将元素附加到 ListModel 后如何更新 QML ListView？

Question

I have a ListView in my QML:

ListView {
    id: listView1
    width: parent.width
    height: parent.height
    interactive: true
    clip: true
    visible: true

    property int elementH: 200

    Component.onCompleted: {
        mSingals.setPlotList(listView1);
    }

    function addImage(src) {
        listModel.append({"imageSrc": src});
    }

    function clear() {
        listModel.clear();
    }

    function redraw() {
        // ?????
        //listView1.update();
    }

    model: ListModel {
        id: listModel
        objectName: lModel
    }

    delegate: Rectangle {
        id: delegateItem
        width: listView1.width; height: listView1.elementH
        color: "blue"
        Image {
            anchors.left: parent.left
            source: imageSrc
            visible: true
        }
    }
}

At first I add some items using an image provider and function addImage(src) :

QVariant qv(QString("image://plots/").append(imageName));
QMetaObject::invokeMethod(plotList, "addImage", Q_ARG(QVariant, qv));

Everything works fine, the ListView updates automatically. Then I clear all items from the ListModel and add some new ones:

QMetaObject::invokeMethod(plotList, "clear");
QVariant qv(QString("image://plots/").append(someNewImage));
QMetaObject::invokeMethod(plotList, "addImage", Q_ARG(QVariant, qv));

The ListView doesn't update even with something like:

QMetaObject::invokeMethod(plotList, "redraw");

So I have to scroll up and down several times to make old elements disappear and new ones appear, but anyway the first element of 20 remains the same, whatever I do.

What should I use in redraw() to force the ListView to update? Maybe somehow emit a dataChanged signal of ListModel ?

Answer 1

Thank you for your answers! In my case the issue was in images' names. The names remained the same, and, though the image provider could provide new images by this names, the ListView didn't request the new images. So now I just change the names for every update.

Answer 2

Well, you shouldn't need to do anything to get the correct behavior. I suppose the problem has to do with doing things from C++ which happens in a blocking manner and doesn't give the objects the time for events to be processed.

Try using a queued connection for invokeMethod() .

QMetaObject::invokeMethod(plotList, "addImage", Qt::QueuedConnection, Q_ARG(QVariant, qv));

However, accessing QML functions from C++ spells "bad design", you shouldn't really be doing that, QML should access C++, not vice versa. If you need to compose the actual strings in C++ that much, you can use a signal and emit the string with it, and connect it to QML handlers, this way you will interact with the model from QML.

Answer 3

This should be solved by setting the cache property of the QML Image to false. From the Image caching section of http://doc.qt.io/qt-5/qquickimageprovider.html :

Images returned by a QQuickImageProvider are automatically cached, similar to any image loaded by the QML engine. When an image with a "image://" prefix is loaded from cache, requestImage() and requestPixmap() will not be called for the relevant image provider. If an image should always be fetched from the image provider, and should not be cached at all, set the cache property to false for the relevant Image