Hello,
Indeed, I had reduced the code too much. John McCall was kind enough to have a
look and here’s the offending code:
func layoutHorizontally(leftRect: inout CGRect, rightRect: inout CGRect) {
let totalWidth = imageRect.size.width + titleRect.size.width +
contentSpacing
rightRect.origin.x = leftRect.maxX + contentSpacing
}
The problem is that imageRect and titleRect are referenced both directly and in
the inout parameters.
But there’s something I’m not understanding. I went back to the ownership
manifesto and re-read the law of exclusivity to make sure I understand it:
If a storage reference expression evaluates to a storage reference that is
implemented by a variable, then the formal access duration of that access may
not overlap the formal access duration of any other access to the same variable
unless both accesses are reads.
So I tried to write a test to trigger the runtime error, but was unsuccessful:
class MyClass {
var p: CGRect = .zero
}
func trigger(a: MyClass, b: MyClass) {
a.p = b.p
}
let m = MyClass()
trigger(a: m, b: m)
Here, a storage reference expression (a.p) evaluates to a storage reference
that is implemented by a variable (the p property of an instance m of MyClass)
and its formal access duration (the trigger function) overlaps the formal
access duration of another access to the same variable (through the b.p storage
reference expression) and both accesses are not reads (a.p is on the LHS of an
assignment).
Why does this not trigger the Law of Exclusivity?
Regards,
David.
> On 27 Jul 2017, at 21:33, Kyle Murray <kyle_mur...@apple.com> wrote:
>
> Hi David,
>
> Are padding or spacing computed properties that access either rectangle? They
> aren't defined in your stripped down code. For example:
>
> var spacing: CGFloat {
> return imageRect.maxX
> }
>
> Without doing something like that, I don't see the same access conflicts that
> you're seeing.
>
> -Kyle
>
>> On Jul 27, 2017, at 5:04 AM, David Hart via swift-users
>> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:
>>
>> Hello,
>>
>> In Xcode 9 beta 4, Swift 4, I’m getting runtime errors popping up for
>> Simultaneous accesses and I think they may be false negatives. Here’s a
>> stripped down version of my code:
>>
>> class MyButton: UIButton {
>> fileprivate var imageRect: CGRect = .zero
>> fileprivate var titleRect: CGRect = .zero
>>
>> func updateRects() {
>> if imageBeforeTitle {
>> layoutHorizontally(leftRect: &imageRect, rightRect:
>> &titleRect)
>> } else {
>> layoutHorizontally(leftRect: &titleRect, rightRect:
>> &imageRect)
>> }
>> }
>>
>> func layoutHorizontally(leftRect: inout CGRect, rightRect: inout
>> CGRect) {
>> leftRect.origin.x = padding
>> rightRect.origin.x = leftRect.maxX + spacing
>> }
>> }
>>
>> While the layoutHorizontally method has two CGRect inout parameters, never
>> in my code do I pass the same CGRect. Any ideas if this is a bug I should
>> post on bugs.swift.org <http://bugs.swift.org/> or if I’m missing something?
>>
>> David.
>> _______________________________________________
>> swift-users mailing list
>> swift-users@swift.org <mailto:swift-users@swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-users
>
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