How to let lldb print the description written in an Objective-C category

Question

An incredible tip for debugging constraint issues print the constraint`s id in lldb

But what should I do to let the lldb print my category`s description?

@interface NSLayoutConstraint (Description)

@end

@implementation NSLayoutConstraint (Description)

-(NSString *)description {
    return [NSString stringWithFormat:@"id: %@, constant: %f", self.identifier, self.constant];
}

@end

Help is really appreciated :)

You should never override a method in a category. It's undefined behavior. — rmaddy, Feb 06 '16 at 03:56
It may work. It may not. That's the problem with undefined behavior. — rmaddy, Feb 06 '16 at 05:04

score 0 · Answer 1 · answered Feb 09 '16 at 00:59

You can use the description method in two ways. One, if in the code in your app somewhere you do:

NSLog(@"My Object: %@", my_object);

then %@ specifier will print the NSString returned by the "description" method of your object.

Another way is in the debugger:

lldb (and gdb before it) has a command called po - which is short for print-object. It's job is to take some expression, evaluate it, interpret the result (in the case of ObjC) as an NSObject and send that object the description method.

So in the debugger you can just call

(lldb) po object

and get your printout.

One detail, though: po actually first checks if the object responds to debugDescription and it will call that in preference to description. So if you are extending an object that implements debugDescription (some Kit objects do this) then po will show you the result of the debugDescription method, not your description.

How to let lldb print the description written in an Objective-C category

1 Answers1