Question

I’m writing a program that uses the Qt Graphics View framework. I have subclassed QGraphicsItem to a class that includes other QGraphicsItem (or other subclasses of it). This class is the parent of the included QGraphicsItem; the idea is to work with composite objects.

From the docs it seems to be a conflict in what I try to achieve:

• Calling ignore() in mousePressEvent will make my object unmovable. I want to move it.
• Calling accept() in mousePressEvent will prevent the event from being propagated to the child object. Some of the child objects should react to mouse events.

How can I make this work?

Answer 1

I think your interpretation of the documentation is incorrect.

Calling ignore() in mousePressEvent will make my object unmovable.

I don’t believe that is true. To me it looks like calling ignore() is like the object saying “I have assessed this event. I have taken all actions I want to in response to this event. I have also decided it was not intended for me, so I will now pass it on to the next object underneath me”. I can’t find anything which suggests the ignore event will unset the QGraphicsItem::ItemIsMovable flag (which is what decides if the QGraphicsItem is movable or not).

I don’t see why you couldn’t make your object move and ignore() the event, but I would advise that this is not a sensible approach (in most instances: obviously you may have cause for it).

Calling accept() in mousePressEvent will prevent the event from being propagated to the child object.

I believe this is true, but the parent can still modify its children. My understanding is calling accept() is like the object saying “I have assessed this event. I have taken all actions I want to in response to this event (which may include modifying my children). I have also decided that the event was intended for me, so I will not be passing the event on”.