Type nesting allows some convenient and straightforward semantics that we see
inside the Swift standard library such as views on String like
String.CharacterView, String.UnicodeScalarView, etc. However a protocol cannot
be nested in a type and gives a non-obvious error that the “Declaration is only
valid at file scope.” Just as other nested types allow proper contextual
scoping, a nested protocol could make a lot sense for a number of patterns. For
example, there are many “Delegate” protocols throughout the Cocoa frameworks.
Here’s a controller/delegate pattern before and after type nesting:
// Without type nesting
protocol MyControllerDelegate : class {
}
class MyController {
weak var delegate: MyControllerDelegate?
}
// With type nesting
class MyController {
weak var delegate: Delegate?
protocol Delegate : class {
}
}
Though the change is mostly semantics, it does allow an explicit association
between My Controller and the Delegate instead of only a named association. It
also cleans up the module name space like other nested types and makes
associated protocols more discoverable in my opinion.
I’d love to hear everyone’s thoughts.
Brad Hilton_______________________________________________
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