Hi folks,
I'm curious to know how others have handled the loss of property observers when
a protocol property has a default implementation.
For example, consider the following simple protocol declaration:
protocol Widget : class {
associatedtype IdentifierType
var identifier: IdentifierType { get set }
}
extension Widget {
var identifier: IdentifierType {
get {
return ...
}
set {
...
}
}
}
A class conforming to the Widget protocol cannot leverage property observers on
the identifier property without overriding the default implementation:
class WidgetFactory: Widget {
typealias IdentifierType = Int
// Overrides the default implementation
var identifier: Int {
didSet {
// ...
}
}
}
This is problematic when the default implementation goes beyond merely
getting/setting a backing variable. The only solution I've come up with is to
mandate a willSet/didSet function pair for each protocol property:
protocol Widget : class {
associatedtype IdentifierType
var identifier: IdentifierType { get set }
func willSetIdentifier(_ newValue: IdentifierType)
func didSetIdentifier(_ newValue: IdentifierType)
}
extension Widget {
var identifier: IdentifierType {
get {
return ...
}
set {
...
}
willSet {
willSetIdentifier(newValue)
}
didSet {
didSetIdentifier(identifier)
}
}
}
I find this to be an ugly solution. Particularly so since non-objc protocols
don't support optional members. Compare this to using a base class:
protocol Widget : class {
associatedtype IdentifierType
var identifier: IdentifierType { get set }
}
class BaseWidget : Widget {
typealias IdentifierType = Int
var identifier: Int {
get {
return ...
}
set {
...
}
}
}
class WidgetFactory : BaseWidget {
// This doesn't override the getter/setter implementation in the base class
override var identifier: Int {
willSet {
...
}
}
}
Has anyone else faced this situation? If so, what was your solution?
-- Neil
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