Question

This is my first question ever so bear with me!

Currently in my program, I have a parent widget which acts as a canvas. The user can add or remove widgets to the parent at run-time. Those widgets are then given an absolute position, that is, they are not positioned by a layout. Once added, a widget can be moved around arbitrarily by the user.

I want the user to be able to select a group of widgets by dragging a box around them. I have already coded the part that displays the rectangle while the user is dragging. Now, I want to be able to retrieve all the widgets within that rectangle (region).

I am aware of the findChild() and findChildren() functions, and they indeed do return the children as they are supposed to. But what I’d really need is a way to limit the search to the boundaries of the region since there will most-likely be quite a lot of widgets within the ‘canvas’. (There could be thousands of widgets spread over a very large area due to the nature of what I’m doing!)

Here is my question: What would be my best option? Should I just go ahead and use findChildren() and loop through the list to find the children within the region manually. Or should I loop through all the pixels within the region using findChild(x, y)? Or perhaps there is an even simpler solution that would speed up the process? Something along the lines of findChildren(x, y, width, height)?

Hopefully my question made sense. I tried to explain things as best as I could. Thanks!

Answer 1

If you had used QGraphicsScene instead of rolling your own, you could have used the items(..) methods to very efficiently find your children in a particular area.

It’s only possible in QGraphicsScene because it uses a BSP spatial acceleration structure, so if you cannot migrate to QGraphicsScene in a reasonable amount of time – you are going to have write your own. It’s not as hard as it sounds, I’ve written numerous bounding volume hierarchy structures and they’re quite straightforward.