Qt 5.2 Model-View-Pattern: How to inform model object about changes in underlying data structure

Question

I have a class used for permanent storage of some item that are organized in a table-like manner. This class is totally unrelated to Qt and comes from a different library. Lets call this class DataContainer for the rest of this question. It provides std-c++ compatible iterators to access and manipulate the content.

I need to display and modify that data through a Qt GUI. My idea was to create a class DataContainerQtAdaptor that inherits from QAbstractTableModel and stores a pointer to the DataContainer object. The DataContainerQtAdaptor serves as an adaptor to the DataContainer object and all manipulation from inside of my Qt app is done through this adaptor. Then I use a QTableView widget to display the information.

Unfortunately the DataContainer might be changed by threads/processes. (For example think of DataContainer as some C++ class that encapsulates a database connection and that database might be changed by someone else.)

Questions:

1) Assume I have a function that is called everytime the internal structur of the DataContainer object has been changed. What is the correct function of the QAbstractTableModel that must be called to inform the model of the underlying change? I need something like "Dear Model, your persistent storage backend changed. Please, update yourself and emit a signal to every attached view in order to reflect this change".

2) Lets say 1) is solved. What is the best way to avoid a "double" GUI update in case the change was triggered through the GUI? E.g: User clicks on a cell in the table widget -> table widget calls setData of the model -> model pushes change to backend -> backend triggers its own "onUpdate" function -> model re-reads complete backend (although it already knows the change) -> GUI is updated a second time

3) The user should be able to insert new rows/columns through the GUI and put data into it. But the position is detemined by this data, because the backend keeps the data sorted. Hence, I have the following problem: The user decides to create a new row at the end and the new data is pushed to the backend. When the backend/model is re-read this data is normally not at the last position, but has been inserted somewhere in the middle and all other data has been moved forward. Ho do I keep all the properties of the the table view widget like "selection of a cell" in sync?

I believe, there must be some simple standard solution to all these question, because it is the same way as QFileSystemModel works. The user selects a file and some other process creates a new file. The new file is displayed in the view and all subsequent rows move forward. The selection moves forward, too.

Matthias

Answer 1

Model Semantics

First of all, you must ensure that the QAbstractItemModel cannot be in an inconsistent state. This means that there are some signals that must be fired on the model before certain changes to the underlying data are done.

There is a fundamental difference between changes to structure and changes to data. Structure changes are the rows/columns of the model being added or removed. Data changes affect the value of existing data items only.

• Structural changes require calling beginXxx and endXxx around the modification. You cannot modify any structure before calling beginXxx. When you're done changing the structure, call endXxx. Xxx is one of: InsertColumns, MoveColumns, RemoveColumns, InsertRows, MoveRows, RemoveRows, ResetModel.

  If the changes affect many discontiguous rows/columns, it's cheaper to indicate a model reset - but be wary that selections on the views might not survive it.

• Data changes that keep the structure intact merely require that dataChanged is sent after the underlying data was modified. This means that there is a window of time when a call to data might return a new value before dataChanged is received by the object that queries the model.

This also implies that non-constant models are almost useless from non-QObject classes, unless of course you implement bridge functionality using observer or similar patterns.

Breaking Update Loops

The Qt-idiomatic way of dealing with update loops on the model is by leveraging the item roles. It's entirely up to you how your model interprets the roles. A simple and useful behavior implemented by QStringListModel is simply to forward the role from the setData call to dataChanged, otherwise ignoring the role.

The stock view widgets react only to dataChanged with the DisplayRole. Yet, when they edit the data, they call setData with the EditRole. This breaks the loop. The approach is applicable both to view widgets and to Qt Quick view items.

Insertion of Data into Sorted Models

As long as the model properly emits the change signals when the sorting is done, you'll be fine.

The sequence of operations is:

1. The view adds a row and calls model's insertRow method. The model can either add this empty row to the underlying container or not. The key is that the empty row index must be kept for now.

2. The editing starts on an item in the row. The view state changes to Editing.

3. Editing is done on the item. The view exits the editing state, and sets the data on the model.

4. The model determines the final position of the item, based on its contents.

5. The model invokes beginMoveRows.

6. The model changes the container by inserting the item at the correct location.

7. The model invokes endMoveRows.

At this point, everything is as you expect it to be. The views can automatically follow the moved item if it was focused prior to being moved. The edited items are focused by default, so that works fine.

