Hooks.m

#import "AppDelegate.h"

/*
 * Note our application delegate doesn't implement -applicationDidEnterBackground:, but it doesn't matter.
 */
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    NSLog(@"Original -application:didFinishLaunchingWithOptions: (self.class is %@)", self.class);
    return YES;
}

- (void)applicationWillEnterForeground:(UIApplication *)application {
    NSLog(@"Original -applicationWillEnterForeground: (self.class is %@)", self.class);
}
@end

@interface ViewController : UIViewController
@end

@implementation ViewController
@end


/*
 * _ClassHooks is a base class for implementing hooks.
 */
@import ObjectiveC.runtime;

@interface _ClassHooks : NSObject
+ (void)hookSelector:(SEL)hookSelector targetSelector:(SEL)targetSelector class:(Class)class;
@end

@implementation _ClassHooks
+ (void)hookSelector:(SEL)hookSelector targetSelector:(SEL)targetSelector class:(Class)class {
    SEL childSelector = targetSelector;

    Method hook   = class_getInstanceMethod(self, hookSelector);
    Method method = class_getInstanceMethod(class, targetSelector);
    if (method) {
        method_exchangeImplementations(hook, method);
        childSelector = hookSelector;
    }

    if (!class_getInstanceMethod(class, childSelector)) {
        class_addMethod(class, childSelector, method_getImplementation(hook), method_getTypeEncoding(hook));
    }
}
@end

/*
 * Our hooks are implemented in _DummyHooks:
 * - First, it hooks UIApplication.setDelegate(), so that in can install the other hooks
 *   after there is an application delegate set, but before -application:didFinishLaunchingWithOptions: is called.
 * - Then, it hooks all the UIApplicationDelegate methods we're interested in. It does not matter whether
 *   the methods are actually implemented by the application delegate or not.
 */
@interface _DummyHooks : _ClassHooks<UIApplicationDelegate>
@end

@implementation _DummyHooks

#pragma mark - UIViewController hooks
- (void)_hooked_viewWillAppear:(BOOL)animated {
    NSLog(@"%@ will be called now on %@", NSStringFromSelector(_cmd), self);
    if ([self respondsToSelector:@selector(_hooked_viewWillAppear:)]) {
        [self _hooked_viewWillAppear:animated];
    }
    NSLog(@"%@ was just called on %@", NSStringFromSelector(_cmd), self);
}

- (void)_hooked_viewWillDisappear:(BOOL)animated {
    NSLog(@"%@ will be called now on %@", NSStringFromSelector(_cmd), self);
    if ([self respondsToSelector:@selector(_hooked_viewWillDisappear:)]) {
        [self _hooked_viewWillDisappear:animated];
    }
    NSLog(@"%@ was just called on %@", NSStringFromSelector(_cmd), self);
}


#pragma mark - UIApplicationDelegate hooks

- (BOOL)_hooked_application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    BOOL ret = YES;
    NSLog(@"%@ will be called now! launchOptions is %@", NSStringFromSelector(_cmd), launchOptions);
    if ([self respondsToSelector:@selector(_hooked_application:didFinishLaunchingWithOptions:)]) {
        ret = [self _hooked_application:application didFinishLaunchingWithOptions:launchOptions];
    } else {
        NSLog(@"%@ does not implement %@.", self.class, NSStringFromSelector(_cmd));
    }
    NSLog(@"%@ was just called!", NSStringFromSelector(_cmd));
    return ret;
}

- (void)_hooked_applicationWillEnterForeground:(UIApplication *)application {
    NSLog(@"%@ will be called now!", NSStringFromSelector(_cmd));
    if ([self respondsToSelector:@selector(_hooked_applicationWillEnterForeground:)]) {
        [self _hooked_applicationWillEnterForeground:application];
    } else {
        NSLog(@"%@ does not implement %@.", self.class, NSStringFromSelector(_cmd));
    }
    NSLog(@"%@ was just called!", NSStringFromSelector(_cmd));
}

- (void)_hooked_applicationDidEnterBackground:(UIApplication *)application {
    NSLog(@"%@ will be called now!", NSStringFromSelector(_cmd));
    if ([self respondsToSelector:@selector(_hooked_applicationDidEnterBackground:)]) {
        [self _hooked_applicationDidEnterBackground:application];
    } else {
        NSLog(@"%@ does not implement %@.", self.class, NSStringFromSelector(_cmd));
    }
    NSLog(@"%@ was just called!", NSStringFromSelector(_cmd));
}

#pragma mark - UIApplication hooks

- (void)_hooked_setApplicationDelegate:(id<UIApplicationDelegate>)delegate {
    NSLog(@"%@ will be called now!", NSStringFromSelector(_cmd));
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // Install the delegate's hooks
        Class cls = delegate.class;
        NSLog(@"Concrete delegate class: %@", cls);

        [_DummyHooks hookSelector:@selector(_hooked_application:didFinishLaunchingWithOptions:)
                   targetSelector:@selector(application:didFinishLaunchingWithOptions:)
                            class:cls];
        [_DummyHooks hookSelector:@selector(_hooked_applicationWillEnterForeground:)
                   targetSelector:@selector(applicationWillEnterForeground:)
                            class:cls];
        [_DummyHooks hookSelector:@selector(_hooked_applicationDidEnterBackground:)
                   targetSelector:@selector(applicationDidEnterBackground:)
                            class:cls];
    });

    [self _hooked_setApplicationDelegate:delegate];
    NSLog(@"%@ was just called!", NSStringFromSelector(_cmd));
}

+ (void)load {
    /*
     * This will install all the hooks
     */
    [_DummyHooks hookSelector:@selector(_hooked_setApplicationDelegate:)
               targetSelector:@selector(setDelegate:)
                        class:UIApplication.class];
    [_DummyHooks hookSelector:@selector(_hooked_viewWillAppear:)
               targetSelector:@selector(viewWillAppear:)
                        class:UIViewController.class];
    [_DummyHooks hookSelector:@selector(_hooked_viewWillDisappear:)
               targetSelector:@selector(viewWillDisappear:)
                        class:UIViewController.class];
}
@end


/*
 * Example log:
 *
 * APP LAUNCH:
 * setDelegate: will be called now!
 * Concrete delegate class: AppDelegate
 * setDelegate: was just called!
 * application:didFinishLaunchingWithOptions: will be called now! launchOptions is (null)
 * Original -application:didFinishLaunchingWithOptions: (self.class is AppDelegate)
 * application:didFinishLaunchingWithOptions: was just called!
 * viewWillAppear: will be called now on <ViewController: 0x7fb1d1c420c0>
 * viewWillAppear: was just called on <ViewController: 0x7fb1d1c420c0>
 * viewWillAppear: will be called now on <UIInputWindowController: 0x7fb1d202da00>
 * viewWillAppear: was just called on <UIInputWindowController: 0x7fb1d202da00>
 *
 * HOME BUTTON:
 * applicationDidEnterBackground: will be called now!
 * AppDelegate does not implement applicationDidEnterBackground:.
 * applicationDidEnterBackground: was just called!
 *
 * TAP APP AGAIN:
 * applicationWillEnterForeground: will be called now!
 * Original -applicationWillEnterForeground: (self.class is AppDelegate)
 * applicationWillEnterForeground: was just called!
 *
 */