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

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.

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.

Well, that's a hack, far from ideal, but can by useful to get runtime warnings without having game-over behavior like in assertions or fatal errors:

func runtimeWarning(_ message: String, file: String = #file, line: Int = #line) {    #if DEBUG    DispatchQueue.global(qos: .userInteractive).async {        // If you got here, please check console for more info        _ = UIApplication.shared.windows        print("Runtime warning: \(message): file \(file.fileName), line \(line)")    }    #endif}fileprivate extension String {    var fileName: String { URL(fileURLWithPath: self).lastPathComponent }}

Unfortunately this leads to quite a big print to console from Main Thread Checker, also does not show on correct line, but at least you have all info needed in console (including line of code you should check).

Usage and result is very similar to assertionFailure or fatalError:

runtimeWarning("Hi, I'm purple")

leads to message in console:

Runtime warning: Hi, I'm purple: file MyFile.swift, line 10

and this, so you won't miss it: