TreeView, how to status on a row?

I am developing an application in Qt (5.10), Linux. It constists of C++, Qml and JavaScript.

I have successfully made a tree structure using TreeView which has clickable rows.
When double clicking on a row, onDoubleClick is handled and a call is made to C++ code to fetch data.
The data is then inserted into the model and then the row will have a + at the beginning of the row.
When clicking the + the row is collapsed and data is visible, can be a few lines or more.
A selected row has by default blue color. The text is black.

All this works perfect but now I have an intention to show when data could not be read, I want the row to be red to indicate failure. The row should stay red after this.
Another thing I want to do is to have some kind of indication when fetching data is ongoing, maybe the row could be purple and when finished loading turn to blue again.

I have tried branchDelegate, rowDelegate, but it seems impossible to make theses changes.

I would be very happy is someone could give me som piece of advice how to go further.

Have you implemented the Qt::BackgroundRole and/or Qt::ForegroundRole cases for your model's QAbstractItemModel::data() method to return a QBrush with the appropriate color to indicate the status?

Thanks for your response.

No I have not done such. Are you confident that it should work using these?
To be hounest, I have no knowledge about how to handle them.

I want to exaplain a little bit more about my stuff.

Here are some pseudo lines from my application.

Code:

---

TreeView {
        id: treeview
        anchors.fill: parent
        model: treemodel
        style: TreeViewStyle {
            branchDelegate: Rectangle {
                //Only activated when a row is populated with children
                width: 15; height: 15
                Image {
                    source: styleData.isExpanded ? "images/minus.svg" : "images/16px/plus.svg"
                }
.
.
.
    onDoubleClicked: {
        //Here is code which calls C++ to get data for children for a specific row (parent)
        //If data was found, it is inserted as children into the model 
    }

---

Only when data is inserted as children branchDelegate is activated and the row will have a +.
When you click on that + it turns to - and children are visible on several rows. Works very well.

But it seems to be impossible to accomplish what I am looking for:
At double click, if no data is found mark the row with a different color or similar, maybe a red * instead of - or +.
In this case branchDelegate is not activated and I am unable to change branch image.

And the other thing am I looking for a solution to, as earlier written, indicate loading of data, that is for example turn row color to purple during loading.
Also seems impossible to achieve.

You said that you wanted the color of the row to indicate the status of that row in some way. It sounds like you have implemented a model for your tree in C++ and you have probably derived it from QAbstractItemModel. When you do that, you have to implement the data() method to return what should be displayed in the tree.

At a minimum, you have to respond to the Qt::DisplayRole value for the "role" argument to data(). If you look at the documentation for other roles, you will see that there are many of them that you can use to change foreground and background colors, icons, check boxes, fonts, and so forth.

This has nothing to do with double-clicking. It simply tells the tree view how to display everything about a row's content. So if you want to color-code the rows based on some kind of status, you have to implement either the ForegroundRole or BackgroundRole case for the data() call, and choose the right color to return (as a QBrush) based on the status.