#NotEmpty annotation in Objective C? - ios

Question

Is there possibility to create custom annotation which will check if parameter of method is empty array or empty string? Something like #NotEmpty in Java. I already use _Nonnull and check parameter with NSParameterAssert but I am curious does we can write custom annotation?
Thanks.

Answer 1

You can use macros to define an inline function.
#define isNil(x) nil ==x

Answer 2

Objective-C has no customizable annotations of this sort, but one of its major strengths is the versatility of its runtime.
And so, if we really wanted to, we could implement this with a wrapper class type:
#interface NotEmpty<Object> : NSProxy
#property(readonly,copy) Object object;
+ (instancetype)notEmpty:(Object)object;
- (instancetype)initWithObject:(Object)object;
#end
#implementation NotEmpty {
id _object;
}
- (id)object {
return _object;
}
+ (instancetype)notEmpty:(id)object {
return [[self alloc] initWithObject:object];
}
- (instancetype)initWithObject:(id)object {
if ([object respondsToSelector:#selector(length)]) {
NSParameterAssert([object length] != 0);
} else {
NSParameterAssert([object count] != 0);
}
_object = [object copy];
return self;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
if (selector == #selector(object)) {
return [NSMethodSignature signatureWithObjCTypes:"#:"];
} else {
return [_object methodSignatureForSelector:selector];
}
}
- (void)forwardInvocation:(NSInvocation *)invocation {
invocation.target = _object;
[invocation invoke];
}
#end
#interface SomeClass : NSObject
#end
#implementation SomeClass
- (void)method:(NotEmpty<NSString*> *)nonEmptyString {
// Call NSString methods, option 1
unsigned long length1 = [(id)nonEmptyString length];
// Call NSString methods, option 2
unsigned long length2 = nonEmptyString.object.length;
// Note that just printing `nonEmptyString` (not nonEmptyString.object)
// will print an opaque value. If this is a concern, then also forward
// #selector(description) from -methodSignatureForSelector:.
NSLog(#"Received: %# (length1: %lu length2: %lu)", nonEmptyString.object, length1, length2);
}
#end
int main() {
SomeClass *sc = [SomeClass new];
[sc method:[NotEmpty notEmpty:#"Not an empty string"]];
// [sc method:[NotEmpty notEmpty:#""]]; // Raises error
}
Do note that this will cause some small performance penalties.