Support file for Inside the Bracket, part 7, the final chapter, part two. This time, it's swizzling.

swizzle.m

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

// clang -g -fobjc-arc -Wall -framework Foundation -o swizzle swizzle.m

// Swap two methods
void SwizzleMethod (Class clas, SEL originalSelector, SEL newSelector) {

    Method originalMethod =
        class_getInstanceMethod (clas, originalSelector);
    Method newMethod =
        class_getInstanceMethod (clas, newSelector);
    
    BOOL addedMethod =
        class_addMethod (clas, originalSelector,
                         method_getImplementation(newMethod),
                         method_getTypeEncoding(newMethod));
    if (addedMethod) {
        class_replaceMethod (clas, newSelector,
                             method_getImplementation(originalMethod),
                             method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations (originalMethod, newMethod);
    }

} // SwizzleMethod


// Naive implementation *sort* of works, but will clober the superclass if
// |clas| doesn't implement the original method but the super does.
// Don't use this.  Really.  It's used as an object lesson on being
// very careful with these tools.
void SwizzleMethodBadly (Class clas, SEL originalSelector, SEL newSelector) {
    Method originalMethod = class_getInstanceMethod (clas, originalSelector);
    Method newMethod = class_getInstanceMethod (clas, newSelector);

    if (originalMethod && newMethod) {
        method_exchangeImplementations (originalMethod, newMethod);
    } else {
        NSLog (@"no swizzle u!");
    }
    
} // SwizzleMethodBadly



// --------------------------------------------------
// Say stuff from Cocoa or some library we want to hack

@interface BaseClass : NSObject

- (void) hornswoggle;
- (void) bamboozle;

- (void) doStuff; // Calls hornswoggle and bamboozle to do its work.
@end

@interface FirstBegat : BaseClass
// implements hornswoggle and bamboozle
@end

@interface SecondBegat : FirstBegat
// implements hornswoggle (not bamboozle)
@end


@implementation BaseClass

- (void) doStuff {
    [self hornswoggle];
    [self bamboozle];
} // doStuff

- (void) hornswoggle {
    NSLog (@"BaseClass hornswoggle");
}

- (void) bamboozle {
    NSLog (@"BaseClass bamboozle");
}

@end // BaseClass


@implementation FirstBegat
- (void) hornswoggle {
    NSLog (@"FirstBegat hornswoggle calling");
    [super hornswoggle];
}

- (void) bamboozle {
    NSLog (@"FirstBegat bamboozle calling");
    [super bamboozle];
}
@end // FirstBegat


@implementation SecondBegat
- (void) hornswoggle {
    NSLog (@"SecondBegat hornswoggle calling");
    [super hornswoggle];
}
@end // SecondBegat


// --------------------------------------------------
// Container for the various swizzled methods


@interface FirstBegat (HackNSlash)
+ (void) hck_hijackMethods;
@end

@implementation FirstBegat (HackNSlash)
+ (void) hck_hijackMethods {
    SwizzleMethod ([self class], @selector(hornswoggle),
                   @selector($hackFirstBegat_Hornswoggle));
}

- (void) $hackFirstBegat_Hornswoggle {
    NSLog (@"I'm in ur FirstBegat doin ur %s", sel_getName(_cmd));
    [self $hackFirstBegat_Hornswoggle];  // Actually calls original implementation
}

@end


@interface SecondBegat (HackNSlash)
+ (void) hck_hijackMethods;
@end

@implementation SecondBegat (HackNSlash)
+ (void) hck_hijackMethods {
    SwizzleMethodBadly ([self class], @selector(bamboozle),
                        @selector($hackSecondBegat_Bamboozle));
}

- (void) $hackSecondBegat_Bamboozle {
    NSLog (@"I'm in ur SecondBegat doin ur %s", sel_getName(_cmd));
}

@end




int main (void) {
    @autoreleasepool {
        FirstBegat *first = [FirstBegat new];
        SecondBegat *second = [SecondBegat new];

        NSLog (@"--------------------------------------------------");
        NSLog (@"FirstBegat, do stuff!");
        [first doStuff];

        NSLog (@"--------------------------------------------------");
        NSLog (@"SecondBegat, do stuff!");
        [second doStuff];


        NSLog (@"==================================================");
        [FirstBegat hck_hijackMethods];

        NSLog (@"FirstBegat, do stuff!");
        [first doStuff];

        NSLog (@"--------------------------------------------------");
        NSLog (@"SecondBegat, do stuff!");
        [second doStuff];


        NSLog (@"==================================================");
        [SecondBegat hck_hijackMethods];

        NSLog (@"FirstBegat, after bad swizzling, do stuff!");
        [first doStuff];

        NSLog (@"--------------------------------------------------");
        NSLog (@"SecondBegat, do stuff!");
        [second doStuff];
    }
    return 0;
} // main