I'm looking at some code from an auto layout library. In it, there is a protocol adopted by UIView:
extension UIView: Constrainable {}
extension UILayoutGuide: Constrainable {
// LayoutGuide doesn't have baseline anchors, so just use the bottom anchor
public var firstBaselineAnchor: NSLayoutYAxisAnchor {
return bottomAnchor
}
public var lastBaselineAnchor: NSLayoutYAxisAnchor {
return bottomAnchor
}
}
public protocol Constrainable {
var topAnchor: NSLayoutYAxisAnchor { get }
var bottomAnchor: NSLayoutYAxisAnchor { get }
var leftAnchor: NSLayoutXAxisAnchor { get }
var rightAnchor: NSLayoutXAxisAnchor { get }
var leadingAnchor: NSLayoutXAxisAnchor { get }
var trailingAnchor: NSLayoutXAxisAnchor { get }
var centerXAnchor: NSLayoutXAxisAnchor { get }
var centerYAnchor: NSLayoutYAxisAnchor { get }
var widthAnchor: NSLayoutDimension { get }
var heightAnchor: NSLayoutDimension { get }
var firstBaselineAnchor : NSLayoutYAxisAnchor { get }
var lastBaselineAnchor : NSLayoutYAxisAnchor { get }
}
What does an empty { get }
accomplish?
These are properties for which the classes that adopt the protocol must supply a getter. The protocol does not specify anything about the setter, so classes could supply a computed property instead of a stored one.
For example, a class that adopts Constrainable
could satisfy a requirement of having topAnchor
by adding
var topAnchor: NSLayoutYAxisAnchor
or by adding
var topAnchor: NSLayoutYAxisAnchor {
...
return ...
}
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