<div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif">><span style="font-family:arial,sans-serif;font-size:12.800000190734863px">Why is the second check false, even if the property is marked as </span><code style="font-family:menlo,consolas,'liberation mono',courier,monospace;font-size:10pt;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgb(248,248,248);border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">unowned</code><span style="font-family:arial,sans-serif;font-size:12.800000190734863px"> for the view?</span></div><div class="gmail_default" style="font-family:georgia,serif"><br></div><div class="gmail_default"><span style="font-size:12.800000190734863px">Please search the mailing list, this is not the first time it comes as a new question. Shortly speaking, it is `false` only because you used `unowned`. If you you can grantee it always exists. Just use it directly, this is what `unowned` for. If you can't grantee that. You should use `weak` and check it with `if let` or `if foo == nil`</span></div><div class="gmail_default"><span style="font-size:12.800000190734863px"><br></span></div><div class="gmail_default"><span style="font-size:12.800000190734863px">Zhaoxin</span></div><div class="gmail_default"><span style="font-size:12.800000190734863px"><br></span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Nov 18, 2016 at 8:05 PM, Adrian Zubarev via swift-users <span dir="ltr"><<a href="mailto:swift-users@swift.org" target="_blank">swift-users@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div class="m_5359467609274298590bloop_markdown"><p>Hi there,</p>
<p>I just can’t get my head around mutable views and COW.</p>
<p>Here is a small example:</p>
<pre><code class="m_5359467609274298590swift">final class Storage {
var keys: [String] = []
var values: [Int] = []
}
public struct Document {
var _storageReference: Storage
public init() {
self._storageReference = Storage()
}
public init(_ values: DocumentValues) {
self._storageReference = values._storageReference
}
public var values: DocumentValues {
get { return DocumentValues(self) }
set { self = Document(newValue) }
}
}
public struct DocumentValues : MutableCollection {
unowned var _storageReference: Storage
init(_ document: Document) {
self._storageReference = document._storageReference
}
public var startIndex: Int {
return self._storageReference.keys.<wbr>startIndex
}
public var endIndex: Int {
return self._storageReference.keys.<wbr>endIndex
}
public func index(after i: Int) -> Int {
return self._storageReference.keys.<wbr>index(after: i)
}
public subscript(position: Int) -> Int {
get { return _storageReference.values[<wbr>position] }
set { self._storageReference.values[<wbr>position] = newValue } // That will break COW
}
}
</code></pre>
<p>First of all the <code>_storageReference</code> property is <code>unowned</code> because I wanted to check the following:</p>
<pre><code class="m_5359467609274298590swift">var document = Document()
print(CFGetRetainCount(<wbr>document._storageReference)) //=> 2
print(<wbr>isKnownUniquelyReferenced(&<wbr>document._storageReference)) // true
var values = document.values
print(CFGetRetainCount(values.<wbr>_storageReference)) //=> 2
print(<wbr>isKnownUniquelyReferenced(&<wbr>values._storageReference)) // false
</code></pre>
<p>Why is the second check false, even if the property is marked as <code>unowned</code> for the view?</p>
<p>Next up, I don’t have an idea how to correctly COW optimize this view. Assume the following scenario:</p>
<p>Scenario A:</p>
<pre><code class="m_5359467609274298590swift">var document = Document()
// just assume we already added some values and can mutate safely on a given index
// mutation in place
document.values[0] = 10
</code></pre>
<p>VS:</p>
<p>Scenario B:</p>
<pre><code class="m_5359467609274298590swift">var document = Document()
let copy = document
// just assume we already added some values and can mutate safely on a given index
// mutation in place
document.values[0] = 10 // <--- this should only mutate `document` but not `copy`
</code></pre>
<p>We could change the subscript setter on the mutable view like this:</p>
<pre><code class="m_5359467609274298590swift">set {
if !isKnownUniquelyReferenced(&<wbr>self._storageReference) {
self._storageReference = ... // clone
}
self._storageReference.values[<wbr>position] = newValue
}
</code></pre>
<p>There is only one problem here. We’d end up cloning the storage every time, because as shown in the very first example, even with <code>unowned</code> the function <code>isKnownUniquelyReferenced</code> will return false for scenario A.</p>
<p>Any suggestions? </p>
<p>PS: In general I also wouldn’t want to use <code>unowned</code> because the view should be able to outlive it’s parent.</p><span class="HOEnZb"><font color="#888888">
<p></p></font></span></div><span class="HOEnZb"><font color="#888888"><div class="m_5359467609274298590bloop_original_html"><div id="m_5359467609274298590bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto"><br></div><br><div id="m_5359467609274298590bloop_sign_1479469826202990080" class="m_5359467609274298590bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div></div><div class="m_5359467609274298590bloop_markdown"><p></p></div></font></span></div><br>______________________________<wbr>_________________<br>
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<br></blockquote></div><br></div>