I'm primarily a .Net developer and have been investigating Qt for a while now. I'm now at the stage of trying to implement the model / view framework in Qt. I think I have a grasp of the basic principles but am unclear of how to hang things together in a more complex UI where widgets need to communicate with each other. Given the following:
// 'domain' model classes
class NestedDomainModel1
{
public:
NestedDomainModel1();
QString name() const;
void setName(const QString& newName);
// other properties
private:
QString m_name;
};
class NestedDomainModel2
{
public:
NestedDomainModel2();
QString name() const;
void setName(const QString& newName);
// other properties
};
class MyDomainModel
{
public:
MyDomainModel();
void addNestedModel1(const NestedDomainModel1& modelToAdd);
NestedDomainModel& nestedObjectModel1At(int index);
int nestedObjectModel1Count() const;
// repeat for model 2
private:
QList<NestedDomainModel1> m_nestedModels1;
QList<NestedDomainModel2> m_nestedModels2;
};
// 'GUI' classes
class MainWindow : public QMainWindow
{
private:
MyDomainModel* m_model;
MyTreeViewWidget* m_treeWidget; // -> this sits in a left dock window
MyInfoDisplayWidget* m_infoWidget; // -> this sits in a right dock window and display details about the item selected in the tree
};
class MyDomainModelTreeModel : public QAbstractItemModel
{
public:
explicit MyDomainModelTreeModel(MyDomainModel* model);
// required overrides for QAbstractItemModel
private:
MyDomainModel* m_model;
};
class MyTreeViewWidget : public QWidget
{
public:
// Take a pointer to the domain model and create a model for the 'view'.
// Will create a tree like:
// Nested Objects 1
// |- object 001
// |- object 002
// |- you get the idea
// Nested Objects 2
// |- other object 001
// |- more of the same
explicit MyTreeViewWidget(MyDomainModel* model);
public slots:
// Used to notify widget when an item is added to the underlying model.
void nestedModel1Added();
void nestedModel2Added();
signals:
void nestedModel1Selected(NestedDomainModel1& selectedModel);
void nestedModel2Selected(NestedDomainModel2& selectedModel);
private slots:
// connect to tree view event when an item is selected and if all ok, emit one of the selected events
void onTreeItemSelectionChanged(const QItemSelection &selected, const QItemSelection &deselected);
private:
QTreeView* m_treeView;
MyDomainModelTreeModel* m_treeModel;
};
class MyNestedClass1ViewModel : QAbstractItemModel
{
public:
explicit MyNestedClass1ViewModel(NestedDomainModel1* model);
setModel(NestedDomainModel1* model);
// required overrides for QAbstractItemModel
private:
NestedDomainModel1* m_model
};
class MyInfoDisplayWidget : public QWidget
{
public:
explicit MyInfoDisplayWidget(QWidget* parent = 0);
public slots:
// this is connected to the 'tree' widget signal in MainWindow
void setModel(NestedDomainModel1& selectedModel);
};
The basic premise of the UI is something similar in feel to Visual Studio. The tree is similar to the Solution Explorer and the 'info display' is similar to the properties window.
Is this how you use the model / view framework? For those familar with WPF / Silverlight development, is the model / view framework similar to MVVM (at a high level) in that it is the 'model of the view' and wraps / contains the domain model?
Is this how you connect the widgets using the model / view framework (ie. one widget passes a pointer or reference of the model to another)? Or should I be using the SelectionModel? Does that work since the tree model contains different types of objects?
How do you identify the root nodes? For instance, when a MyNestedObject1 is created and needs to be added to tree do I rely on the knowledge that root node is at a model index QModelIndex(0, 0) (ie. row 0 with an invalid parent index)?
I'm finding the terminology you're using a bit awkward, for example MyNestedClass1ViewModel is just a model. I'm not sure what a ViewModel would be.
