Relative to parent

In the basic widget functions I showed that setPosition and setSize could be given constants or a value relative to the size of the parent widget.

widget->setPosition("10%", "5%");
widget->setSize("30%", "10%");

The layout system will make sure that when the size of the parent changes, the position and size of the widget will be updated as well. This way you don’t have to update the position and sizes yourself each time the window size changes.

Because you can refer to the parent widget, one way to keep a widget centetered on the screen would be the following:

widget->setPosition("(parent.innersize - size) / 2");

A better solution would be to change the origin though. Setting the position to (“50%, “50%”) doesn’t work by default because it will place the top-left corner of the widget in the center of the parent, but the origin can be changed from the top-left to the center of the widget to truly center the widget:

widget->setPosition("50%", "50%");
widget->setOrigin(0.5f, 0.5f);

The parameters passed to setOrigin are values from 0 to 1 that indicate which point of the widget is specified with the position (with 0 being left/top and 1 being bottom/right). The following code would place the widget at the right side of the parent while having the center of the widget at 75% of the parent size.

widget->setPosition("100%", "75%");
widget->setOrigin(1.f, 0.5f);

If the last paragraph isn’t clear enough then here is a calculation with numbers. Imagine the parent is a panel of size (800, 600) while the widget is a button of size (150, 40). The button would be placed on position (800 - 200, 0.75 * 600 - 0.5 * 40) = (650, 430). When the panel is resized to a width of 400 pixels then the button will automatically be moved to a left position of 250.

Binding other widgets

You can not only refer to the parent, but also to other widgets. In the example below, the button is positioned 50 pixels to the right of another button.

button2->setPosition({bindRight(button1) + 50.0f, bindTop(button1)});

You can achieve the same when using strings. The name of the widget used in the layout string has to match with the one given to the add function when the widget was added to the mutual parent.

group->add(button1, "ButtonName");
button2->setPosition({"ButtonName.right + 50", ""});