Required Container Functionality

Your DataContainer doesn't have enough functionality to make it work unless all access to it were to be done through the model. If you want to access the container directly, either make the container explicitly inherit QAbstractXxxxModel, or you'll have to add a notification system to the container. The former is an easier option.

Your core question reduces to: can I have model functionality without implementing some variant of the model notification API. The obvious answer is: no, sorry, you can't - by definition. Either the functionality is there, or it isn't. You can implement the notification API using an observer pattern if you don't want the container to be a QObject - then you'll need your model shim class. There's really no way around it.

The QFileSystemModel gets notified by the filesystem about individual directory entries that have changed. Your container has to do the same - and this amounts to providing a dataChanged signal, in some shape or form. If the model has items that get moved around or added/removed - its structure changes - it has to emit the xxxAboutToBeYyy and xxxYyy signals, by calling the relevant beginZzz and endZzz methods.

Indices

The most important underdocumented aspect of QModelIndex is: its instances are only valid for as long as the model's structure hasn't changed. If your model is passed an index that was generated prior to a structure change, you're free to behave in an undefined way (crash, launch a nuclear strike, whatever).

The whole reason for the existence of QModelIndex::internalPointer() is your use case of having an underlying, complex-indexed data container. Your implementation of the model's createIndex method must generate index instances that store references to the DataContainer's indices in some form. If those indices fit in a pointer, you don't need to allocate the data on the heap. If you need to allocate the container index storage on the heap, you must retain a pointer to this data and delete it any time the container's structure changes. You're free to do it, since nobody is supposed to use the index instance after a structure change.

Answer 2

From the documentation of method bool QAbstractItemModel::insertRows(int row, int count, const QModelIndex & parent = QModelIndex()):

If you implement your own model, you can reimplement this function if you want to support insertions. Alternatively, you can provide your own API for altering the data. In either case, you will need to call beginInsertRows() and endInsertRows() to notify other components that the model has changed.

The same goes for removeRows() and moveRows() (they have their own begin*() and end*() methods). For modifying data of existing item there's a dataChanged() signal.

Here's how it goes (answer for question 1):

Implement your own methods for inserting/deleting/modifying data, where each of those methods must look like this:

beginInsertRows(parentIndex, beginRow, endRow);
// code that modifies underlying data
endInsertRows();

beginRow and endRow must be provided to inform which where the rows will be inserted and how many of them (endRow-beginRow).

For beginDeleteRows() and beginMoveRows() it's the same.

When you have a method which simply modified data in existing item, then this method must emit signal at the end: dataChanged().

If you do a lot of changes in the data, it sometimes is simpler to just call beginResetModel() and endResetModel() in the method performing this huge modification. It will cause all views to refresh all data in it.

Answer for question 2:

This is up to the View class implementation if it will "double-update". When you enter data in the View, data is sent to the model through one of the edition methods in model (insertRows(), setData(), etc). Default implementation of those methods always use begin*() and end*() methods and so the proper notification signals are emitted by the model. All Views listen to those signals, including the one you used for entering the data, therefore the "double-update" will be performed.

The only way to define this behaviour is to inherit the View and reimplement its protected slots (like dataChanged() and similar) to avoid updating if the value was detected to be provided by this view.

I'm not sure if Qt views do that already or not. Answer to this requires someone more educated in Qt internals, or looking into Qt source code (which I don't have at the moment). If somebody knows this, please comment and I will update the answer.

I think it's not that bad to reload the data from model - it guarantees that what you see is indeed the value from the model. You avoid possible problems with the Editor and the View bugs.

Answer for question 3:

When you reload whole model, then there is no simple way to keep track of selection. In that case you need to ask view->selectionModel() about current selection and try to restore it after reload.

However if you do partial refreshing (using methods I described in answer 1), then the View will track the selection for you. Nothing to worry about.

Final comments:

If you want to edit data from outside of model class, you can do it. Just expose begin*() and end*() methods as public API, so other code that edits data can notify model and views about changes.

While it can be done, it's not a good practice. It may lead to bugs, because it's easy to forget about calling notification everywhere you modify the data. If you have to call model API to notify about changes, why not already move all editing code insise the model and expose editing API?