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Objective C >> Is there a way to check the return value of the Selector?

let's say I have a selector that can be assigned to several different methods - each has a different return value.

Is there a way to check what is the return value of the method holding the selector before calling "performSelector"?

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4 answers


Is there a way to check what is the return value of the method holding the selector before calling "performSelector"?

Value? Not. A type? Jap. It seems that you want the return type of the method (or your question doesn't make sense).



Method m = class_getInstanceMethod([SomeClass class], @selector(foo:bar:));
char type[128];
method_getReturnType(m, type, sizeof(type));

Then you can check the returned type string in type

. For example "v"

means void (google full list).

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you can use NSInvocation which is recommended by Apple Docs for this purpose

Here is some sample code for using NSInvocation



    SEL selector = NSSelectorFromString(@"someSelector");
if ([someInstance respondsToSelector:selector]) {
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:
                                [[someInstance class] instanceMethodSignatureForSelector:selector]];
    [invocation setSelector:selector];
    [invocation setTarget:someInstance];
    [invocation invoke];
    float returnValue;
    [invocation getReturnValue:&returnValue];
    NSLog(@"Returned %f", returnValue);
}
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If the method doesn't return an object (it returns a primitive type) then use NSInvocation instead.

@implementation NSObject(SafePerformSelector)
-(id) performSelectorSafely:(SEL)aSelector;
{
    NSParameterAssert(aSelector != NULL);
    NSParameterAssert([self respondsToSelector:aSelector]);

    NSMethodSignature* methodSig = [self methodSignatureForSelector:aSelector];
    if(methodSig == nil)
        return nil;

    const char* retType = [methodSig methodReturnType];
    if(strcmp(retType, @encode(id)) == 0 || strcmp(retType, @encode(void)) == 0){
        return [self performSelector:aSelector];
    } else {
        NSLog(@"-[%@ performSelector:@selector(%@)] shouldn't be used. The selector doesn't return an object or void", [self class], NSStringFromSelector(aSelector));
        return nil;
    }
}
@end
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performSelector: always returns id

. The actual type returned is determined by the method you are calling; so there is no way to know this beforehand.

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