How does Apptimize \ Optimizely work on iOS?

Question

I\'m trying to figure out a few things about the implementation going on \"behind the scene\" for manipulating UI elements on the fly, straight from the web console on Apptimize

Answer 1

I wonder the same and couldn't find a definite answer, so here is my (hopefully) educated guess:

Thanks to the runtime environment it is actually not that hard to use Aspect-Orientated-Programming (AOP) in Cocoa(-Touch), in which rules are written to hook in in other classes method calls.

If you google for AOP and Objective-C, several libraries pop up that wrap the runtime code nicely.

For example steinpete's Aspect library:

[UIViewController aspect_hookSelector:@selector(viewWillAppear:) withOptions:AspectPositionAfter usingBlock:^(id aspectInfo, BOOL animated) {
    NSLog(@"View Controller %@ will appear animated: %tu", aspectInfo.instance, animated);
} error:NULL];

This method call

+ (id)aspect_hookSelector:(SEL)selector
                      withOptions:(AspectOptions)options
                       usingBlock:(id)block
                            error:(NSError **)error {
    return aspect_add((id)self, selector, options, block, error);
}

calls aspect_add()

static id aspect_add(id self, SEL selector, AspectOptions options, id block, NSError **error) {
    NSCParameterAssert(self);
    NSCParameterAssert(selector);
    NSCParameterAssert(block);

    __block AspectIdentifier *identifier = nil;
    aspect_performLocked(^{
        if (aspect_isSelectorAllowedAndTrack(self, selector, options, error)) {
            AspectsContainer *aspectContainer = aspect_getContainerForObject(self, selector);
            identifier = [AspectIdentifier identifierWithSelector:selector object:self options:options block:block error:error];
            if (identifier) {
                [aspectContainer addAspect:identifier withOptions:options];

                // Modify the class to allow message interception.
                aspect_prepareClassAndHookSelector(self, selector, error);
            }
        }
    });
    return identifier;
}

which again calls several other quite frightening looking functions that do the heavy lifting in the runtime

static void aspect_prepareClassAndHookSelector(NSObject *self, SEL selector, NSError **error) {
    NSCParameterAssert(selector);
    Class klass = aspect_hookClass(self, error);
    Method targetMethod = class_getInstanceMethod(klass, selector);
    IMP targetMethodIMP = method_getImplementation(targetMethod);
    if (!aspect_isMsgForwardIMP(targetMethodIMP)) {
        // Make a method alias for the existing method implementation, it not already copied.
        const char *typeEncoding = method_getTypeEncoding(targetMethod);
        SEL aliasSelector = aspect_aliasForSelector(selector);
        if (![klass instancesRespondToSelector:aliasSelector]) {
            __unused BOOL addedAlias = class_addMethod(klass, aliasSelector, method_getImplementation(targetMethod), typeEncoding);
            NSCAssert(addedAlias, @"Original implementation for %@ is already copied to %@ on %@", NSStringFromSelector(selector), NSStringFromSelector(aliasSelector), klass);
        }

        // We use forwardInvocation to hook in.
        class_replaceMethod(klass, selector, aspect_getMsgForwardIMP(self, selector), typeEncoding);
        AspectLog(@"Aspects: Installed hook for -[%@ %@].", klass, NSStringFromSelector(selector));
    }
}

including method-swizzling.

---

It is easy to see that here we have a tool that will allow us to send the current state of an app to re-build this in a web-page but also to manipulate objects in an existing code.
Of course this is just a starting point. You will need a web service that assembles the app and sends it to the users.

---

Personally I never used AOP for such a complex task, but I used it for teaching all view controllers tracking capabilities

- (void)setupViewControllerTracking
{
    NSError *error;
    @weakify(self);
    [UIViewController aspect_hookSelector:@selector(viewDidAppear:)
                              withOptions:AspectPositionAfter
                               usingBlock:^(id < AspectInfo > aspectInfo) {
                                   @strongify(self);
                                   UIViewController *viewController = [aspectInfo instance];
                                   NSArray *breadCrumbs = [self breadCrumbsForViewController:viewController];

                                   if (breadCrumbs.count) {
                                       NSString *pageName = [NSString stringWithFormat:@"/%@", [breadCrumbs componentsJoinedByString:@"/"]];
                                       [ARAnalytics pageView:pageName];
                                   }
                               } error:&error];
}

---

update

I played a bit and was able to create a prototype. If added to a project, it will changes all view controllers background color to blue and after 5 seconds all living view controllers background to orange, by using AOP and dynamic method adding.

source code: https://gist.github.com/vikingosegundo/0e4b30901b9498ae4b7b

The 5 seconds are triggered by a notification, but it is obvious that this could be a network event.

