CTObjectiveCRuntimeAdditions

Some Objective-C runtime additions.

Swizzeling methods with blocks

If you wanted to swizzle a method before, you would have to declare a category on a class (usually resulting in two extra files).

CTObjectiveCRuntimeAdditions introduces a new method void class_swizzleSelectorWithBlock(Class class, SEL originalSelector, SEL unusedSelector, id block); which takes the class, the selector you want to swizzle, an unused selector in which the previous implementation will be stored and a block that will be called instead of the original implementation.

The block must come with a defined argument list as follows:

• return type of previous implemention must match the return type of the block.
• first argument must be an object which will be self
• second argument must be an implementation pointer, which will be passed to the block to be able to call the previous implementation
• further arguments must be the as of the original implementation after its SEL _cmd argument

Example:

If we take the following class as an example

@interface CTTestSwizzleClass : NSObject {

- (NSString *)helloWorldStringFromString:(NSString *)string;

@end

@implementation CTTestSwizzleClass

- (NSString *)helloWorldStringFromString:(NSString *)string
{
    return [@"Hello World" stringByAppendingFormat:@" %@", string];
}

@end

one could swizzle the selector @selector(helloWorldStringFromString:) as follows:

class_swizzleSelectorWithBlock([CTTestSwizzleClass class], @selector(helloWorldStringFromString:), @selector(__hookedHelloWorldStringFromString:), ^NSString *(CTTestSwizzleClass *blockSelf, IMP originalImplementation, NSString *string) {
    return [originalImplementation(blockSelf, @selector(__hookedHelloWorldStringFromString:), string) stringByAppendingFormat:@" Hooked"];
});

Now the following code

CTTestSwizzleClass *testObject = [[CTTestSwizzleClass alloc] init];
NSLog(@"%@", [testObject helloWorldStringFromString:@"Oli"]);

would produce the output Hello World Oli Hooked instead of just Hello World Oli.

Getting runtime information about blocks

CTBlockDescription lets you inspect blocks including arguments and compile time features at runtime.

One could use CTBlockDescription for the following test block:

// a test block.
BOOL(^testBlock)(BOOL animated, id object) = ^BOOL(BOOL animated, id object) {
    return YES;
};

// allocating a block description
CTBlockDescription *blockDescription = [[CTBlockDescription alloc] initWithBlock:testBlock];

// getting a method signature for this block
NSMethodSignature *methodSignature = blockDescription.blockSignature;
/**
<NSMethodSignature: 0x253f080>
    number of arguments = 3
    frame size = 12
    is special struct return? NO
    return value: -------- -------- -------- --------
        type encoding (c) 'c'
        flags {isSigned}
        modifiers {}
        frame {offset = 0, offset adjust = 0, size = 4, size adjust = -3}
        memory {offset = 0, size = 1}
    argument 0: -------- -------- -------- --------
        type encoding (@) '@?'
        flags {isObject, isBlock}
        modifiers {}
        frame {offset = 0, offset adjust = 0, size = 4, size adjust = 0}
        memory {offset = 0, size = 4}
    argument 1: -------- -------- -------- --------
        type encoding (c) 'c'
        flags {isSigned}
        modifiers {}
        frame {offset = 4, offset adjust = 0, size = 4, size adjust = -3}
        memory {offset = 0, size = 1}
    argument 2: -------- -------- -------- --------
        type encoding (@) '@'
        flags {isObject}
        modifiers {}
        frame {offset = 8, offset adjust = 0, size = 4, size adjust = 0}
        memory {offset = 0, size = 4}
*/

License

MIT

Thanks goes to the Block Implementation Specification and the A2DynamicDelegate project for some good libffi samples and convertion from encodings to libffi types.