Qt C++ : How to get event target object? - c++

Question

There is a container Widget instance A and there are contained Widget instances C1, C2, C3, ... etc within A.
There is a slot that handles Widget A's action_triggreed() signal.
Is there a way to determine which of the target Widgets C1, C2, C3, ... was clicked?
The reason for this necessity is that there are numerous contained widgets, and it doesn't make sense to use connect() method on each, which would require 50+ lines of extra connect statements, one for each!
For example: Consider a QToolBox with lots of Buttons. How would you determine which Button is pressed by using a QToolBox action_triggered or similar signals, without using signals and slots for individual Buttons separately?

Answer 1

You can use the QObject::sender() function.
void A::triggeredSlot() {
QObject* obj = sender();
QButton* but = qobject_cast<QButton*>(obj);
}
Just use the correct types etc.

Answer 2

There are a few general approaches.
The first one is straightforward - you should connect each widget's signal to the slot and check what sender() is. Yes, lines of code for connecting.
The second one is to use a kind of mapper. Every widget is connected to the mapper and only mapper is connected to the target slot. Lines of code again, at least in the part where you connect widgets to the mapper. There is a generic QSignalMapper or you can use more specific one suited for your widgets. For example, if they are buttons then you can use QButtonGroup class. Every button is registered in the group and only one signal/slot connection is required.
QButtonGroup group;
group->addButton(buttonC1,C1_ID);
...
group->addButton(buttonC1,Cn_ID);
connect(&group,SIGNAL(buttonClicked(QAbstractButton*),this,SLOT(buttonClicked(QAbstractButton*));
The third approach is to detect mouse event only on the mother's widget A and then iterate over all its children and find which one is under mouse. Less code, you can easily add new widgets, but the cost is iterating over all widgets in runtime. Below is an example. Note, that you can add specific QObject names or properties to the widgets C1... so that you could filter them if you are interested only in a part of children widgets of the given type.
void mousePressEvent(QMouseEvent* event)
{
if (event->button()==Qt::LeftButton)
{
QList<QToolButton*> buttons=findChildren<QToolButton*>(); // you can also use specific object names on the widgets under your interest
foreach (QToolButton* button, buttons)
{
if (button->underMouse()) // you could try isDown() for button, but I'm not sure if that will work here
{
emit buttonClicked(button);
break;
}
}
}
}
Well, you could try another approach also. Detect the mouse event, get cursor position and find the child widget on that position. Not costly in runtime.
void mousePressEvent(QMouseEvent* event)
{
if (event->button()==Qt::LeftButton)
{
QPoint pt=mapFromGlobal(QCursor::pos());
QWidget* child=childAt(pt);
if (child)
{
emit childClicked(child);
}
}
}