Unsolved Reordering rows of QTableView with drag and drop
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@kshegunov said in Reordering rows of QTableView with drag and drop:
Yes, but the UI doesn't tell the model "move this row", it tells it, "hey I have this thing, please do what it says inside".
Ah yes, you're right. And maybe where the API is lacking. For example, what is the significance of of a
MoveAction
if one doesn't know where the data is being moved from? In fact the default ItemModels implementations pretty much ignore theaction
attribute indropMimeData()
except to validate that's it's not a Link or no-op. And then the UI Views try to compensate for that. I haven't used the default behaviors in so long, I forget how limited they are.Instead I like to encode meta data about the source data (which is being dragged/copied) in
mimeData()
, typically as a "header" of another MIME type, or sometimes the data itself has enough details. Then the "canDrop" and "drop" methods know exactly "move this row," like you said. Not some ambiguous "here's some data, plop it in." In fact if data is only going to be moved/copied within the same model (no external destinations), one doesn't even need to encode the data itself, just where it currently lives in the model (itsQ[Persistent]ModelIndex
, essentially, or some other unique record ID which is already common in structured data, like a database), which is also much more efficient and easier compared to the default routines. OTOH if the data may move between model instances, then again the destination model can know that the source is external, in which case a "move" operation becomes an insert/replace operation.Personally I think none of this should be handled by the UI at all. Whether the model itself should handle it, or some other helper/proxy class is an implementation detail (personal taste, needs, etc). That's really the main point I was questioning in my original reply to Rodrigo. I don't want to have to create a custom UI for each data model's preferred behavior, after all, I'd rather centralize that functionality, just like the data model is centralized.
The one place this breaks down somewhat is when moving data out of one model and into another. The source model needs to be notified that the data can be safely removed. So the UI, or some other mechanism, does need to do that. But this is pretty basic, eg. if the remote drop (not onto itself) event was accepted as a move, then tell the local model it's OK to delete that data. What is harder is to coordinate cut/paste operations, since there's nothing like a
QDrag
to monitor.To clarify, I'm talking about manipulating well-structured data sets. If one wants to, say, drop some selected text onto a field and have that text become the new value, that's really a whole different thing (and simpler, I think). Or arbitrarily copy data between models, including adding columns if necessary, which is what using all the built-in defaults basically allows.
The moment you start pushing unsanitized user-supplied data over opaque datatypes to a model, that's the moment you break any three-tier validation you may've hoped to achieve. If the data comes through a network channel, you'd not push it directly to the model, be it mime-compatible or not, would you?
I think this is veering even further off into OT territory... :-) From a data manipulation perspective, cut/copy/paste is just like DnD but with a clipboard object. If one is concerned about validating the MIME data, then that is a factor regardless of how the data is originally obtained (to varying degrees, perhaps, eg. local clipboard vs. Internet stream :-). I certainly don't want any UI component to do that, but again where/how/if this happens is an implementation detail up to the author(s). The validation needs of any particular data/project may vary from just checking the MIME type to running it through a virus checker... who knows.
Best,
-Max -
@Max-Paperno said in Reordering rows of QTableView with drag and drop:
The one place this breaks down somewhat is when moving data out of one model and into another.
Or when the consumer of the model wants to handle input differently, be it a View (i.e. UI element) or something else.
OTOH if the data may move between model instances, then again the destination model can know that the source is external, in which case a "move" operation becomes an insert/replace operation.
Data moves between the consumers of the data storage (in this case that'd be the view), not between the actual storages (which is the model in this particular case) is my main point. You wouldn't just expect copying a MySQL data file into a PgSQL engine to simply work, you have that boundary in between them through which you channel all inputs and outputs.
Ah yes, you're right. And maybe where the API is lacking.
Yes, I think I am, but this whole discussion is really irrelevant, simply because that stuff ain't gonna change any time soon. So whatever the "verdict" is, it means nothing in the grand scheme of things.
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Well, maybe it's semantics, but to me data moves between storage containers, based on how it is manipulated via the UI, and preferably with integrity checks in the actual storage mechanism before it is written there. Not "between consumers." That's how actual structured DBs are designed, anyway. The "UI" I would edit or run queries in to transfer data between DBs knows nothing about the actual data, nor should it. And just because I'm telling it to "channel" some data from some other DB (using my queries), doesn't mean the destination should just blindly accept that data (eg. even in well-formed data there may be possible relational constraints and other minutiae).
