Assertion macro that works exactly like NSAssert(), with the modification that a code block will be run if the condition is not met. Helps a lot to prevent repetitions when handling the condition a second time for release code (NSAssert will be stripped by default.) Thanks to @ merowing_ for https://gist.github.com/krzysztofzablocki/5921645 !

Assertions.h

// See http://stackoverflow.com/a/257424/283482
// Normally a "do {…} while (0)" is used to prevent the problem described in the linked answer,
// but this would prevent us from using "continue" or "break" in the block parameter.
// The problem is fixed thanks to the "if (1) {…}", doing the same as the "do {…} while (0)".

#define AssertTrueOrRunBlock(condition, block, description, ...)\
if (1) {\
	__PRAGMA_PUSH_NO_EXTRA_ARG_WARNINGS\
	BOOL check = !!(condition);\
	NSCAssert(check, (description), ##__VA_ARGS__);\
	if (!check) {\
		NSString *format = [NSString stringWithFormat:@"%@%@%@", @"Assertion failure in %s, %s:%d\nCondition not satisfied: %s, reason: '", description, @"'"];\
		NSLog(format, __PRETTY_FUNCTION__, __FILE__, __LINE__, #condition, ##__VA_ARGS__);\
		block\
	}\
	__PRAGMA_POP_NO_EXTRA_ARG_WARNINGS\
}

Example.m

- (void)pushScene:(SKScene *)scene withTransition:(SKTransition *)transition {
	AssertTrueOrRunBlock(![self.stack containsObject:scene], {
		// You can do whatever you want here!
		// continue, break work as expected.
		return;
	}, @"Pushed scene already on stack!");
	
	[super presentScene:scene transition:transition];
	[self.stack addObject:scene];
}

// Output:
/*Assertion failure in -[Example pushScene:withTransition:transition:], /…/Example.m:6
Condition not satisfied: ![self.stack containsObject:scene], reason: 'Pushed scene already on stack!'*/

Using this macro turns off code highlighting in whole source file. This is likely a bug in Xcode that I already encountered and it happens always if you put some code inside a macro. Does it happen to others? Is there a solution?

In addition, if you use NSCAssert you prevent retaining self inside of a block (I don't mean your block argument, but ^{}).

@iMartinKiss thanks for pointing me to NSCAssert. I fixed this, the possible wrong use of description and the possible double evaluation of condition.

Docs for NSAssert and NSCAssert indicate that these macros should be be called only for Objective-C or C code, respectively. I think it's probably a bad idea to rely on NSCAssert use in strict Objective-C code.