---

update 2

I taught my prototype to open a network interface and accept rgb values for the background.
Running in simulator this would be

http://127.0.0.1:8080/color////
http://127.0.0.1:8080/color/50/120/220/

I use OCFWebServer for that

//
//  ABController.m
//  ABTestPrototype
//
//  Created by Manuel Meyer on 12.05.15.
//  Copyright (c) 2015 Manuel Meyer. All rights reserved.
//

#import "ABController.h"

#import 
#import 
#import 
#import 


#import 
#import "UIViewController+Updating.h"
#import "UIView+ABTesting.h"


@import UIKit;

@interface ABController ()
@property (nonatomic, strong) OCFWebServer *webserver;
@end
@implementation ABController


void _ab_register_ab_notificaction(id self, SEL _cmd)
{
    [[NSNotificationCenter defaultCenter] addObserver:self selector:NSSelectorFromString(@"ab_notifaction:") name:@"ABTestUpdate" object:nil];
}


void _ab_notificaction(id self, SEL _cmd, id userObj)
{
    NSLog(@"UPDATE %@", self);
}


+(instancetype)sharedABController{
    static dispatch_once_t onceToken;
    static ABController *abController;
    dispatch_once(&onceToken, ^{

        OCFWebServer *server = [OCFWebServer new];

        [server addDefaultHandlerForMethod:@"GET"
                              requestClass:[OCFWebServerRequest class]
                              processBlock:^void(OCFWebServerRequest *request) {
                                  OCFWebServerResponse *response = [OCFWebServerDataResponse responseWithText:[[[UIApplication sharedApplication] keyWindow] listOfSubviews]];
                                  [request respondWith:response];
                              }];

        [server addHandlerForMethod:@"GET"
                          pathRegex:@"/color/[0-9]{1,3}/[0-9]{1,3}/[0-9]{1,3}/"
                       requestClass:[OCFWebServerRequest class]
                       processBlock:^(OCFWebServerRequest *request) {
                           NSArray *comps = request.URL.pathComponents;
                           UIColor *c = [UIColor colorWithRed:^{ NSString *r = comps[2]; return [r integerValue] / 255.0;}()
                                                        green:^{ NSString *g = comps[3]; return [g integerValue] / 255.0;}()
                                                         blue:^{ NSString *b = comps[4]; return [b integerValue] / 255.0;}()
                                                        alpha:1.0];

                           [[NSNotificationCenter defaultCenter] postNotificationName:@"ABTestUpdate" object:c];
                           OCFWebServerResponse *response = [OCFWebServerDataResponse responseWithText:[[[UIApplication sharedApplication] keyWindow] listOfSubviews]];
                           [request respondWith:response];
                       }];

        dispatch_async(dispatch_queue_create(".", 0), ^{
            [server runWithPort:8080];
        });

        abController = [[ABController alloc] initWithWebServer:server];
    });
    return abController;
}

-(instancetype)initWithWebServer:(OCFWebServer *)webserver
{
    self = [super init];
    if (self) {
        self.webserver = webserver;
    }
    return self;
}


+(void)load
{
    class_addMethod([UIViewController class], NSSelectorFromString(@"ab_notifaction:"), (IMP)_ab_notificaction, "v@:@");
    class_addMethod([UIViewController class], NSSelectorFromString(@"ab_register_ab_notificaction"), (IMP)_ab_register_ab_notificaction, "v@:");

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(.00001 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self sharedABController];
    });
    [UIViewController aspect_hookSelector:@selector(viewDidLoad)
                              withOptions:AspectPositionAfter
                               usingBlock:^(id aspectInfo) {

                                   dispatch_async(dispatch_get_main_queue(),
                                                  ^{
                                                      UIViewController *vc = aspectInfo.instance;
                                                      SEL selector = NSSelectorFromString(@"ab_register_ab_notificaction");
                                                      IMP imp = [vc methodForSelector:selector];
                                                      void (*func)(id, SEL) = (void *)imp;func(vc, selector);
                                                });
                               } error:NULL];

    [UIViewController aspect_hookSelector:NSSelectorFromString(@"ab_notifaction:")
                              withOptions:AspectPositionAfter
                               usingBlock:^(id aspectInfo, NSNotification *noti) {

                                   dispatch_async(dispatch_get_main_queue(),
                                                  ^{
                                                      UIViewController *vc = aspectInfo.instance;
                                                      [vc updateViewWithAttributes:@{@"backgroundColor": noti.object}];
                                                  });
                               } error:NULL];
}
@end

---

//
//  UIViewController+Updating.m
//  ABTestPrototype
//
//  Created by Manuel Meyer on 12.05.15.
//  Copyright (c) 2015 Manuel Meyer. All rights reserved.
//

#import "UIViewController+Updating.h"

@implementation UIViewController (Updating)
-(void)updateViewWithAttributes:(NSDictionary *)attributes
{
    [[attributes allKeys] enumerateObjectsUsingBlock:^(NSString *obj, NSUInteger idx, BOOL *stop) {

        if ([obj isEqualToString:@"backgroundColor"]) {

                [self.view setBackgroundColor:attributes[obj]];
        }
    }];
}
@end

---

the full code: https://github.com/vikingosegundo/ABTestPrototype