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Creating a calendar using QTableWidget with QItemDelegate

  • Hi,

    I coded a little calendar using a QTableWidget. When the user edits a QListWidget (every row is an hour of the corresponding day), the text is automatically stored into the corresponding QTableWidgetItem.

    Well, after storing three or more things into the QListWidget, the data in the QTableWidget cell is going to look really chaotic. Also, I'd like to have some more stuff stored into the table cells, like little pictograms on the right top corner of the cell.

    My aim is to filter the data in the to-do list (QListWidget) and only list the first three points in the corresponding table cell. These three strings should be listed vertically as a block and the block itsself should be aligned horizontally left and vertically centered.
    On the right top corner of the cell, I'd like to place one ore more pictograms (horizontally listed).

    For these purposes I need something more than just a simple QTableWidgetItem. After googleling for a while, I think that QItemDelegate/QStyledItemDelegate is the thing I need for it.

    My problem now is, I don't have a clue of it. I didn't get it. I think I have to subclass it and reimplement some functions like QItemDelegate::paint(...) and stuff.

    But I don't know how to use them and when they are called and so on... Also I don't know how to realize my purposes with the delegate class.. How can I render the cell like I want it, positioning strings and pictograms in different aligned spaces into the same cell...

    Maybe someone could give me a hint how to do this...

    Thank you in anticipation!

  • Lifetime Qt Champion


    The delegate class is called each time a cell must be painted, or an editor is requested.

    You can find several links to good examples in the details of QStyledItemDelegate documentation

    Hope it helps

  • Hi SGaist,

    thank you for your post. Unfortunatelly, your link did not help me. I already read the documentation, but I can't find any examples. Qt docu tells me that QItemDelegate provides diverse roles for displaying stuff like a role for bitmaps, a role for strings and so on and Qt docu mentions the important functions how to display it.

    My problem is, that I don't know how to position the text/bitmaps together in a cell. How would I have to use the paint(..) function to create a QTableWidget cell with QStrings and pictograms in the following constellation ("-" means empty space):


    I have seen layouts like the one above in many calendars/tables of applications. It is a common layout for table cells and I hope, there will be someone who knows how to realize this. I don't need a complete compilable solution, but I have reached a point where I don't know how to go on, I don't get the information I need while reading Qt's docu or google links where the displaying of QSpinBoxes is explained...

  • Lifetime Qt Champion

    You will be working with drawImage and drawText. So you'll be painting on a rectangle. Calculate the appropriate position and you're good to go.

  • Sounds like something to work with. So the fantastic class I was always looking for is the QPainter! Reading its documentation shows me alle the functionality I was looking for in the QItemDelegate class...

    So, do I have to subclass QItemDelegate/QStyledItemDelegate or can I simply create an instance of QPainter and pass it to my table cells?
    If I do have to subclass it, where should I create the QPainter instance? In the paint() function? I still don't understand how Qt manages the call of paint(...) while creating/updating my QTableWidget... Does any QTableWidget cell has its own QPainter that is passed to the paint(..) function? Does that mean that the paint function is called 50 times if I have a table with 50 cells?
    Sorry, but documentation lacks a bit with the description of delegate handling (through the eyes of an unexperienced user). :-)

  • Lifetime Qt Champion

    You have to subclass QStyledItemDelegate. You receive the QPainter to use in the paint function (it's the first argument of the function)

    The delegate will be called for each cell since each cell can contain something different.

  • Hi and sorry for the belated response.

    Well, I subclassed QStyledItemDelegate now, reimplemented the paint function and tried to display a string on the top left corner of each cell in column 2 of a table. To ensure that only column 2 uses the custom delegate, I set the delegate only for column 2.

