On Apr 19, 2017, at 10:56 , Charles Srstka <cocoa...@charlessoft.com> wrote:
>
> 2. Stored properties need to call willChangeValue(forKey:) and
> didChangeValue(forKey:).
> a. In most cases, just add “dynamic” to the property declaration, and
> Cocoa will automagically insert the needed calls.
The problem with saying it this way is that it can be taken to imply that the
calls will be inserted statically *into the setter’s implementation*, which is
not true. It is true that Cocoa will automagically *execute* the needed calls.
It also can be taken to imply that Cocoa does this automagically because the
property is dynamic, which is also not really true. Cocoa does this because
it’s a setter for an Obj-C property which returns true from its corresponding
automaticallyNotifiesObserversOfFoo class method. That is, “dynamic” doesn’t
turn the automagic behavior off or on, it’s simply a requirement to ensure that
the property is fully in the Obj-C domain.
> 3. Computed properties do not need to be dynamic, […].
This is also not exactly true. Computed properties that have only a getter do
not need to be dynamic, because they don’t generate any KVO notifications via a
setter, and so “dynamic” is irrelevant.
However, a computed property that has a setter will *not* produce the expected
notifications unless it is marked “dynamic”, even if marked “@objc”. (I tested
this to be sure.) There’s nothing special about computed vs. stored properties
in this regard.
I think you were “really” talking about derived properties, which are typically
computed properties with only a getter, no setter.
In addition, “dynamic” is documented as needed, in the place I linked to before:
https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html#//apple_ref/doc/uid/TP40014216-CH7-ID6
<https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html#//apple_ref/doc/uid/TP40014216-CH7-ID6>
under the heading Key-Value Observing where it says: "Add the dynamic modifier
to any property you want to observe.” This may be overly conservative, but it
is at least official.
For all those reasons, I think it’s pointless to try to figure out the contexts
where “dynamic” is unnecessary. It seems to me that the best advice is that
from the documentation: unconditionally add “dynamic” to any property you want
to observe. (Well, in the future, add “@objc dynamic”.)
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