What you're missing in this example is an actual view. MyTreeViewWidget is just a dumb widget that isn't actually a view in Qt terms at all, it's essdentialy just a dumb 'canvas' that you want to display data in. So the way to do this is:
You have underlying data in ordinary objects such as NestedDomainModel2. These are not Models in the Qt sense though and I wouldn't name them as such. They're just ordinary objects and don't implement any of the MVC interfaces.
Your MyNestedClass1ViewModel, which is a Qt model class. It accesses the underlying data objects above (1) in the implementation of it's data() and setData() methods.
A view class subclassed from QAbstractItemView. This is what you're missing. It has all the magic hooks to plug into the API of the model class from (2) above. It gets signals from the model telling it when there have been changed, which invoke methods such as dataChanged(), rowsInserted(). You implement these methods to make appropriate changes in your display widget below in point (4).
Your display widget. It doesn't implement any of the model/view API itself and is updated by your view. If it's interactive and can be used to change model data, you do that by calling setData(), insertRows(), removeRows(), etc on the model. The display changes will automatically propagate back to the widget via the view. Be careful not to generate infinite loops of changes propagating from widget->model->view->widget->model->view etc.
I have done a similar thing to use a QGraphicsScene/QGraphicsView to display items in a model. Despite it's name QGraphicsView isn't part of the model/view framework, so I implemented a custom view class which drew the model data on the QGraphicsScene.
Here's my code, in Python. It draws worlds on a map for an SF wargame:
class WorldItemView(QtGui.QAbstractItemView):
""" Hidden view which interfaces between the model and the scene.
"""
def __init__(self, model, parent=None):
QtGui.QAbstractItemView.__init__(self, parent)
self.hide()
self.setModel(model)
self.my_model = model
self.scene = MapScene(self.my_model)
self.resetWorlds()
def dataChanged(self, topLeft, bottomRight):
top_row = topLeft.row()
bottom_row = bottomRight.row()
#debug_log("Top row " + str(top_row) + " Bottom row " + str(bottom_row))
for row in range(top_row, (bottom_row + 1)):
self.scene.worldChanged(row)
def rowsInserted(self, parent, start, end):
for row in range(start, (end + 1) ):
pmi = self.my_model.getPMI(row)
self.scene.insertWorld(pmi)
def rowsAboutToBeRemoved(self, parent, start, end):
for row in range(start, (end + 1)):
self.scene.removeWorld(row)
def resetWorlds(self):
self.scene.clearWorlds()
# Add worlds to scene
last_row = self.my_model.rowCount() - 1
self.rowsInserted(None, 0, last_row)
I hope that helped.
Related
I would like to have a qt QML var accessible globally, and anywhere else in my qml files. Is this possible?
I know that upon creating a variable in a C++ object can be accessed in QML by exposing its getter function, but this only works if you know the type of the data type e.g. string, int, bool.
Is there a variable data type (or class) that can handle a QML var in C++, so that I can only call it in the other parts of the QML files?
AS Amfasis said, you can use the rootContext, so you can access it from anywhere in QML - as long as you do not shadow the name. Alternatively you can also register a Singleton to QML.
For both, you first need to create a QObject
public class MyContextObject: public QObject {
Q_OBJECT
Q_PROPERTY(QVariant myVar READ myVar NOTIFY myVarChanged)
QVariant m_myVar;
public:
MyContextObject(QObject* parent = nullptr) : QObject(parent) {}
QVariant myVar() { return m_myVar; }
void setMyVar(QVariant var) {
if (var == m_myVar) return;
m_myVar = var;
emit myVarChanged();
}
signals:
void myVarChanged();
}
This object you create in your main and set it as a contextProperty
MyContextObject* mctx = new MyContextObject();
view.rootContext()->setContextProperty("myCtx", mctx);
To set it from C++ use the setter. On the QML-side just bind to myCtx.myVar
Expose the setter also, if you want to modify it from QML also
This is not tested, I don't have a Qt development environment available right now.