I've been designing DBs and various UIs to interact with them for 25 years, so I think I have a pretty good handle on it by now, or at least what works for me. IMHO, the UI should be as oblivious to the data as possible while still providing a good/appropriate UX. In fact I try to drive the UI with the data (or rather meta-data) as much as possible. Like the QtDesigner property editor, for example (some interesting code there if one were ever inclined to check it out). Because let's face it, who wants to re-write the same basic CRUD UI over and over. This is what the Qt views try to provide, an abstract UI which can be somewhat controlled via meta data from the data/model via an agreed-upon API. And they're relatively easy to customize further, as needed.
Anyway I would also like to point out that the Qt Item model classes do not necessarily need to store any data ("data" is not even in the name ;-). In fact one could argue that they shouldn't, and if one needs persistent data, then they obviously just can't. They're already interfaces to the data, not the actual data. The most obvious examples of this are the
QSQL*Table
models, or theQFileSystemModel
. Also proxy models, which are clearly just further interface abstraction, not actual data.Cheers,
-Max -
@Rodrigo-B said in Reordering rows of QTableView with drag and drop:
Actually, if I keep the method but replace the delayed row selection by selectRow(m_dropRow) (as shown below), I get the anomalous behavior. If I remove this line, things work properly but without selecting the moved row afterwards.
It seems the delayed row selection does not work in the Mac (makes sense since probably different OSs deal with events differently). Then the row selection happens before the dropMimeData and changes the targeted row, causing the bizarre behavior we observed.Firstly, sorry about hijacking your thread.
Second, I think you're right and the queued event is delivered to the view object "too soon" on the Mac. This sort of invocation doesn't come with any promise of linearity. Instead you could try replacing the
invokeMethod()
part inMyTableView
with a single-shot timer (tested and works here, Win10 MSVC17 Qt 5.14.2):// mytableview.h #include <QTimer> ... void dropEvent... { .... QTimer::singleShot(1, this, [this]() { selectRow(m_dropRow); }); }
The
1
is milliseconds and should be enough, but 10ms won't be noticeable to a user either.The whole thing seems like an awkward workaround, but does seem to work. If I think of anything better, I'll let you know :-)
HTH,
-MaxPS. what was with the
and
s intablemodel.cpp
if()
clauses? Does that actually build on something w/out errors? Just curious! -
@Max-Paperno said in Reordering rows of QTableView with drag and drop:
Well, maybe it's semantics, but to me data moves between storage containers, based on how it is manipulated via the UI, and preferably with integrity checks in the actual storage mechanism before it is written there. Not "between consumers." That's how actual structured DBs are designed, anyway. The "UI" I would edit or run queries in to transfer data between DBs knows nothing about the actual data, nor should it. And just because I'm telling it to "channel" some data from some other DB (using my queries), doesn't mean the destination should just blindly accept that data (eg. even in well-formed data there may be possible relational constraints and other minutiae).
This isn't what I meant. The database doesn't spill its data raw, does it? And even if does, nobody's expecting filling in that raw data into another database engine to work. What I meant was, that software's designed as layers over layers, and as data goes trough it gets transformed to prepare it for the next layer.
Firstly, sorry about hijacking your thread.
He specifically asked for this discussion, but if desired I could always fork it.
PS. what was with the ands in tablemodel.cpp if() clauses? Does that actually build on something w/out errors? Just curious!
I disapprove of it, but it's valid c++, every compliant compiler should build it.
