Constraints Layout

Enaml widgets come in two basic types: Containers and Controls. Controls are conceptually single UI elements with no other Enaml widgets inside them, such as labels, fields, and buttons. Containers are widgets which contain other widgets, usually including information about how to layout the widgets that they contain. Examples of containers include top-level windows, scroll areas and forms.

Enaml uses constraints-based layout implemented by the Cassowary layout system. Constraints are specified as a system of linear inequalities together with an error function which is minimized according to a modified version of the Simplex method. The error function is specified via assigning weights to the various inequalities. The default weights exposed in Enaml are 'weak', 'medium', 'strong', 'required', and 'ignored', but other values are possible within the system, if needed. While a developer writing Enaml code could specify all constraints directly, in practice they will use a set of helper classes, functions and attributes to help specify the set of constraints in a more understandable way.

Every widget knows its preferred size, usually by querying the underlying toolkit, and can express how closely it adheres to the preferred size via its hug_width, hug_height, resist_width and resist_height, limit_width and limit_height attribute which take one of the previously mentioned weights. These are set to reasonable defaults for most widgets, but they can be overriden. The hug attributes specify how strongly the widget resists deformation by adding a constraint of the appropriate weight that specifies that the dimension be equal to the preferred value, while the resist attributes specify how strongly the widget resists compression by adding a constraint that specifies that the dimension be greater than or equal to the preferred value. The limit attributes specify how strongly the widget resists expansion by adding a constraint that specifies that the dimension be smaller than or equal to the preferred value

Containers can specify additional constraints that relate their child widgets. By default a container simply lays out its children as a vertical list and tries to expand them to use the full width and height that the container has available. Layout containers, like Form, specify different default constraints that give automatic layout of their children, and may provide additional hooks for other widgets to use to align with their significant features.

Additional constraints are specified via the constraints attribute on the container. The simplest way to specify a constraint is with a simple equality or inequality. Inequalities can be specified in terms of symbols provided by the components, which at least default to the symbols for a basic box model: top, bottom, left, right, v_center, h_center, width and height. Other components may expose other symbols: for example the Form widget exposes midline for aligning the fields of multiple forms along the same line, and a Container exposes various contents symbols to account for padding around the boundaries of its children.

enamldef Main(Window):
    Container:
        constraints = [
            # Pin the first push button to the top contents anchor.
            pb1.top == contents_top,

            # Relate the left side of the push button to the width
            # of the container.
            pb1.left == 0.3 * width,

            # Relate the width of the push button to the width of
            # the container
            pb1.width == 0.5 * width,

            # Pin the second push button to the left contents anchor.
            pb2.left == contents_left,

            # Relate the top of the push button to width of the first
            # push button.
            pb2.top == 0.3 * pb1.width + 10
        ]
        PushButton: pb1:
            text = 'Horizontal'
        PushButton: pb2:
            text = 'Long Name Foo'

However, this can get tedious, and so there are some helpers that are available to simplify specifying layout. These are:

spacer

A singleton spacer that represents a flexible space in a layout with a minimum value of the default space. Additional restrictions on the space can be specified using ==, <= and >= with an integer value.

spacer.flex()

A flexible spacer that has a hard minimum but also a weaker preference to be no larger than that minimum.

horizontal(*items) or hbox(*items)

vertical(*items) or vbox(*items)

These four functions take a list of symbols, widgets and spacers and create a series of constraints that specify a sequential horizontal or vertical layout where the sides of each object in sequence abut against each other.

align(variable, *items)

Align the given string variable name on each of the specified items.

grid(*rows, **config)

A function which takes a variable number of iterable rows and arranges the items in a grid according to the configuration parameters.

factory(func, *args, **kwargs)

A function which takes a function which should return the set of constraints to use. The factory function is called each time the layout can change (widget addition, deletion, etc).The arguments are passed are passed to function.

By using appropriate combinations of these objects you can specify complex layouts quickly and clearly.

enamldef Main(Window):
    Container:
        constraints = [
            # Arrange the Html Frame above the horizontal row of butttons
            vbox(
                html_frame,
                hbox(
                    add_button, remove_button, spacer,
                    change_mode_button, spacer, share_button,
                ),
            ),

            # Weakly align the centers of the Html frame and the center
            # button. Declaring this constraint as 'weak' is what allows
            # the button to ignore the constraint as he window is resized
            # too small to allow it to be centered.
            align('h_center', html_frame, change_mode_button) | 'weak',

            # Set a sensible minimum height for the frame
            html_frame.height >= 150,
        ]
        Html: html_frame:
            source = '<center><h1>Hello Enaml!</h1></center>'
        PushButton: add_button:
            text = 'Add'
        PushButton: remove_button:
            text = 'Remove'
            clicked :: print('removed')
        PushButton: change_mode_button:
            text = 'Change Mode'
        PushButton: share_button:
            text = 'Share...'

Alternatively one can override the layout_constraints function in the enaml definition.

enamldef Main(Window):
    title = 'Custom Constraints'
    Container:
        layout_constraints => ():
            rows = []
            widgets = self.visible_widgets()
            row_iters = (iter(widgets),) * 2
            rows = list(zip_longest(*row_iters))
            return [grid(*rows)] + [align('v_center', *row) for row in rows]
        Label:
            text = 'Name'
        Field:
            pass
        Label:
            text = 'Surname'
        Field:
            pass
        PushButton:
            text = 'Click me'