How to log a warning that shows up as a runtime issue in Xcode?

Question

Xcode 8 or 9 started displaying runtime issues. You see a purple icon at the top of the window, and a list in Issue Navigator, next to buildtime issues like compilation warnings and errors.

The runtime issues I've seen are created by the system libraries. Is there a way for my own application code to generate these?

Answer 1

Yes! You'll see these if you do something that a sanitizer catches, like performing certain UI operations a background thread with Thread Sanitizer enabled. Having an ambiguous layout and pausing in the view debugger is also a way to get this to occur. Either way, seeing this occur in Apple's libraries isn't a good thing…

UPDATE: I looked into how these warnings are hit, and LLDB essentially sets a breakpoint on a set of magic functions (__asan::AsanDie(), __tsan_on_report, __ubsan_on_report, __main_thread_checker_on_report) inside of a sanitizer dynamic library that are called when issues occur. If you define your own version of these functions and call them, you'll get the warning to show up. First, create the symbol somewhere in your project:

void __main_thread_checker_on_report(char *message) { } 

Then you can trigger it at will, just by putting this code anywhere in your application (you probably want to hide it behind a macro):

((void (*)(char *))dlsym(dlopen(NULL, RTLD_LAZY), "__main_thread_checker_on_report")))("This will show up as a warning") 

Needless to say, this will almost certainly not play nicely with the actual sanitizer if you choose to use it. You should probably compile the above conditionally based on whether you are using a sanitizer or not.

Answer 2

In XCode 8.3 and earlier you can use set breakpoint into any method of UIKit class like setNeedsDisplay() like below.

Also there is library in objective-c steipete class in which #import <objc/runtime.h> is used.

But in Xcode 9 below library Xcode.app/Contenets/Developer/usr/lib/libMainThreadChecker.dylib is available, Which handle for any relevant issues potentially performed out-of-main thread at runtime.