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@Max-Paperno said in Reordering rows of QTableView with drag and drop:
The whole thing seems like an awkward workaround, but does seem to work. If I think of anything better, I'll let you know :-)
@Rodrigo-B I think the version below is better. No funky timers and such. And it can preserve multiple selections (if dropping multiple rows). The comments are longer than the code, so hopefully there's enough detail in there, but let me know if anything is unclear. I cut this down to the bare minimum, everything else acts like a default
QTableView
and can be controlled externally.#ifndef MYTABLEVIEW_H #define MYTABLEVIEW_H #include <QTableView> #include <QDropEvent> class MyTableView: public QTableView { Q_OBJECT QItemSelection m_droppedSelection; bool m_dropAccepted = false; public: using QTableView::QTableView; protected: void startDrag(Qt::DropActions supportedActions) override { // Clear the accepted flag, this will be set to true in dropEvent() if the drop was accepted. m_dropAccepted = false; // Note that startDrag() will, essentially, end up calling dropEvent() if/when the drag is dropped. // dropEvent() may or may not accept the drop, which is why we use the m_dropAccepted flag above. // Actually dropEvent() is called by the event processing system, but since all the GUI // stuff runs in one thread anyway, this is essentially the same thing. QTableView::startDrag(supportedActions); // We've now returned from possibly executing dropEvent(). // If the drop was accepted, re-select the dropped rows, which were obtained during dropEvent(). if (m_dropAccepted) selectionModel()->select(m_droppedSelection, QItemSelectionModel::ClearAndSelect | QItemSelectionModel::Rows); } void dropEvent(QDropEvent *e) override { // Clear any stored selections. m_droppedSelection.clear(); // This will determine what the "new" selection will be after a drop is completed. // Connecting to rowsInserted works because when data is dropped on the model it will be // inserted as new rows.. At least that is the default behavior of QTableModel. // We also keep the connection handle to disconnect it later. const QMetaObject::Connection monitor = connect(model(), &QAbstractItemModel::rowsInserted, this, [this](const QModelIndex&, int first, int last) { m_droppedSelection = QItemSelection(model()->index(first, 0), model()->index(last, 0)); }); // Now do the default stuff. Here is where the event may or may not be accepted, // and the new data insert would happen if it was. QTableView::dropEvent(e); // Let startDrag() know we accepted the drop and there's something to re-select. if (e->isAccepted() && e->dropAction() == Qt::MoveAction && !m_droppedSelection.isEmpty()) m_dropAccepted = true; // We probably don't want m_droppedSelection populating every time rows are added, and // definitely don't want multiple connections. disconnect(monitor); } }; #endif // MYTABLEVIEW_H
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@kshegunov said in Reordering rows of QTableView with drag and drop:
PS. what was with the ands in tablemodel.cpp if() clauses? Does that actually build on something w/out errors? Just curious!
I disapprove of it, but it's valid c++, every compliant compiler should build it.
That's insane, I have literally never noticed this used or mentioned anywhere in over 20 years of C[++]! Or maybe I did and blocked it out :-) And after some rudimentary searching through my usual C++ references, I can't find this mentioned anywhere now either. Ironically the only place I did find a reference to it is in the MSVC docs. Where I also see that MSVC requires a special switch to enable this usage. Good on them (for once!). I might have never noticed it this time if it wasn't for that.
Thanks for the education!
-Max -
@Max-Paperno
We debated this a while ago in some thread here. There is at minimumand
,or
andnot
, I think there are some others. This is standard C++, not just MSVC, but I forget which C++ it came into.Actually I think we found it was quite an old C++, not recent, so it's been around a long time. It turned out (can't find the reference[*]) this was not so much to do with making it "readable" (if that's your view), it is actually because some terminals/languages(?) have trouble with the symbol characters in
||
etc. so they allowed words instead...!Recently I saw code posted here by someone in another thread translating to C++ and using
not
, probably because s/he thought that was normal for C++. I shuddered ;-)[*] OK, it's to do with "digraphs" & "trigraphs": see https://en.wikipedia.org/wiki/Digraphs_and_trigraphs, https://dev.to/delta456/modern-c-and-or-and-not-as-boolean-operators-2jgf, and others.
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For funsies, and to support my PoV that the item model is the proper interface to the data ;-), here's an option to toggle the drop behavior between move and overwrite using only the item model. It's @kshegunov 's solution but with a toggle switch to control the behavior. Items can still be dropped between lines to move them, in either mode. Most importantly, the
QTableView::dragDropOverwriteMode
remainsfalse
regardless (more on this below, after the code break).In
tablemodel.h
I added:public: using QAbstractTableModel::QAbstractTableModel; // for proper constructor bool m_dropOverwriteEnabled = false; // could be private void setDropOverwriteEnabled(bool enable = true) { m_dropOverwriteEnabled = enable; } // a "slot"
In
tablemodel.cpp
,dropMimeData()
becomes:bool TableModel::dropMimeData(const QMimeData *data, Qt::DropAction action, int row, int /*column*/, const QModelIndex &parent) { QModelIndex newParent; // If dropping onto another item and overwrite mode is disabled, // move the dropped data to the next row down (as if it was dropped // between rows). if (!m_dropOverwriteEnabled && parent.isValid()) row = qMin(parent.row() + 1, rowCount()); else newParent = parent; // ensure drops on empty space in the view go into the last row if (!newParent.isValid() && row == -1) row = rowCount(); return QAbstractTableModel::dropMimeData(data, action, row, 0, newParent); }
Then connect some toggle trigger like your checkbox to the
setDropOverwriteEnabled()
"slot."
