Model View ViewModel framework for large C++ Qt applications

Overview

This model-view-viewmodel framework is intended for large Qt based scientific applications written in C++. Project was created as a playground toward GUI refactoring of BornAgain project.

Main features of the framework are:

• Application model to store arbitrary data of GUI session.
• Serialization of application models to json.
• Undo/redo based on command pattern.
• View model to show parts of application model in Qt widgets. Depends on Qt.
• Scientific plotting based on qcustomplot.
• Automatic generation of widgets from model content.
• Property editors.
• Flexible layout of Qt's trees and tables.

Requirements

• C++-17
• CMake 3.14
• Qt 5.12

Installation instructions

git clone --recurse-submodules  https://github.com/gpospelov/qt-mvvm.git
mkdir <build-dir>; cd <build-dir>
cmake <source>; make -j8; ctest

# run one of examples
<build-dir>/bin/collidingmice

Example

If you are familiar with Qt's reach example section you might saw it's funny collidingmice example showing basics of QGraphicsScene.

To demonstrate the idea behind qt-mvvm library the code of the example was slightly modified. The mice data has been moved into the dedicated model, the content of the model was shown both in the QGraphicsScene and in QTreeView. It is possible now to save the application state in json file and later load the session back with saved mice positions. Additionally, it is possible to go back in time and watch how mice are moving in opposite directions by dragging a slider:

See short animation here.

The demo shows that qt-mvvm library allows to equip the GUI with the serialization and undo/redo and to provide proper model/view relations via relatively small modifications to the original code. Implementing similar features from the scratch in bare metal Qt would take much more time and the resulting code wouldn't be easily transferable to another project.

This and other examples can be found in examples sub-directory of qt-mvvm package.

The context

Qt, naturally, has model view to manage the data and its presentation. According to some, Qt is misusing the model view terminology and more correct naming would be

• Data -> Model
• Model -> ViewModel
• Delegate -> Controller
• View (that's ok)

Whatever the right terminology is, Qt doesn't tell much about the architecture of complex applications, how to structure the Data and where to put business logic. Qt's model view is rather a way to adapt the user data for Qt's trees and tables and adjust their behavior with delegates.

Given library is an attempt to understand how to deal with the application data.

More explanations

The framework consists of three libraries: libmvvm_model.so, libmvvm_viewmodel.so, and libmvvm_view.so.

libmvvm_model.so defines tree-like structure SessionModel to store any data of the GUI session. This part of the framework is intentionally made Qt-independent. The idea behind is the following:

In large Qt applications, the business logic gets quickly spoiled with presentation logic. Qt classes like QModelIndex start to appear everywhere, even in places that have nothing to do with Qt graphics. Attempt to store GUI session data in QAbstractItemModel leads to inflexible layout in Qt trees and tables. Attempt to fix this with QAbstractProxyModel leads to the appearance of overwhelmingly complicated proxy models. Removing Qt from dependencies allows focusing more on common needs (i.e. objects construction, property editing, etc) of GUI applications rather than on presentation details.

SessionModel has a concept of properties, undo/redo, serialization, and it's own minimal signaling to handle business logic. Thus, the intention here is to build an application model to handle data and logic of GUI while being independent on any particular GUI library.

Strictly speaking, libmvvm_model.so still relies on QVariant but eventually, it will be replaced with std::variant.

Second library, libmmv_viewmodel.so, contains ViewModel and serves as a thin counterpart of SessionModel in the Qt world. ViewModel doesn't own the data but simply acts as a proxy to different parts of SessionModel. It is derived from QAbstractItemModel and intended to work together with Qt's trees and tables. The layout of ViewModel (i.e. parent/child relationships) doesn't follow the layout of the original SessionModel. It is generated on the fly by using strategy who-is-my-next-child provided by the user. In practice, it allows generating Qt tables and trees with arbitrary layouts, based on a common data source, without diving into the nightmare of QAbstractProxyModel. Particularly, the aforementioned machinery allows having something in the line of the ancient Qt property browser framework.

Third library, libmmv_view.so, contains few widgets for plotting and property editing.

Size of the framework

• 20k loc of libraries (libmvvm_model.so, libmmv_viewmodel.so and libmmv_view.so)
• 15k loc of tests
• 10k of user examples

Disclaimer and afterword

The library is intended for large GUI applications. The definition of large is quite arbitrary and means something in the range 20k - 200k lines of code. The main logic here is that using the additional library for smaller Qt applications is redundant, Qt has everything that may be required. If small GUI becomes messy with time, it can always be refactored or even rewritten from scratch.

However, when the number of views to show the same data is getting large, and the GUI enters the range 20k - 200k, this is were a given library might help in proper separation of data, logic, and UI. When the GUI grows even further, well, developers of such large GUI know already what they need and probably have already implemented similar machinery.

The project is under active development.