<div style="white-space:pre-wrap">I want to reiterate that I have objective-c code, others have objc code, and the cocoa, etc. frameworks have code that depend on optional protocol for things like (but not limited to) delegates. This is of course obvious but what seems to get lost in the discussion is that you can't always replace the non-existence of an implementation of an optional protocol method with a default implementation.<br><br>I have code that probes a delegate when registered and based on the what subset of the optional protocol methods it handles configures its runtime state to optimize itself to that reality. For example it may avoid allocating and maintaining potentially complex state if one or more methods are not implemented by the delegate (since no one is interested in it). If we just blindly provide default implementation for optional methods then this optimization couldn't take place.<br><br>I know others - including I believe Apple framework code - do similar optimizations based on what methods an object implements.<br><br>I think we should maintain the optional concept in support of bridging existing objc code into swift (confined to @objc)... unless a way to bridge things can be defined that avoids the loss of optimization potential I outlined above.<br><br>Optional protocols don't need to be expanded into Swift itself since I believe alternate methods and patterns exists to solve the same type of need.<br><br>-Shawn</div><br><div class="gmail_quote"><div dir="ltr">On Thu, Apr 7, 2016 at 5:12 PM Douglas Gregor via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word">Hi all,<div><br></div><div>Optional protocol requirements in Swift have the restriction that they only work in @objc protocols, a topic that’s come up a <a href="http://thread.gmane.org/gmane.comp.lang.swift.devel/1316/focus=8804" target="_blank">number</a> of <a href="http://thread.gmane.org/gmane.comp.lang.swift.evolution/13347/focus=13480" target="_blank">times</a>. The start of these threads imply that optional requirements should be available for all protocols in Swift. While this direction is implementable, each time this is discussed there is significant feedback that optional requirements are not a feature we want in Swift. They overlap almost completely with default implementations of protocol requirements, which is a more general feature, and people seem to feel that designs based around default implementations and refactoring of protocol hierarchies are overall better.</div><div><br></div><div>The main concern with removing optional requirements from Swift is their impact on Cocoa: Objective-C protocols, especially for delegates and data sources, make heavy use of optional requirements. Moreover, there are no default implementations for any of these optional requirements: each caller effectively checks for the presence of the method explicitly, and implements its own logic if the method isn’t there.</div><div><br></div><div><b>A Non-Workable Solution: Import as optional property requirements</b></div><div>One suggestion that’s come up to map an optional requirement to a property with optional type, were “nil” indicates that the requirement was not satisfied. For example, </div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">@protocol NSTableViewDelegate</font></div><div><font face="Menlo">@optional</font></div><div><font face="Menlo">- (nullable NSView *)tableView:(NSTableView *)tableView viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row;</font></div><div><font face="Menlo">- (CGFloat)tableView:(NSTableView *)tableView heightOfRow:(NSInteger)row;</font></div><div><font face="Menlo">@end</font></div></blockquote><div><br></div><div>currently comes in as</div><div><br></div><div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><font face="Menlo">@objc protocol NSTableViewDelegate {</font></div><div><font face="Menlo"> optional func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?</font></div><div><font face="Menlo"> optional func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat</font></div><div><font face="Menlo">}</font></div><div><font face="Menlo"><br></font></div></blockquote></div><div>would come in as:</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">@objc protocol NSTableViewDelegate {</font></div><div><font face="Menlo"> var tableView: ((NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?)? { get }</font></div><div><font face="Menlo"> var tableView: ((NSTableView, heightOfRow: Int) -> CGFloat)? { get }</font></div><div><font face="Menlo">}</font></div></blockquote><div><br></div><div>with a default implementation of “nil” for each. However, this isn’t practical for a number of reasons:</div><div><br></div><div>a) We would end up overloading the property name “tableView” a couple dozen times, which doesn’t actually work.</div><div><br></div><div>b) You can no longer refer to the member with a compound name, e.g., “delegate.tableView(_:viewFor:row:)” no longer works, because the name of the property is “tableView”.</div><div><br></div><div>c) Implementers of the protocol now need to provide a read-only property that returns a closure. So instead of</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">class MyDelegate : NSTableViewDelegate {</font></div><div><font face="Menlo"> func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView? { … }</font></div><div><font face="Menlo">}</font></div></blockquote><div><br></div><div>one would have to write something like</div><div><br></div><div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><font face="Menlo">class MyDelegate : NSTableViewDelegate {</font></div><div><font face="Menlo"> var tableView: </font><span style="font-family:Menlo">((NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?)