Here's a simplifiedMainWindow
widget which brings all the above together (including the previousMyTableView
):#ifndef MAINWINDOW_H #define MAINWINDOW_H #include "tablemodel.h" #include "mytableview.h" #include <QWidget> #include <QCheckBox> #include <QTableView> #include <QVBoxLayout> class MainWindow : public QWidget { Q_OBJECT public: MainWindow(QWidget *parent = nullptr) : QWidget(parent) { setWindowTitle("Drag and Drop app"); MyTableView *tableView = new MyTableView(this); tableView->setSizeAdjustPolicy(QAbstractScrollArea::AdjustToContents); tableView->setSelectionMode(QAbstractItemView::ExtendedSelection); tableView->setSelectionBehavior(QAbstractItemView::SelectRows); tableView->setDragDropMode(QAbstractItemView::InternalMove); tableView->setDragEnabled(true); tableView->setAcceptDrops(true); tableView->setDropIndicatorShown(true); tableView->setDragDropOverwriteMode(false); TableModel *model = new TableModel(this); tableView->setModel(model); QCheckBox *dropOverwriteModeCheckbox = new QCheckBox("Drop overwrite mode", this); connect(dropOverwriteModeCheckbox, &QCheckBox::toggled, model, &TableModel::setDropOverwriteEnabled); QVBoxLayout *layout = new QVBoxLayout(this); layout->addWidget(tableView); layout->addWidget(dropOverwriteModeCheckbox); } }; #endif // MAINWINDOW_H
Now regarding
QTableView::dragDropOverwriteMode
, when set totrue
the view just does strange things. For one thing one can no longer drop between rows for some reason. And dropping on top of another row acts differently depending on if one is moving an item down vs. up. Moving up sorta works but creates a new blank record at the bottom. Moving down doesn't work at all. Honestly I have no idea what this is supposed to do, nor do I remember ever actually having it enabled on a view with DnD. Even the docs seem sorta confused on the subject:If its value is true, the selected data will overwrite the existing item data when dropped, while moving the data will clear the item. If its value is false, the selected data will be inserted as a new item when the data is dropped. When the data is moved, the item is removed as well.
Sounds pretty vague to me. And nothing about not being able to drop between items (I think?). Maybe there's some combination of flags and other model or view settings which are needed for this to actually work in some sensible fashion.
Anyway, HTH,
-Max -
@Max-Paperno said in Reordering rows of QTableView with drag and drop:
For funsies, and to support my PoV that the item model is the proper interface to the data ;-), here's an option to toggle the drop behavior between move and overwrite using only the item model. It's @kshegunov 's solution but with a toggle switch to control the behavior. Items can still be dropped between lines to move them, in either mode. Most importantly, the QTableView::dragDropOverwriteMode remains false regardless (more on this below, after the code break).
Thanks for the code. However it doesn't address @Rodrigo-B's question (which we both missed in fact):
@Rodrigo-B said in Reordering rows of QTableView with drag and drop:
Would it be possible to obtain the same behavior but overriding view methods only?
So, I'd say yes, but you have to completely reimplement the
dropEvent
handler in the view and not delegate to the model. -
@kshegunov said in Reordering rows of QTableView with drag and drop:
So, I'd say yes, but you have to reimplement the dropEvent handler in the view and not delegate to the model.
Agreed. Which wouldn't be so bad, except the part one really wants to re-use is private (
d->dropOn()
) which further calls some other private utility methods and also updates a private member variable (dropIndicatorPosition
). Everything except setting that private member could just be copied over (I think, and I"m not sure what the fallout would be from not settingdropIndicatorPosition
properly), but... blech. I get why stuff should stay private (ind
), but something likedropOn()
(and a few other utilities) could be protected members of the main class w/out much danger of breaking BC down the line.-Max
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Which gets me to my original position. I'd rather have this whole d&d ripped out of the model, and probably out of the view and put inside a delegate (i.e. one that's aggregated) which handles the logic and propagates down to the model how and what should be modified. But as said, such things aren't happening any time soon.