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Anchors are an efficient way to handle different aspect ratios for basic multiple resolution handling in GUIs,
For more complex user interfaces, they can become difficult to use.
This is often the case of games, such as RPGs, online chats, tycoons or simulations. Another common case where more advanced layout features may be required is in-game tools (or simply just tools).
All these situations require a more capable OS-like user interface, with advanced layout and formatting. For that, Containers are more useful.
Containers provide a huge amount of layout power (as an example, the Godot editor user interface is entirely done using them):
When a Container-derived node is used, all children Control nodes give up their own positioning ability. This means the Container will control their positioning and any attempt to manually alter these nodes will be either ignored or invalidated the next time their parent is resized.
Likewise, when a Container derived node is resized, all its children will be re-positioned according to it, with a behavior based on the type of container used:
Example of HBoxContainer resizing children buttons.
The real strength of containers is that they can be nested (as nodes), allowing the creation of very complex layouts that resize effortlessly.
When adding a node to a container, the way the container treats each child depends mainly on their container sizing options. These options can be found by inspecting the layout of any Control that is a child of a Container.
Sizing options are independent for vertical and horizontal sizing and not all containers make use of them (but most do):
Fill: Ensures the control fills the designated area within the container. No matter if a control expands or not (see below), it will only fill the designated area when this is toggled on (it is by default).
Expand: Attempts to use as much space as possible in the parent container (in each axis). Controls that don't expand will be pushed away by those that do. Between expanding controls, the amount of space they take from each other is determined by the Stretch Ratio (see below). This option is only available when the parent Container is of the right type, for example the HBoxContainer has this option for horizontal sizing.
Shrink Begin When expanding, try to remain at the left or top of the expanded area.
Shrink Center When expanding, try to remain at the center of the expanded area.
Shrink End When expanding, try to remain at the right or bottom of the expanded area.
Stretch Ratio: The ratio of how much expanded controls take up the available space in relation to each other. A control with "2", will take up twice as much available space as one with "1".
Experimenting with these flags and different containers is recommended to get a better grasp on how they work.
Godot provides several container types out of the box as they serve different purposes:
Arranges child controls vertically or horizontally (via HBoxContainer and VBoxContainer). In the opposite of the designated direction (as in, vertical for an horizontal container), it just expands the children.