Question
Answer and Explanation
Hooking all load methods in Objective-C is not straightforward due to the nature of the runtime and the fact that load methods are class methods called very early in the loading process. These methods are intended for initial class setup and should be used with care. However, if you need to intercept the execution of all load methods for purposes such as logging, testing, or debugging, here’s how you can approach it.
Understanding the Challenge
The load method is automatically called by the runtime when a class is loaded into memory. It's called only once per class, and it's a class method (not an instance method). Because it happens so early, you can't easily hook it using traditional method swizzling techniques which typically operate on instance methods.
Method 1: Using the Objective-C Runtime API with `_objc_init`
One of the more robust but advanced approaches involves using the internal `_objc_init` function, which is part of the Objective-C runtime. This function is called at the very beginning of the runtime loading process. We can attempt to intercept this call by using a symbolic breakpoint and then hooking all load methods.
Code Snippet:
#import <objc/runtime.h>
#import <dlfcn.h>
static void (original_objc_init)();
static void hooked_objc_init()
{
original_objc_init();
// Iterate through all loaded classes and hook load method
int numClasses = objc_getClassList(NULL, 0);
Class classes = (__unsafe_unretained Class)malloc(sizeof(Class) numClasses);
numClasses = objc_getClassList(classes, numClasses);
for (int i = 0; i < numClasses; i++) {
Class currentClass = classes[i];
Method loadMethod = class_getClassMethod(currentClass, @selector(load));
if(loadMethod) {
// Perform your hooking or logging here
Method originalMethod = class_getClassMethod(currentClass, @selector(load));
void (originalImp)(id, SEL) = (void ()(id, SEL))method_getImplementation(originalMethod);
IMP myLoadImp = imp_implementationWithBlock(^(id self) {
NSLog(@”load method of %s was called”,class_getName(self));
originalImp(self, @selector(load)); // Call the original load method
});
method_setImplementation(loadMethod,myLoadImp);
}
}
free(classes);
}
__attribute__((constructor))
static void initialize_hooks()
{
void handle = dlopen(NULL, RTLD_NOW);
original_objc_init = dlsym(handle, "_objc_init");
dlclose(handle);
if (original_objc_init)
{
original_objc_init = (void()()) dlsym(RTLD_DEFAULT,"_objc_init");
// hook objc_init
Dl_info info;
dladdr(original_objc_init, &info);
// we hook objc_init
struct rebinding rebind_objc_init;
rebind_objc_init.name = "_objc_init";
rebind_objc_init.replacement = (void)hooked_objc_init;
rebind_objc_init.replaced = (void)&original_objc_init;
rebind_symbols((struct rebinding[1]){rebind_objc_init},1);
}
}
Explanation:
- `_objc_init` is intercepted at the very start of app initialization using the `rebinding_symbols` method. The function `hooked_objc_init` is called before the original objc_init function.
- Inside `hooked_objc_init`, we first call the original `_objc_init` to proceed with normal initialization. Then, we iterate through all loaded classes using the runtime API and get the class method named `load`.
- If a `load` method is found, we use `imp_implementationWithBlock` to create a new implementation that logs the class name and calls the original implementation.
- Finally we replace the original implementation of the load method with our custom implementation
Method 2: Using an Interceptor Class with a Category
Another, less intrusive approach involves creating a category on `NSObject` and overriding the `load` method there. Then, in your other classes you may call the super method.
Code Snippet:
// NSObject+LoadHook.h
#import <Foundation/Foundation.h>
@interface NSObject (LoadHook)
@end
// NSObject+LoadHook.m
#import "NSObject+LoadHook.h"
#import <objc/runtime.h>
@implementation NSObject (LoadHook)
+ (void)load {
NSLog(@"load method of NSObject (or subclasses) was called %s",class_getName(self));
}
@end
//Other classes
@interface MyClass : NSObject
@end
@implementation MyClass
+ (void)load {
[super load];
NSLog(@”load method of MyClass was called”);
}
@end
Explanation:
- By implementing a load method in the NSObject category and calling super in your other classes you can intercept when the load method is being called.
Important Considerations
- Order of Execution: Be aware that `load` methods are called in an unpredictable order and before the application has fully initialized. This means that you should not rely on other classes being fully initialized.
- Potential Instability: Method swizzling and runtime manipulation can be unstable, especially if not done correctly. Thorough testing is crucial.
- Alternatives: Before resorting to this level of interception, carefully consider other options. Often, a more structured design approach or alternative hooks (such as notification observers for specific events) might provide a better solution.
- Production Readiness: Be extra cautious when doing this kind of hooking in a production environment. The code provided is only for illustrative and debugging purposes, and it may break your code or the underlying system.
In conclusion, while it’s technically possible to hook all load methods, the methods involve delving into low-level runtime operations. Always consider whether a simpler, safer approach can address your needs. Use the above methods judiciously and with a thorough understanding of their implications.