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.send { color:#77bb77; }
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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_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="swift">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.startIndex
}
public var endIndex: Int {
return self._storageReference.keys.endIndex
}
public func index(after i: Int) -> Int {
return self._storageReference.keys.index(after: i)
}
public subscript(position: Int) -> Int {
get { return _storageReference.values[position] }
set { self._storageReference.values[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="swift">var document = Document()
print(CFGetRetainCount(document._storageReference)) //=> 2
print(isKnownUniquelyReferenced(&document._storageReference)) // true
var values = document.values
print(CFGetRetainCount(values._storageReference)) //=> 2
print(isKnownUniquelyReferenced(&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="swift">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="swift">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="swift">set {
if !isKnownUniquelyReferenced(&self._storageReference) {
self._storageReference = ... // clone
}
self._storageReference.values[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>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_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="bloop_sign_1479469826202990080" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div></div><div class="bloop_markdown"><p></p></div></body></html>