    This is how my paint function looks like:
    void testDelegate::paint(QPainter *pPainter, const QStyleOptionViewItem &soviOption, const QModelIndex &miIndex) const
    pPainter->drawText(QRect(0,0,50,20), "TestString");
    QStyledItemDelegate::paint(pPainter, soviOption, miIndex);
    Well, the interesting thing now is, that the strings (5 strings because I have 5 cells in column 2) are all positioned on the top left corner of the top left cell (col 1, row 1). That is suspicious, because the coordinates should be relative for each cell...
    Also , when I manually change the window size, the table and its cells adjust to the window but the strings don't move! Hence, they are not stored into their corresponding cells but hover them as a z-stack... They don't respect cell borders...
    What's the matter with that behaviour??

    Instead, when I use the QRect from the option variable of the argument list of the paint function, I get the results I expected!
    pPainter->drawText(soviOption.rect, "TestString");
    Where is the difference between these two methods and why does the first example position the data "over" the cells without respecting their borders and absolute positioning at the top left corner of the whole table??

  • Lifetime Qt Champion

    Because the same painter is used to paint the whole widget. The option parameter gives you all the information needed to paint one cell.

  • But option.rect also only returns a QRect, doesn't it? So, where is the difference between the following examples:
    painter->drawText(option.rect, "TestString"); // assuming that option.rect returns a QRect(100,50,20,20)

    painter->drawText(QRect(100,50,20,20), "TestString");
    For the compiler, it should be tha same, isn't it?

  • Lifetime Qt Champion

    The rect changes for each cell

  • yeah I know, I catched the coordinates of option.rect for the 5 cells with qDebug:
    x is alway 185, y = index.row * 51, width is always 91 and height is always 50.

    Hence, the following two methods do exactly the same positioning (I tested it):
    painter->drawText(option.rect, "TestString");

    painter->drawText(QRect(185,index.row()*51,91,50), "TestString");

    BUT the difference is, that the first example stores the strings INTO the cells (changing window size also lets the strings move with its corresponding cells). The second example doesn't. In the second example, the strings are layed over the table, they don't belong to a cell and fly over their borders when resizing the window... They are like ghosts!

    Can you explain this behaviour?

  • Lifetime Qt Champion

    Again: option.rect changes for each cell. It will be adapted to changes of column size, row size etc.

  • Oh yes, this makes sense... I forgot the fact that paint() is also called while resizing... ok everything is clear now. Thanks

  • Maybe one last Question:
    Does it make any difference to the appeareance when using a custom item delegate in combination with QTableWidgetItem instead of using the custom delegate with plain cells? Does the custom delegate also effect to QTableWidgetItems or will they use their own display methods?

  • Lifetime Qt Champion

    No, these are just items holding data, the delegate will act just the same

  • Ah, I see. So a delegate is really just a layer that is not interested in data that is already stored into a table. So I think it will not be possible for me to place data via delegate that should be clickable like a link, right? The click would go "through" the delegate and would only hit the underlying QTableWidgetItem...

  • Lifetime Qt Champion

    eventFilter comes to mind to handle that

  • alright, thank you for your live support! I think I can handle the rest by myself.


  • Well, maybe I have one more question:
    In my code, I subclass QItemDelegate but I only reimplement the paint function. So the editor behaviour should be the same as in default delegate mode.

    What I'm asking myself now is, how the delegate (which, in my eyes, can be seen as a layer of data over the table) interacts with QTableWidgetItems. For a example, I inserted QTableWidgets in every table cell with a text "itemText". Also, I used the custom QItemDelegate and let the paint function paint a text "delegateText" over every cell.
    The result is (as expected), that the "delegateText" is displayed over the "itemText". So the delegate is like a layer that is not interested in what the cell does.

    BUT, what happens if I try to edit the QTableWidgetItem by double click? As expected, an editable cursor appears. But is this now the editor of the QTableWidgetItem or is it the editor of the QItemDelegate ?? And what would happen if I disable one of them?

  • Lifetime Qt Champion

    The QTableWidgetItem is essentially a data container, it doesn't know about any editor.

    It's the delegate that uses a factory that will return the corresponding editor based on the item content type.