方法Swizzling

Question

在尝试对NSString类使用Swizzle isEqualToString:方法时，我遇到了一些奇怪的行为。下面是有问题的代码：

#import <Foundation/Foundation.h>
#import <objc/objc-runtime.h>

@interface NSString (SwizzleString)
- (BOOL) custom_isEqualToString:(NSString *)aString;
- (NSRange)custom_rangeOfString:(NSString *)aString;
@end

@implementation NSString (SwizzleString)

- (BOOL) custom_isEqualToString:(NSString *)aString;
{
    NSLog(@"Inside custom_isEqualToString method definition");
    return [self custom_isEqualToString:aString];
}

- (NSRange)custom_rangeOfString:(NSString *)aString;
{
    NSLog(@"Inside custom_rangeOfString method definition");
    return [self custom_rangeOfString:aString];
}

@end

int main(int argc, const char * argv[])
{

    Method m1, m2;

    m1 = class_getInstanceMethod([NSString class], @selector(isEqualToString:));
    m2 = class_getInstanceMethod([NSString class], @selector(custom_isEqualToString:));
    method_exchangeImplementations(m1, m2);

    m1 = class_getInstanceMethod([NSString class], @selector(rangeOfString:));
    m2 = class_getInstanceMethod([NSString class], @selector(custom_rangeOfString:));
    method_exchangeImplementations(m1, m2);

    NSString *foo = @"Foo";

    // Does not log anything, is still using isEqualToString: implementation
    [foo isEqualToString:@"Foo"];
    // Also does not log anything, since it is using the method implementation from isEqualToString:
    [foo custom_isEqualToString:@"Foo"];

    // Does log something because rangeOfString now uses custom_rangeOfString IMP
    [foo rangeOfString:@"Foo"];
    // Does not log anything because it uses the method implementation from rangeOfString:
    [foo custom_rangeOfString:@"Foo"];
}

isEqualToString:和rangeOfString:都是在NSString上一个名为(NSStringExtensionMethods)的类别中定义的，所以我包含了rangeOfString swizzle，以表明我正确地混合了方法，并且特别是一个NSString对象，这样我就可以排除有关class cluster问题的问题。

当我为上面的代码生成程序集时，我看到的不是普通的objc_msgSend调用，而是类似于l_objc_msgSend_fixup_isEqualToString_的东西。这让我发现了更多关于objective-c vtable的信息，其中似乎可以找到isEqualToString:：

static const char * const defaultVtable[] = {
    "allocWithZone:", 
    "alloc", 
    "class", 
    "self", 
    "isKindOfClass:", 
    "respondsToSelector:", 
    "isFlipped", 
    "length", 
    "objectForKey:", 
    "count", 
    "objectAtIndex:", 
    "isEqualToString:", 
    "isEqual:", 
    "retain", 
    "release", 
    "autorelease", 
};

我一整天都在钻研objective-c源码和互联网，想知道怎样才能以某种方式仍然能够和isEqualToString:打交道。

Answer 1

如您所知，这是一个类集群。您需要的是实际的类，而不是公共类，所以只需询问您拥有的string对象：

NSString *foo = @"Foo";

m1 = class_getInstanceMethod([foo class], @selector(isEqualToString:));
m2 = class_getInstanceMethod([foo class], @selector(custom_isEqualToString:));
method_exchangeImplementations(m1, m2);

还有一种脆弱的替代方法，使用类名本身：

NSClassFromString(@"__NSCFConstantString")
NSClassFromString(@"__NSCFString")