How do you set a priority on auto-layouts default spacing metric in a visual format?

Question

Auto-layout uses the - symbol to indicate that a constraint should use the default spacing. Is there a way to set the priority on that spacing without having to explicitly set a metric value?

For example, the following visual format will create a leading constraint between myView and its superview equal to the default Cocoa/CocoaTouch value. But that constraint has a priority of required.

@"H:|-[myView]"

The following will create a leading constraint with a lower priority but an explicit metric value must be specified:

@"H:|-(10@750)-[myView]"

I want to be able to give the leading constraint a priority, but not an explicit metric. The reasoning being that 1) I don't want to have to guess what Apple's value is and 2) To future proof if Apple changes that value.

Ideally, I'd like something like that following:

@"H:|-(@750)-[myView]" or @"H:|-(-@750)-[myView]"

But neither are accepted grammar. Is there a secret format string that would achieve this?

(I don't think there's a difference between iOS and MacOS for something like this, but if there is, I'm targetting MacOS.)

Answer 1

You cannot set priority for default spacing in visual format constraints. You can only do it when you create a constraint explicitly.

let constraint = NSLayoutConstraint(item: myView, attribute: .leadingMargin, relatedBy: .equal, toItem: superview, attribute: .leadingMargin, multiplier: 1, constant: 0)
constraint.priority = 750

You can get the default spacing by specifiying attributes as leadingMargin and trailingMargin instead of using just leading and trailing.