? = {</span></div><div><span style="font-family:Menlo"> </span><font face="Menlo">… except you can’t refer to self in here unless you make it lazy ...</font></div><div><span style="font-family:Menlo"> }</span></div><div><span style="font-family:Menlo">}</span></div></blockquote></div><div><br></div><div>d) We’ve seriously considered eliminating argument labels on function types, because they’re a complexity in the type system that doesn’t serve much of a purpose.</div><div><br></div><div>One could perhaps work around (a), (b), and (d) by allowing compound (function-like) names like tableView(_:viewFor:row:) for properties, and work around (c) by allowing a method to satisfy the requirement for a read-only property, but at this point you’ve invented more language hacks than the existing @objc-only optional requirements. So, I don’t think there is a solution here.</div><div><br></div><div><b>Proposed Solution: Caller-side default implementations</b></div><div><br></div><div>Default implementations and optional requirements differ most on the caller side. For example, let’s use NSTableView delegate as it’s imported today:</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">func useDelegate(delegate: NSTableViewDelegate) {</font></div><div><font face="Menlo"> if let getView = delegate.tableView(_:viewFor:row:) { // since the requirement is optional, a reference to the method produces a value of optional function type</font></div><div><font face="Menlo"> // I can call getView here</font></div><div><font face="Menlo"> }</font></div><div><font face="Menlo"><br></font></div><div><font face="Menlo"> if let getHeight = delegate.tableView(_:heightOfRow:) {</font></div><div><font face="Menlo"> // I can call getHeight here</font></div><div><font face="Menlo"> }</font></div><div><span style="font-family:Menlo">}</span></div></blockquote><div><br></div><div>With my proposal, we’d have some compiler-synthesized attribute (let’s call it @__caller_default_implementation) that gets places on Objective-C optional requirements when they get imported, e.g.,</div><div><br></div><div><div><font face="Menlo">@objc protocol NSTableViewDelegate {</font></div><div><font face="Menlo"> @__caller_default_implementation func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView?</font></div><div><font face="Menlo"> </font><span style="font-family:Menlo">@__caller_default_implementation</span><font face="Menlo"> func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat</font></div><div><font face="Menlo">}</font></div></div><div><font face="Menlo"><br></font></div><div>And “optional” disappears from the language. Now, there’s no optionality left, so our useDelegate example tries to just do correct calls:</div><div><br></div><div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><font face="Menlo">func useDelegate(delegate: NSTableViewDelegate) -> NSView? {</font></div><div><font face="Menlo"> let view = delegate.tableView(tableView, viewFor: column, row: row)</font></div><div><font face="Menlo"> let height = delegate.tableView(tableView, heightOfRow: row)</font></div><div><font face="Menlo">}</font></div><div><font face="Menlo"><br></font></div></blockquote></div><div>Of course, the code above will fail if the actual delegate doesn’t implement both methods. We need some kind of default implementation to fall back on in that case. I propose that the code above produce a compiler error on both lines *unless* there is a “default implementation” visible. So, to make the code above compile without error, one would have to add:</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">extension NSTableViewDelegate {</font></div><div><font face="Menlo"> @nonobjc func tableView(_: NSTableView, viewFor: NSTableColumn, row: Int) -> NSView? { return nil }</font></div><div><font face="Menlo"> </font></div><span style="font-family:Menlo"> @nonobjc </span><span style="font-family:Menlo">func tableView(_: NSTableView, heightOfRow: Int) -> CGFloat { return 17 }</span><div><font face="Menlo">} </font></div></blockquote><div><br></div><div>Now, the useDelegate example compiles. If the actual delegate implements the optional requirement, we’ll use that implementation. Otherwise, the caller will use the default (Swift-only) implementation it sees. From an implementation standpoint, the compiler would effectively produce the following for the first of these calls:</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="Menlo">if delegate.responds(to: #selector(NSTableViewDelegate.tableView(_:viewFor:row:))) {</font></div><div><font face="Menlo"> // call the @objc instance method with the selector tableView:viewForTableColumn:row:</font></div><div><font face="Menlo">} else {</font></div><div><font face="Menlo"> // call the Swift-only implementation of tableView(_:viewFor:row:) in the protocol extension above</font></div><div><font face="Menlo">}</font></div></blockquote><div><br></div><div>There are a number of reasons why I like this approach:</div><div><br></div><div>1) It eliminates the notion of ‘optional’ requirements from the language. For classes that are adopting the NSTableViewDelegate protocol, it is as if these requirements had default implementations.</div><div><br></div><div>2) Only the callers to these requirements have to deal with the lack of default implementations. This was already the case for optional requirements, so it’s not an extra burden in principle, and it’s generally going to be easier to write one defaulted implementation than deal with it in several different places. Additionally, most of these callers are probably in the Cocoa frameworks, not application code, so the overall impact should be small.</div><div><br></div><div>Thoughts?</div><div><br></div><div><span style="white-space:pre-wrap"> </span>- Doug</div><div><br></div></div>_______________________________________________<br>
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