By exploiting a property of the Objective-C runtime, it’s possible to have a class living and breathing in memory without it ever have been imported, allocated, nor initialized anywhere else in the code base. The property in question is the +load message that is sent to every NSObject shortly after a class is loaded into memory. Consider the following implementation of a hypothetical class called MyClass:
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@implementation MyClass + (void)load { [[self sharedInstance] run]; } + (instancetype)sharedInstance { static MyClass *sharedClass; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ sharedClass = [self new]; }); return sharedClass; } - (void)run { // Do stuff } @end |
Upon MyClass getting loaded into memory, +load is called by the runtime, which allocates, instantiates, and retains an instance of MyClass and then calls -run to do some arbitrary work. MyClass is not #imported from anywhere else, and it is also not retained by anyone else either. It’s there but you can’t see it.
In practice this is probably a bad idea because of the sneaky nature of things. So have a good reason before deciding to use something like this.
