anchor - UI constraints for Pythonista
Python wrapper for Apple iOS UI view layout constraints, available as
anchor.pyon GitHub. Run the file to see a sample constraint-driven layout.
Constraints are used to determine how views are laid out in your UI. They are an alternative to the
framemethod used in Pythonista by default.
Constraints are defined as equations, which are dynamically evaluated as the dimensions or views of your UI change. For example, the following constraint places the Cancel button always beside the Done button:
cancel_button.at.trailing == done_button.at.leading_padding
(Here, 'trailing' and 'leading' are same as 'right' and 'left', but automatically reversed if your device is set for a right-to-left language.)
Constraints can use the following attributes:
left, right, top, bottom, width, height
left_margin, right_margin, top_margin, bottom_margin, leading_margin, trailing_margin
- Use these when you want to leave a standard margin between the view and the edge of its superview (inside margin).
left_padding, right_padding, top_padding, bottom_padding, leading_padding, trailing_padding
- Use these when you want to leave a standard margin between the view and the view next to it (outside margin).
Why would I need them?
It depends on your style and preferences regarding building UIs.
You can create pretty much all the same layouts and achieve the same level of dynamic behavior just using Pythonista's regular
flexattribute and the
The reason to consider constraints is that they, and the convenience methods in this wrapper, provide perhaps a more human way of expressing the desired layout. You can use one-liners for "keep this view below that other view, no matter what happens", or "this view takes over the top half of the screen, with margins", without fiddling with pixel calculations or creating several ui.Views just for the layout.
Anatomy of a constraint
Constraints have this syntax:
target_view.at.attribute == source_view.at.attribute * multiplier + constant
targetview is now constrained and unaffected by setting
center- but you can read these values if you need to know the absolute shape and position of a view.
sourceview is unaffected and remains in the 'frame mode', until used on the left side of constraint.
- Relationship can be
>=(but nothing else).
- You can also
/a multiplier or
-a constant, and have several multipliers and constants, but they will only be combined per type (i.e.
* 6 + 1 / 3 - 5is the same as
* 2 - 4).
- Multiplier can be zero or the source left out of the equation, but only if the target attribute is a size attribute, e.g.
target.at.height == 100
- Target and source attributes cannot mix:
- size and position attributes
- vertical and horizontal position attributes
- absolute and relative position attributes (e.g.
These are all Apple restrictions, and the wrapper checks for them to avoid an ObjC exception and a Pythonista crash. Please let me know if you find other crashing combos.
Pythonista UI views do not natively support constraints, of course, so we need to enable them.
The explicit option is to call
enableon the UI view, maybe at view creation. For example:
import anchor, ui label = anchor.enable(ui.Label(alignment=ui.ALIGN_CENTER)) label.at.width == 100
An alternative is to use already-enabled versions of every Pythonista UI view class, defined in anchor.py, so you can save a little typing by importing it like this:
from ui import * from anchor import * label = Label(alignment=ALIGN_CENTER) label.at.width == 100
When you constrain a view, you have to unambiguously constrain both its position and size. If you miss something, the view usually is not visible at all. To debug constraints, you can either check an individual view for problems with:
Or check your whole view hierarchy by:
This will print out the whole hierarchy, and return any ambiguous views as a list.
To be continued
@JonB, @shinyformica, @cvp, thank you for the encouragement. Even though this was independently initiated, has different code and API, @zrzka has ”prior art” in the form of his proof of concept code from a while back.
Convenient view alignment
Enabled views have an
alignattribute that supports aligning matching attributes of views. For example, aligning the heights of two views:
alignis especially convenient when you need to align several views at once:
In addition to all the regular constraint attributes like
center_xin the examples above,
alignsupports aligning the composite attributes
Convenient view placement within superview
Creating individual constraints can quickly become a bit of a bore. Thus the wrapper includes a number of methods for "docking" views.
For example, the following places constraints to the top and both sides, leaving height still undefined:
Following docking methods are available:
all, center, horizontal, vertical, horizontal_between, vertical_between, top, bottom, leading, trailing, top_leading, top_trailing, bottom_leading, bottom_trailing
The most specialized of these are the
_betweenmethods, which dock the view to the sides in one direction, and between the two given views in another. Here's an example:
result_area.dock.horizontal_between( search_button, done_button)
dockmethods leave a margin between the edges of the superview and the view. This can be adjusted with the
Dock.MARGIN(the default) - standard margin
Dock.TIGHT- no margin
Dock.SAFE- align to the safe area insets, if applicable
You can also change the default by setting the
Dock.default_fit = Dock TIGHT
shareparameter can be used to define how much of the superview's area the view should take:
This is only exact if you use
TIGHTfit, as there is no way to dynamically account for the space taken by margins.
constantparameter can be used to adjust the margins manually, although I feel that this is probably bad layout design.