<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hello all,<div class=""><br class=""></div><div class="">I’ve discovered a bug that’s causing crashes when Swift code interoperates with AppKit in a certain way. I intend to file a bug report, but I’m not sure whether the bug should be filed as a Swift bug, or as an AppKit bug since a case could probably be made for either, and I thought I would ask the list first.</div><div class=""><br class=""></div><div class="">What’s happening, basically, is that when an NSTextFieldCell subclass contains Objective-C-compatible reference-type properties, and is loaded from a .nib file in which its containing text field has its Baseline aligned to some other object via autolayout, its properties get over-released when the cell is deallocated. A simple example that will cause the crash is:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">@objc</span><span style="font-variant-ligatures: no-common-ligatures" class="">(Foo) </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">class</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Foo: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">NSObject</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {}</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">@objc</span><span style="font-variant-ligatures: no-common-ligatures" class="">(CustomTextFieldCell) </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">class</span><span style="font-variant-ligatures: no-common-ligatures" class=""> CustomTextFieldCell: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">NSTextFieldCell</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> foo = </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Foo</span><span style="font-variant-ligatures: no-common-ligatures" class="">()</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">The equivalent code in Objective-C works properly and does not crash:</span></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span style="font-family: Helvetica; font-size: 12px;" class=""><br class=""></span></span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #78492a" class="">#import </span><span style="font-variant-ligatures: no-common-ligatures" class=""><Cocoa/Cocoa.h></span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">@interface</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Foo: NSObject</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@end</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@implementation</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> Foo</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@end</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">@interface</span><span style="font-variant-ligatures: no-common-ligatures" class=""> CustomTextFieldCell: NSTextFieldCell</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@property</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> (</span><span style="font-variant-ligatures: no-common-ligatures" class="">nonatomic</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures" class="">strong</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">) Foo *foo;</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@end</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">@implementation</span><span style="font-variant-ligatures: no-common-ligatures" class=""> CustomTextFieldCell</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">- (</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">instancetype</span><span style="font-variant-ligatures: no-common-ligatures" class="">)initWithCoder:(NSCoder *)coder {</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">->_foo = [Foo new];</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><br class="webkit-block-placeholder"></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> [</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">super</span><span style="font-variant-ligatures: no-common-ligatures" class=""> initWithCoder:coder];</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="font-family: Menlo; font-size: 11px; margin: 0px; line-height: normal; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">@end</span></div><div style="font-family: Menlo; font-size: 11px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures;" class="">The problem seems to occur, as far as I can tell, because when the Baseline layout relation is applied, AppKit copies the text field’s cell. Subsequently, NSCell’s -copyWithZone: method calls NSCopyObject, which in turn calls a private function named “fixUpCopiedIvars.” With an Objective-C cell class, fixUpCopiedIvars calls class_getIvarLayout, and retains all its instance variables, so both the original cell and the copy have an owning reference to all of them. This retain is then balanced by a release when the cell is deallocated. With a Swift cell class, however, class_getIvarLayout returns NULL, so the ivars are never retained; however, this nonexistent retain is still balanced by a release when the cell is deallocated. The result is that the program accesses freed memory, leading to a crash or worse.</span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""><br class=""></span></div><div class="">A sample project demonstrating all this is here: <a href="http://www.charlessoft.com/bug_examples/Crash_Swiftly.zip" class="">http://www.charlessoft.com/bug_examples/Crash_Swiftly.zip</a></div><div class=""><br class=""></div><div class="">So, there’s clearly a bug here, but I’m not sure which of these three possibilities is correct:</div><div class=""><br class=""></div><div class="">- This is a bug in AppKit, because NSCell should not be using the deprecated NSCopyObject or assuming that class_getIvarLayout will work.</div><div class=""><br class=""></div><div class="">- This is a bug in the Swift<->Objective-C bridge, because Swift can be used to write Objective-C objects, and legacy Objective-C code like NSCopyObject that interacts with said objects may assume that it can access instance variables via class_getIvarLayout; thus, the latter should work.</div><div class=""><br class=""></div><div class="">- Or option 3: The Swift team considers it to be a bug in AppKit, and the AppKit team considers this to be a bug in Swift, or maybe the Foundation team gets involved to make this into a triangle. This is obviously the worst-case scenario. ;-)</div><div class=""><br class=""></div><div class="">What does the community think? Should I file this as a bug on Swift? Foundation? AppKit? All three?</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Charles</div><div class=""><br class=""></div></span></div></div></body></html>