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SGVSuperMessagingProxy

An NSProxy/SwiftObject subclass for invoking superclass method implementations of any Objective-C object. Also works with dynamic methods on Swift objects.

What

A proxy object that allows one to invoke method implementations from any class higher in the inheritance hierarchy for any dynamically dispatched method.
On creation, the proxy is passed the object and optionally a class in that object's inheritance hierarchy.
Any message sent to the proxy will be executed as if it was invoked with a super keyword from inside that object's class declaration.

Objective-C example:

  
@interface Cat : NSObject

@property (nonatomic, readonly) NSString *exclamation;

@end

@implementation Cat

- (NSString *)exclamation {
    return @"Meouw!";
}

@end

@interface NyanCat : Cat

@end

@implementation NyanCat

- (NSString *)exclamation {
    return @"Nyan!";
}

@end

NyanCat *cat = [NyanCat new];
NSLog(@"%@", [cat exclamation]); // -> Nyan!
NSLog(@"%@", [[cat sgv_super] exclamation]); // -> Meouw!

Swift example:

import Foundation

class Cat {
    dynamic func says() -> String {
        return "Purr"
    }
}

class NyanCat: Cat {
    dynamic override func says() -> String {
        return "Nyan"
    }
}

class NyanNyanCat: NyanCat {
    dynamic override func says() -> String {
        return "Nyan-nyan"
    }
}

extension Cat: SuperMessageable {}

let nyanNyanCat = NyanNyanCat()
print(nyanNyanCat.says()) // -> Nyan-nyan
let catProxy = nyanNyanCat.superProxy(forAncestor: Cat.self)!
print(catProxy) // -> Purr

How

The proxy is initialized with a pointer to the original object, which can be optionally retained. It uses the dynamic method resolving mechanism of Objective-C runtime (by implementing +resolveInstanceMethod:) to dynamically add implementations for all selectors invoked on the proxy instance.
These dynamically added implementations are trampoline functions written in assembly that modify original method arguments and then do a tail call to one of objc_msgSendSuper(), objc_msgSendSuper_stret(), objc_msgSendSuper2(), or objc_msgSendSuper2_stret() Objective-C runtime functions.
The modification of the arguments involves substituting the original message receiver (self pointer to the proxy instance) with a pointer to an objc_super structure as expected by the objc_msgSendSuper functions. The rest of the original method arguments remain unchanged and are passed to objc_msgSendSuper verbatim.
The objc_super structure contains a pointer to the original object and a class pointer for method implementation lookup. It is stored as an ivar in the proxy.

Points of interest:

• preserving register state while modifying arguments in inline assembly
• proper handling of both normal methods and methods returning large/struct values (by using trampolines to objc_msgSendSuper and objc_msgSendSuper_stret as appropriate)
• funcation argument modification for all currently used platforms (arm and arm64 for iOS devices, i386 and x86-64 for various iOS Simulator flavors)
• Swift support

Usage

For more Objective-C and Swift examples, one can quickly install and run the unit tests project using pod try SGVSuperMessagingProxy.

Supported platforms

Objective-C:

• iOS: 7.0+
• OSX: 10.8+
• watchOS: 1.0+
• tvOS: 9.0+

Swift:

• iOS: 8.0+
• OSX: 10.9+
• watchOS: 2.0+
• tvOS: 9.0+

Installation

SGVSuperMessagingProxy is available through CocoaPods and contains separate Objective-C and Swift subspecs.

For Swift-only or Swift/Objective-C projects, use the Swift subspec in your Podfile:

pod "SGVSuperMessagingProxy/Swift", "~> 2.0"

For Objective-C-only projects, use the Objective-C subspec:

pod "SGVSuperMessagingProxy/Objective-C", "~> 2.0"

Author

Aleksandr Gusev
@sanekgusev
[email protected]

License

SGVSuperMessagingProxy is available under the MIT license. See the LICENSE file for more info.