Rationale:
As a everybody knows, “optional” keyword is present in swift solely to maintain
compatibility with objective-c protocols. Optional methods in protocols are
neither a swift capability, nor a wanted one. In fact, proposal [0070] intends
to begin the transition to an optionals-free language. In fact, optionals, as
presented by objective-c, are a manner to avoid interface segregation.
Nonetheless, in many occasions, optionals are used to model customized behavior
vs default one. For instance, if you take a look at the documentation of
UITableViewDataSource or delegate, you’ll see that optional methods are not
required, since the framework provides a default behavior that can be
customized by implementing the corresponding optional methods.
Consequently, is most cases, optional methods in objective-c are a means to
replace the semantics of default protocol method implementations, as supported
through extensions in swift.
Therefore, my proposal is to modify the semantics of “optional” keyword in
swift to mean: “you must provide a default implementation of this method
through an extension”. This is different from objective-c semantics, which mean
“the implementation of this method may not be provided”.
Detailed design:
In this proposal, protocols could be defined like this:
protocol Datasource {
associatedtype Element
var count:Int {get}
func elementAt(index:Int) -> Element
optional func color(elementIndex:Int) -> UIColor
}
However, this definition enforces the developer to create an extension a
provide a default implementation of the optional method:
extension Datasource {
func color(elementIndex:Int) -> UIColor {
return UIColor.blackColor()
}
}
In this way, what we are achieving is that we are avoiding objective-c optional
semantics (which is a way to avoid interface segregation), but we are making
explicit that a method in a protocol requires a default implementation, thus
not requiring the developer to re-implement the method in any entity adopting
the protocol (as it currently happens when we provide a default method
implementation). Moreover, we are making explicit that a certain protocol
method has a default implementation, which can be confusing right now.
Note that in this proposal, the intention is to keep both "@objc optional” and
simply “optional” keywords, to highlight both semantics. However, in order to
avoid @objc optional semantics as much as possible (to be able to remove it in
a future swift release), new annotations could be incorporated to optional
methods in objective-c code, to specify the default returned value. For
instance, the annotations could be like this:
@protocol Datasource
-(NSInteger) numberOfElements;
-(NSObject*) elementAtIndex:(NSInteger)index;
@optional
-(UIColor*) colorOfElementAtIndex:(NSInteger)index
__attribute__((swift_default_value(“UIColor.blackColor()")));
@end
Note that this annotation also allows to better understand the default behavior
in case the method is not implemented without reading the documentation. This
annotation should produce a swift code similar to the above one.
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