[swift-evolution] [Pitch] Changing NSObject dispatch behavior

Charles Srstka cocoadev at charlessoft.com
Fri Dec 16 08:30:33 CST 2016

> On Dec 16, 2016, at 12:36 AM, Kevin Ballard <kevin at sb.org> wrote:
> On Thu, Dec 15, 2016, at 03:01 PM, Charles Srstka wrote:
>>> On Dec 15, 2016, at 4:33 PM, Kevin Ballard <kevin at sb.org <mailto:kevin at sb.org>> wrote:
>>> The problem with that code isn't that `dynamic` doesn't work for computed properties. It does; if you mutate the `foo` property, you'll get the KVO notifications. The problem is you have one property that depends on another and you didn't set up the KVO machinery properly using automaticallyNotifiesObservers(forKey:) or automaticallyNotifiesObserversOf<key>() (incidentally in Swift you can write the latter as a `static let`, since that becomes a class method in Obj-C).
>> You’ll only get the notifications if you mutate ‘foo’ directly. This, however, is fairly useless, because if you are watching ‘foo’, you want to be notified every time the value changes, not just when someone hits one particular accessor. Code relying on observation of ‘foo’ in the example I provided would be prone to breaking in mysterious and possibly horrible ways.
> No, if you implement keyPathsForValuesAffecting<key>() then you get "foo" KVO notifications when "bar" is mutated.  That's the whole point of that method, and this is exactly what you have to do in Obj-C as well.

Right… the sentence I was quoting was talking about code which uses ‘dynamic’ but *doesn’t* use keyPathsForValuesAffecting<key>. You’ll get notifications if someone calls that one particular accessor, but otherwise you won’t.

>>> So yes, `dynamic` by itself doesn't mean that the property supports KVO. But there are very few reasons to use `dynamic` outside of supporting KVO, so it's a pretty good signal that the property does support it. And conversely, not having `dynamic` doesn't mean that it doesn't support KVO, though if it does have manual KVO support using will/didChangeValue(forKey:) then it should be documented as such.
>> Use of the ‘dynamic’ keyword enables all manner of runtime hackery which someone may be employing. The trick to automatically add KVO conformance to accessors is probably the most common, but it’s hardly the only one. One also might want to declare things ‘dynamic’ when working with Objective-C frameworks not under one’s control which might assume the ability to do metaprogramming on your classes
> That is exceedingly rare. I can't even remember the last time I used such a thing.

You used such a thing the last time you used KVO. ;-)

>> I know it’s commonplace to use ‘dynamic’ all over the place wherever Core Data is involved.
> It is? Why? Maybe you're confusing this with Obj-C's @dynamic keyword, which is completely unrelated to Swift's `dynamic`. When writing Swift NSManagedObject subclasses, you use the @NSManaged property attribute, not the `dynamic` keyword (@NSManaged does effectively the same thing that Obj-C's @dynamic, except it's reserved for integration with CoreData instead of being as generic as Obj-C's @dynamic is).

@NSManaged implies dynamic, though. Core Data is entirely built on the dynamic runtime, and is using it pretty much everywhere.

>> Long story short, ‘dynamic’ does not guarantee KVO conformance in any way, shape, or form.
> And declaring that your property returns a String doesn't guarantee that it actually does either. You can always write broken code. But `dynamic` is required for automatic KVO conformance, and it's extremely rare to have a reason to use `dynamic` outside of KVO, so it's a really really strong signal that the property supports KVO. If you're using `dynamic` on a property but don't support KVO correctly, as you showed in your code example, that's a bug with your code.

Correlation does not imply causation. Dynamic properties often support KVO, but the developer may have just declared it dynamic in order to use Objective-C’s dynamism to solve some other problem. You don’t know, and thus I certainly wouldn’t call it a bug in the code.


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