To expose it as singleton, you can use this function:
https://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterSingletonType-1
I have a 2 widgets inherited from QDialog. One of these widgets is called by another widget.
I need to pass data from the parent widget to the child. For example, I want passing QStringList.
I can make signals and slots in both classes. Slot of parent widget class - transferList(QStringList) - filling my QStringList.
How should I make the signal and slot connection? The child widget, of course, knows nothing about the parent.
// .h-file of parent widget.
class ElectricIndicationDialog : public QDialog {
#include "PollIndication.h" // class of child widget
QSharedPointer <PollIndication> pollInd;
public slots:
void transferList(QStringList);
signals:
void listTfansfer(QStringList);
private:
QStringList sendList;
};
// .cpp-file of parent widget
pollInd = QSharedPointer <PollIndication>(new PollIndication());
pollInd->show();
void ConfIndication::transferList(QStringList lst) {
lst.append("str1");
lst.append("str2");
}
// .h-file of child widget
class PollIndication : public QDialog {
public slots:
void getList(QStringList);
signals:
void listGet(QStringList);
private:
QStringList recList; // We transfer data to it
}
You don't need a signal/slot for that: your parent knows the type of its child and has a pointer on it. So, you can call a method of PollIndication when you need to send data to your dialog.
void ConfIndication::transferList(QStringList lst) {
lst.append("str1");
lst.append("str2");
pollInd->changeTransferList(lst);
}
If your dialog is modal, you can also create your dialog only when needed and give your list as parameter of the constructor.
void ConfIndication::transferList(QStringList lst) {
lst.append("str1");
lst.append("str2");
PollIndication* pollInd = new PollIndication(lst, this);
pollInd->exec();
}
It is normally a bad idea to make a parent class to know what are their children....
you can in the parent class define an abstract method (think about some pure virtual) so every childclass is forced to implement it... after that, the parent class can invoke the method and the child will implement the login depending on how it must react to it...
According to wikipedia, I need to
In the Decorator class, redirect all "Component" methods to the
"Component" pointer;
But QWidget has too many methods to redirect.
For example I want to create a moveable widget decorator:
class Moveable : public QWidget
{
Q_OBJECT
public:
explicit Moveable(QWidget * widgetToBeDecorated){
this->widgetToBeDecorated = widgetToBeDecorated;
}
~Moveable();
protected:
void mousePressEvent(QMouseEvent* e) override {
lastPoint = e->pos();
widgetToBeDecorated->mousePressEvent(e);
}
void mouseMoveEvent(QMouseEvent* e) override {
auto offset = e->pos()-lastPoint;
lastPoint = e->pos();
widgetToBeDecorated->move(widgetToBeDecorated->pos()+offset);
widgetToBeDecorated->mouseMoveEvent(e);
}
void mouseReleaseEvent(QMouseEvent* e) override {
widgetToBeDecorated->mouseReleaseEvent(e);
}
private:
QWidget* widgetToBeDecorated;
QPoint lastPoint;
};
Do I need to redirect other methods like show(), paintEvent(QPaintEvent*)?
The decorator pattern is a guideline, not a rule. You are fully allowed to modify patterns that do not suit your purposes. In this case, you only need to concern yourself with the part of QWidget that are involved with positioning, so there is no advantage to overriding any other of it's methods.
It looks like you only want to modify the behaviour of certain events in your QWidget. Lucky for you, there is already a mechanism in QObject to do just such a thing, it's called an Event Filter.
There is a nice example of how to use it in the Qt documentation, and a good advantage that improves on your class above is that you can use the same object to filter many QWidgets, instead of having one filter for each widget.
