Enaml is a programming language and framework for creating professional-quality user interfaces with minimal effort. It relies on Kiwi to layout widgets.

To implement its layout, Enaml uses a nestable model. Containers widgets handle the constraints generation used to layout their children. Furthermore, they pass to their their children a representation of the bounding box in which they should live which allows the widgets to position themselves inside their parent. Since each leaf component has a “preferred size”, the system can be solved from the bottom-up and set the size the parents based on the required space of the children. If at a later time the parent is resized, this new input can be used to solve the layout problem.

The following sections will describe in more details how the constraints are generated and the preferred size estimated.

Widget variables

In Enaml, each widget that can be constrained defines a bunch of Kiwi Variable: left, top, width and height. In addition, it provides easy access to combination of those: right, bottom, h_center, v_center. Those variables will be used to define the layout.

In addition, because each widget has a preferred size, it defines a set of constraints related to that preferred size on which the user can act upon by modifying their strength: - hug_width: equivalent to (width == hint) | hug_width - hug_height: equivalent to (height == hint) | hug_height - resist_width: equivalent to (width >= hint) | resist_width - resist_height: equivalent to (height >= hint) | resist_height - limit_width: equivalent to (width <= hint) | limit_width - limit_height: equivalent to (height <= hint) | limit_height

Finally, widget that can contain other widgets define a set of variables that they expose to their children to allow to place themselves relative to their parents. Those are: contents_left, contents_top, contents_width, contents_height, contents_right, contents_bottom, contents_h_center, contents_v_center. Those are usually not equivalent to the non-prefixed variables (even-though they are related) because of the container margins.

The base classes used a mixin to implement those behaviors are defined in: https://github.com/nucleic/enaml/blob/master/enaml/layout/constrainable.py

Constraints definition

Using the above variable, one can express any constraints. However, even for simple vertical or horizontal boxes, the constraints to define, in particular if one needs to introduce some spacing around the objects, become quite painful to write by hand.

To make constraints definition easier, Enaml relies on helpers function and classes. In the following, we will focus on how horizontal and vertical boxes constraints are handled, by studing the following example in details:


Here we consider a container widget with three child widgets. The outer black frame represents the limit of the container. The dashed frame represents the contents visible to children when defining their constraint. The container uses the margin definition to relate the outer left, top, width and height to their ‘contents’ equivalent.

The three widgets are arranged according to a horizontal box for which the constraints are created using the hbox helper function which simply accepts a list of widgets and spacers . To define the constraints from that list, Enaml relies on the spacers represented here in orange. Each spacer has a given size and a policy regarding that size (is it a minimum value, maximum, how strongly to enforce that size). For each orientation, hbox add spacers so that there is a spacer between each widget and between the widgets and the parent boundaries. Some spacers can have a zero size, simply meaning that widgets should be in contact.

When generating the constraints, hbox will be passed the container and use the spacers to generate the constraints by simply glueing the anchors of surrounding widgets. Each spacer can generate multiple constraints which gives this process a lot of flexibility. Furthermore, those helpers define the same variable as the widgets allowing for to position groups with respect to one another.


In practice, hbox itself relies on some helpers but the above gives you the general idea.

For further details you can have a look at the source of the helpers described in this section which can be found in the Enaml source:

Setting up the solver

So far we have only defined the constraints that represent the layout, we will now turn to how Enaml pass those to the solver and how it handle updates and solver resets.

By default, each container manages its own solver independently. This has the advantage of keeping the system relatively smalls and hence allow for faster updates. When setting up the solver, the container will add for each widget a set of constraints reflecting the preference of the widget regarding its size as reported by the widget, and add to those the constraints defining the layout. It will also add two edit variable representing the width and height of the container.

Once the solver has been set up it can be used to compute different values, such as the best size for the container (requesting a size of 0 with a 0.1*weak strength), its min size (0 size, medium strength) and max size (max size, medium strength).

When the parent is resized, the solver is invoked again with the new width and height as suggestion. On the other hand, if the constraints change either because widgets have been added or removed or because the users modified them, the solver is reset and the constraints are rebuilt-from scratch. This means that we never keep the solver around long enough to have to worry about memory consumption due to unused variables in the solver.

In a complex hierarchy, the top parent will request the sizes of the nested containers which will trigger the solving of their constraints. At some point in the nested structure, we will only find widgets which provides a size hint without requiring to solve constraints (ex: a button). This will allow to solve the system and then propagate back upward.

Hopefully this brief introduction will have clarified how Enaml make use of kiwi to layout its widgets. Some fine mechanics have been simplified for the sake of this description but you can check Enaml sources for a more in depth description.