What makes a Qt widget and its layout behave properly (in regard to its size)?

Question

I'm having all sorts of size problems with Qt. I am creating my own widgets and using different layouts (generally, I need my own to make them work properly without spending hours on the "powerful" default layouts... which don't lay things out as intended.)

Once I'm done with a widget and its layout though, it doesn't work right. The size is never getting set properly unless I call widget->resize(1, 1); which finally forces a "resize" and makes the widget look correct (i.e. recompute the geometry.) Even the updateGeometry() call has no effect.

This is a dreadful problem when the resize() needs to be called on the parent widget (yuck!) and from what I'm reading should not be necessary were the layouts properly programmed.

Is there a sample that works and is not several thousand of lines long, or does Qt require several thousand lines to make anything work perfectly, even the simplest widget?

What are the minimal functions to be called to make a widget & its layout work at once?

Thank you. Alexis

P.S. I tried to implement the sizeHint(), minimumSize(), maximumSize(), others that I'm missing? I was hoping that would be enough. Obviously, I also implement the setGeometry() on the layout to resize the children appropriately.

--- addition 1

There is a sample image with a layout that clearly isn't available as is in Qt. The positioning, functions, and colors of the different keys is XML driven and works for any keyboard in the world.

On screen keyboard

(note, this sample doesn't show the Enter key displayed on two rows and wider below than at the top; more or less, not doable at all with the regular layouts; of course, it works with my version.)

--- clarification

I'm not too sure how to describe the problem better. I was thinking to write a test widget next to see how I can reproduce the problem and then post that and eventually fix it. 8-)

The default layout function that the internal Qt layouts make use of require a lot of coding. I would like to avoid having to copy/paste all of that because for maintenance, it makes it close to impossible.

--- today's findings

As I needed to tweak one of the widgets, I decided to add a VBoxLayout and make it work.

I actually found the problem... One of the widgets in my tree is a QScrollArea and that sizeHint() returns (-1, -1). Not exactly what I'd expect but... whatever you put inside that widget has better know how to compute its width and height or else... it fails.

Looking at the code closely, I could actually compute the width by using the widest width found. Once I used that, the widget would appear (and it actually resizes itself as things change in the list, kinda cool.)

This being said, my earlier comment about having a tree of widgets that auto-resize themselves stands. From the root up to the parents of the leaves in your tree, all of those widgets will need a valid layout. Once I added one in the top widget it resized itself and its children properly (well... in my case up to the QScrollArea, the rest required a bottom to top resizing. Funny how that works!)

--- ah! ha! moment (or: what you find reading the implementation code!)

Today I bumped in another problem which just needed the correct call... I just couldn't find anything worth it in the documentation.

All the objects have a layout now, but a certain parent would not resize properly. Plain simple.

I had a call to the parent as following:

// changes to the children are changing the geometry
parentWidget()->updateGeometry();

Yeah. The docs says that's what you have to do. Nothing happens at all with that call. No idea what it's supposed to do, I did not look at that function. It never did anything for me anyway.

So... I looked at the layout to try to understand how it would send the info up/down. I did not see much except for one interesting comment:

// will trigger resize

This is said of the SetFixedSize mode. To reach that function you need to make the layout for update. Ah! Yes... the layout, not the parent widget... let's try that instead:

parentWidget()->layout()->update();

And voila! It resizes correctly in all cases I have. Quite incredible that the widget updateGeometry() doesn't trigger the same effect...

What is it that you're trying to accomplish that the built-in layout system can't do for you? It's difficult to answer a question that will be of any use to you when when it essentially boils down to "how do I make things work?" — Chris
For GUI/Layout quetions, including an annotated screen shot can help a lot. There's no shortage of people here that like to figure things out when they have solid info :) — Arnold Spence
if you don't have a problem your code then you can share it on www.ideone.com :D — blueskin
@blueskin I cannot share that code, no... but was thinking to create a test case that fails so I can share that. Maybe it's because I am missing layouts at every levels... Only the lower widgets don't need a layout, and I guess that all the ancestors do need a layout if we want them to work. — Alexis Wilke

Kaleb Pederson Kaleb Pederson · Accepted Answer · 2011-09-09T04:23:46

Although it's possible to do what you want it sounds like the problems you are having are because you're using Qt in a way that it's not meant to be used. Why do you need separate widgets for each key represented on the keyboard?

I see two options, both of which are better in some way:

Use QGraphicsScene and QGraphicsView.
A single custom widget that uses custom drawing to display the keyboard (and likely uses hover for hints).

The first option is probably better. Your keys could then be represented by QGraphicsSimpleTextItem's or even a QGraphicsSvgItem. It also provides a number of standard layouts or you could choose to write your own layout. By default you can use the keyPressEvent or mouseReleaseEvent to respond to user interactions.

I'd highly recommend you take a look at the QGraphicsView examples to get an idea what you can do.

If you go the second route you'll need to record the different key locations so you can respond accordingly as the user moves the mouse around, clicks, etc.

What makes a Qt widget and its layout behave properly (in regard to its size)?

2 Answers