Forcing subclass creation through method swizzling not airtight?
Forcing subclass creation through method swizzling not airtight?
- Subject: Forcing subclass creation through method swizzling not airtight?
- From: Antonio Nunes <email@hidden>
- Date: Sun, 11 Mar 2012 07:05:56 +0000
In the latest public release of my software, I used method swizzling to force creation of my subclass when the system tries to create instances of its superclass. However, I have received a very few reports where apparently instances of the superclass are still created, and wreak havoc (i.e. they cause a crash). In what way is it still possible for instances of the superclass to be created?
The swizzling is performed early on during the application startup:
+ (void)initialize
{
// To force PDFDocuments to be created as NMD_PDFDocuments
// Necessary because of an issue with PDFThumbnailView
Method originalMethod = class_getClassMethod([PDFDocument class], @selector(allocWithZone:));
Method superMethod = class_getClassMethod(class_getSuperclass([PDFDocument class]), @selector(allocWithZone:));
Method replacedMethod = class_getClassMethod([PDFDocument class], @selector(allocWithZoneReplacement:));
if (superMethod == originalMethod) {
class_addMethod(object_getClass([PDFDocument class]), @selector(allocWithZone:),
method_getImplementation(replacedMethod),
method_getDescription(replacedMethod)->types);
}
// To force PDFPages to be created as NMD_PDFPage
originalMethod = class_getClassMethod([PDFPage class], @selector(allocWithZone:));
superMethod = class_getClassMethod(class_getSuperclass([PDFPage class]), @selector(allocWithZone:));
replacedMethod = class_getClassMethod([PDFPage class], @selector(allocWithZoneReplacement:));
if (superMethod == originalMethod) {
class_addMethod(object_getClass([PDFPage class]), @selector(allocWithZone:),
method_getImplementation(replacedMethod),
method_getDescription(replacedMethod)->types);
}
}
The replacement method lives in a category on PDFPage:
@implementation PDFPage (PDFPage_AllocWithZoneReplacement)
// Forces PDFPages to be created as NMD_PDFPages
+ (id)allocWithZoneReplacement:(NSZone *)zone
{
if ([self class] == [PDFPage class]) {
return [NMD_PDFPage allocWithZoneReplacement:zone];
} else {
return [super allocWithZone:zone];
}
}
@end
The only change I can think of that might make a difference is to change:
if ([self class] == [PDFPage class])
to
if ([self class] != [NMD_PDFPage class])
but why would this code ever be called for any classes other than either PDFPage or NMD_PDFPage? Maybe someone can point out circumstances under which this whole setup can fail, or be sidestepped.
-António
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A merry heart does good like medicine
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