginRemoveRows(QModelIndex(), position, position+rows-1);
 
    for (int row = 0; row < rows; ++row) {
        stringList.removeAt(position);
    }
 
    endRemoveRows();
    return true;
}

bool StringListModel::removeRows(int position, int rows, const QModelIndex &parent)
{
    beginRemoveRows(QModelIndex(), position, position+rows-1);

    for (int row = 0; row < rows; ++row) {
        stringList.removeAt(position);
    }

    endRemoveRows();
    return true;
}

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[wysota]

But setData isn't a slot/visible either =)
And it uses QModelIndex too just fine. In ListView case it contains index of the row.

setData() is not exposed directly in QML anywhere so I don't see your point. QAbstractItemModel subclass instance is exposed to QML using a regular mechanism which allows you to modify properties of a QObject or call its slots. The object is not "translated" in any way so there is no (general) way to add artificial methods to the model class unless you implement them as scriptable methods in C++.

But parent is empty anyway. It is subclass of listmodel - there are only rows. There shouldn't be any parents (it's by default QModelIndex()).

As I said, if it doesn't work then you either didn't implement something correctly or you didn't call it correctly. Does it update if you use it with QListView? Does your method even get called?

I even found almost the same implementation in qt docs

It's the "almost" part that worries me

[flashmozzg]

setData() is not exposed directly in QML anywhere so I don't see your point. QAbstractItemModel subclass instance is exposed to QML using a regular mechanism which allows you to modify properties of a QObject or call its slots. The object is not "translated" in any way so there is no (general) way to add artificial methods to the model class unless you implement them as scriptable methods in C++.

My point is: why not have the same mechanism for deleting/adding items as there is for setting items?
Currently if the right flag is passed and setData is implemented you can modify item properties corresponding to roles.
I don't see why you can't in the same they if removeRows/Columns implemented remove items.

As I said, if it doesn't work then you either didn't implement something correctly or you didn't call it correctly. Does it update if you use it with QListView? Does your method even get called?
It's the "almost" part that worries me

My app is in QtQuick so I'm obviously using qml ListView.

Anyway, it seems to be working now for no apparent reason. It was probably some another weird bug with desync of mocs/precompiled files and etc. whic miraculously cures itself after rebuilding/running qmake again.

[wysota]

My point is: why not have the same mechanism for deleting/adding items as there is for setting items?

Because there is no standard way to do this.

Currently if the right flag is passed and setData is implemented you can modify item properties corresponding to roles.

You have fixated yourself on using editable QAbstractItemModel subclasses. This is a miniscule of models used in QML.

I don't see why you can't in the same they if removeRows/Columns implemented remove items.

Because in case of editing you are not accessing the model but rather a special object which internally has C++ access to the model. This special proxy represents a single record in the model. It would be counter-intuitive to equip the record with abilities to add other records to a model or remove other records from the model.

My app is in QtQuick so I'm obviously using qml ListView.

I don't see how that matters. Check the model with a QListView to see if it is updated when you call the method which you say doesn't work if you call it from QtQuick.