I have a screen class as
class Screen : public QLayout
{
public:
Screen();
~Screen();
void paintEvent(QPaintEvent *e);
};
When I am creating the object I got an error that can not create an object for pure abstract class. Since QLayoput is pure abstract , How can I create an object for a class which is inherits the QLayout ?
definitions:
Screen::Screen( )
{
}
Screen::~Screen()
{
delete this ;
//Screen(new QSize (100,100));
}
void Screen::paintEvent(QPaintEvent *e)
{
}
QLayout is pure abstract, meaning it has virtual members without a definition. To subclass it, you need to provide definitions for all such methods in your class. Specifically, Qt Docs state that
To make your own layout manager, implement the functions addItem(),
sizeHint(), setGeometry(), itemAt() and takeAt().
For more, see there (there are additional optional advices for further functions which should be implemented): http://qt-project.org/doc/qt-4.8/qlayout.html
I created a public QGraphicsItem Node class which has getter/setter methods for a bunch of properties relevant to my application. The application is a diagram editor in which the users design models. This Node class is used to populate an also public QGraphicsScene Diagram class.
I'm now trying to implement a load/save mechanism in the app that enables users to save and reuse the models - editing them as they were when they saved. I'm having some trouble figuring out how to attack this problem.
I already know I have to get the relevant properties for the QGraphicsItem objects and save those to file, and then on load, reconstruct the scene using the data I saved. My question is: when I use the QGraphicsScene::items() function, it returns a QList<QGraphicsItem *>. What can I do to have it return a QList<Node *>?
If I get a list of all the nodes I appended to the scene, I know what to do next.
I began to realise that I will probably have to reimplement the items() function in my Diagram class, but I was hoping I could get away with it more easily. In this case, can someone please explain how to do it?
I would suggest to implement the QGraphicsItem::type() method and using qgraphicsitem_cast to cast into the desired class. In your case, you could subclass all your custom GraphicsItems from a common base class.
Your base class would look like this:
class MyGraphicsItem: public QGraphicsItem
{
enum { Type = UserType + 1 };
int type() const { return Type; }
};
Your Node and your Link class would inherit this base:
class Node : public MyGraphicsItem
{
// ...
};
class Link : public MyGraphicsItem
{
// ...
};
Somewhere else you could cast a QGraphicsItem into your base class MyGraphicsItem like this for example:
QList<QGraphicsItem*> allItems = graphicsScene->items();
foreach (QGraphicsItem *item, allItems) {
// Using qgraphicsitem_cast
MyGraphicsItem* graphicsItem = qgraphicsitem_cast<MyGraphicsItem*>(item);
if (graphicsItem) {
// Do something with MyGraphicsItem
}
}
While the items function returns a list of QGraphicsItem pointers, you could try either dynamic_casting to check if it's a Node pointer or use the Qt metadata and the type() function.
However, the method I often use, which can help you in other ways too, is to maintain a unique id for every type of object and a static lists of item Ids. For example: -
class BaseItem : public QGraphicsItem
{
public:
BaseItem(QGraphicsItem* parent)
: QGraphicsItem(parent), m_Id(m_NextId++) // initialising the object's unique Id
{
}
unsigned int ID() const
{
return m_Id;
}
private:
static unsigned int m_NextId; // the next Object Id, initialised to 0 in implementation
unsigned int m_Id; // the object's unique Id
}
class Node : public BaseItem
{
public:
Node(QGrahicsItem* parent)
: BaseItem(parent)
{
m_NodesList.push_back(m_Id);
}
private:
static QList<unsigned int> m_sNodesList; // all Node ids in the scene
QList<unsigned int> m_linkedNodeList; // linked nodes
}
All items added to the GraphicsScene are inherited from the BaseItem
You also have a static list of Node Ids, so you can iterate through all of them for loading / saving and you can add a static helper function in the base class to return a list of Nodes, by searching through the List of Node Ids and matching them to their node pointers in the scene.
This architecture also allows you to include a list of linked node ids in each Node object, rather than using the parent / child system of Qt, which in the case of a node diagram, isn't always what is needed.
Overall, I've used this architecture for many QGraphicsScene applications and it has really made development very easy for items that have complex links to other items.