> On 9 Mar 2017, at 01:49, Quincey Morris <quinceymor...@rivergatesoftware.com>
> wrote:
>
> So, the correct answer to your original question, I think, is that if your
> model data has changed in such a way that the representation in custom views
> no longer reflects the data, then you should set “needsDisplay” on *every*
> custom subview that uses “draw(_:)”.
>
> If you just need your changed view to be re-composited with the same subview
> contents as previously, then you don’t need to set “needsDisplay” on the
> subviews, just the parent view.
OK - that disagrees with Ben’s conclusion:
"Thus, it seems to follow that so long a custom view's display() calls super,
then all of its subviews should also be drawn when its needsDisplay is true.”
But maybe it is covered by the “as necessary” clause in Apple’s documentation:
display(): "Displays the view and all its subviews if possible, invoking each
of the NSView methods lockFocus(), draw(_:), and unlockFocus() as necessary.”
I have a custom view that has two custom subviews, and is a subclass of a more
general custom view. The code that tells the general (superclass) view that it
needs to be redrawn shouldn’t have to know which kind of (subclass) view it is
dealing with. So it would be necessary for the subclass view to override
needsDisplay with an observer that sets needDisplay to be true for its subviews:
override var needsDisplay: Bool
{
didSet
{
for view in subviews
{
view.needsDisplay = needsDisplay
}
}
}
Currently, I don’t actually need to do this because I’m not trying to use
layer-backed views (that happened accidentally through a bug or unexpected
behaviour in Interface Builder), but the fact that Apple is unclear about how
needsDisplay affects subviews means that I probably should do this in case they
introduce new drawing optimisations in future versions of macOS.
The other puzzle in all this is why it should be the case that setting
needsDisplay to be true on a layer-backed view actually doesn’t set it to be
true:
> One other strange behaviour with the problem xib occurred when I added some
> debugging code to try and find out why the subview wasn’t being redrawn:
>
> view.needsDisplay = true
> print("view.needsDisplay: \(view.needsDisplay)”)
>
> and this consistently printed “ view.needsDisplay: false” after
> view.needsDisplay had been set to true. Despite this, “draw” (= drawRect in
> ObjC) was being called for “view” - but not for view’s subviews.
Before I discovered that IB had changed one of my views to be layer-backed, I
tried using the didSet observer to pass on needsDisplay to subviews and found
that it was setting the value to be false for subviews when I had set it to be
true for the superview (that’s why I added the debugging code above).
So it seems that for layer-backed views, setting needsDisplay does cause the
view to be redrawn - but the value is never actually set. If I needed to work
with layer-backed views, it might be possible to get around this by overriding
willSet instead of didSet.
I think NSView needs a “Redraws subviews” property (similar to “Autoresizes
subviews”) that will take away the guesswork and hackery that we currently need
to use in order to get some kind of consistent behaviour.
Jeremy
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