On Fri, Apr 22, 2016 at 7:35 PM, Douglas Gregor via swift-evolution <swift-evolution@swift.org> wrote:
Proposal link:
https://github.com/DougGregor/swift-evolution/blob/objc-optional/proposals/NNNN-optional-requirements.md
After a whole lot of discussion and thrashing on optional requirements, I
have a draft for a modest proposal: change the ‘optional’ keyword to
something that indicates that this feature is only for compatibility with
Objective-C and will not be supported on Swift protocols. Comments welcome!
- Doug
Make Optional Requirements Objective-C-only
Proposal: SE-NNNN
Author(s): Doug Gregor
Status: Awaiting review
Review manager: TBD
Introduction
Swift currently has support for "optional" requirements in Objective-C
protocols, to match with the corresponding feature of Objective-C. We don't
want to make optional requirements a feature of Swift protocols (for reasons
described below), nor can we completely eliminate the notion of the language
(for different reasons also described below). Therefore, to prevent
confusion about our direction, this proposal changes the optional keyword
objcoptional to indicate that this is an Objective-C compatibility feature.
Swift-evolution threads: eliminate optional requirements, make Swift
protocols support optional requirements and make optional protocol
requirements first class citizens.
Motivation
Having optional only work for Objective-C requirements is very weird: it
feels like a general feature with a compiler bug that prevents it from
working generally. However, we don't want to make it a feature of Swift
protocols and we can't eliminate it (see alternatives considered), so we
propose to rename the keyword to make it clear that this feature is intended
only for compatibility with Objective-C.
Proposed solution
Rename the optional contextual keyword to objcoptional. Note that:
It would read better as objc_optional or objcOptional, but keywords in Swift
run the words together, and
It should not be an attribute @objcOptional because it changes the effective
type of the declaration. Referencing an optional requirement wraps the
result in one more level of optional, which is used to test whether the
requirement was implemented.
This means that:
@objc protocol NSTableViewDelegate {
optional func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int)
-> NSView?
optional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat
}
becomes:
@objc protocol NSTableViewDelegate {
objcoptional func tableView(_: NSTableView, viewFor: NSTableColumn, row:
Int) -> NSView?
objcoptional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat
}
Impact on existing code
Any code that declares @objc protocols with optional requirements will need
to be changed to use the objcoptionalkeyword. However, it is trivial for the
migrator to update the code and for the compiler to provide Fix-Its, so the
actual impact on users should be small.
Alternatives considered
It's a fairly common request to make optional requirements work in Swift
protocols (as in the aforementioned threads, hereand here). However, such
proposals have generally met with resistance because optional requirements
have significant overlap with other protocol features: "default"
implementations via protocol extensions and protocol inheritance. For the
former case, the author of the protocol can provide a "default"
implementation via a protocol extension that encodes the default case
(rather than putting it at the call site). In the latter case, the protocol
author can separate the optional requirements into a different protocol that
a type can adopt to opt-in to whatever behavior they customize. While not
exactlythe same as optional requirements, which allow one to perform
per-requirement checking to determine whether the type implemented that
requirement, the gist of the threads is that doing so is generally
considered an anti-pattern: one would be better off factoring the protocol
in a different way. Therefore, we do not propose to make optional
requirements work for Swift protocols.
The second alternative would be to eliminate optional requirements entirely
from the language. The primary challenge here is Cocoa interoperability,
because Cocoa's protocols (primarily delegates and data sources) have a
large number of optional requirements that would have to be handled somehow
in Swift. These optional requirements would have to be mapped to some other
construct in Swift, but the code generation model must remain the same
because the Cocoa frameworks rely on the ability to ask the question "was
this requirement implemented by the type?" in Objective-C code at run time.
The most popular approach to try to map optional requirements into existing
Swift constructs is to turn an optional method requirement into a property
of optional closure type. For example, this Objective-C protocol:
@protocol NSTableViewDelegate
@optional
- (nullable NSView *)tableView:(NSTableView *)tableView
viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row;
- (CGFloat)tableView:(NSTableView *)tableView heightOfRow:(NSInteger)row;
@end
which currently imports into Swift as:
@objc protocol NSTableViewDelegate {
optional func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int)
-> NSView?
optional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat
}
would become, e.g.,
@objc protocol NSTableViewDelegate {
var tableView: ((NSTableView, viewFor: NSTableColumn, row: Int) ->
NSView?)? { get }
var tableView: ((NSTableView, heightOfRow: Int) -> CGFloat)? { get }
}
Unfortunately, this introduces an overloaded property named tableView. To
really make this work, we would need to introduce the ability for a property
to have a compound name, which would also let us take the labels out of the
function type:
@objc protocol NSTableViewDelegate {
var tableView(_:viewFor:row:): ((NSTableView, NSTableColumn, Int) ->
NSView?)? { get }
var tableView(_:heightOfRow:): ((NSTableView, Int) -> CGFloat)? { get }
}
By itself, that is a good feature. However, we're not dont, because we would
need yet another extension to the language: one would want to be able to
provide a method in a class that is used to conform to a property in the
protocol, e.g.,
class MyDelegate : NSObject, NSTableViewDelegate {
func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) ->
NSView? { ... }
func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat { ... }
}
Indeed, the Objective-C implementation model effectively requires us to
satisfy these property-of-optional-closure requirements with methods so that
Objective-C clients can use -respondsToSelector:. In other words, one would
not be able to implement these requirements in by copy-pasting from the
protocol to the implementing class:
class MyDelegate : NSObject, NSTableViewDelegate {
// Note: The Objective-C entry points for these would return blocks, which
is incorrect
var tableView(_:viewFor:row:): ((NSTableView, NSTableColumn, Int) ->
NSView?)? { return ... }
var tableView(_:heightOfRow:): ((NSTableView, Int) -> CGFloat)? { return
... }
}
That is both a strange technical restriction that would be limited to
Objective-C protocols and a serious usability problem: the easiest way to
stub out the contents of your type when it conforms to a given protocol is
to copy the declarations from the protocol into your type, then fill in the
details. This change would break that usage scenario badly.
There have been other ideas to eliminate optional requirements. For example,
Objective-C protocols could be annotated with attributes that say what the
default implementation for each optional requirement is (to be used only in
Swift), but such a massive auditing effort is impractical. There is a
related notion of caller-site default implementations that was not
well-received due to its complexity.
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