Bdtjtk

Lets imagine that we have two classes:

@interface First : NSObject



@end



@interface Second : NSObject



@end



@implementation First

+(void)load

{

    NSLog(@"This must be called first");

}

@end



@implementation Second

+(void)load

{

    NSLog(@"And this must be called second");

}

@end

We have +load methods in each class. If we run this code, This must be called first will be first and And this must be called second will be second.

What determines the order in which the +load methods of this classes are called? In my experiment, if I move @implementation of second class before @implementation of first class - And this must be called second is printed first and This must be called first is printed second. Is this means that +load order depends only from order in source code?

In my real case I have precompiled framework with custom +load (some code are called before main() and I see logs from it), and I need to execute my code before this code (and as I understand - I can place it into +load, but I don't know how to change order). Or may be I can call my code before framework code with some other technique?

You really can't rely on order, nor can you effectively control order. By design. +load should happen before the +initialize of that class, but they order of the two is seemingly indeterminate across multiple classes (which I find slightly surprising, but well within the rules).

This is a big part of why you shouldn't do any heavy lifting in +load or +initialize. They really should only be used sparingly and only for initializing a small bit of highly localized state. Touching other significant subsystems is dangerous because you'll be changing initialization order and behavior in ways that might break the system. Shouldn't, but it might, it can, and it has in the past.

Instead, you really should try to have a "start here" point in your framework code that the client explicitly calls into.

+load methods are invoked by the objc runtime as part of the image loading process (you can see this by breaking in your load method and printing a stacktrace).

The order in which +load methods are invoked seems to depend on the order of the objc class lists generated by clang.

If you look at the source code of the objc runtime, you'll see that load_images (the function called by dyld), calls prepare_load_methods to get a list of all objc classes in an image. prepare_load_methods calls _getObjc2NonlazyClassList, which fetches the objc classlist from the __objc_nlclslist section in the image.

load_images then calls call_load_methods, which goes through all loaded classes and invokes their +load methods.