Return value for performSelector:

Question

What will the return value for performSelector: if I pass a selector that returns a primitive type (on object), such as 'week' on NSDateComponents (which will return an int)?

Restaurant

Answer 1

An example of using NSInvocation to return a float:

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);
}

Lepidosiren

Apple's guide on using NSInvocation – Everara

Although this was a very useful code snippet, I would recommend against the test of [someInstance respondsToSelector:x]. I would much rather have my code crash than silently bypass an expected invocation. – Hereditable

@Hereditable as with every part of code, there are many possible use cases. Some people need it for something far less important, so it's safer to silently bypass than crash. It's good that dizy presented safe-check, as people may select to use it or not. Otherwise some devs could not think to check (or they don't know how and just quickly code without research) while they should in their case. – Hultin

Answer 2

I think you cannot get the return value from performSelector. You should look into NSInvocation.

Mispronounce

I might have to use NSInvocation, but you can get the return value of performSelector:, at least if it's an object. – Restaurant

The docs specifically state that you must use NSInvocation if the method you're calling via ‑performSelector: returns something other than an object. – Chellman

Answer 3

New answer to old question~

There is a more concise way of getting the return value from performSelector

NSInvocationOperation *invo = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(height) object:nil];
[invo start];
CGFloat f = 0;
[invo.result getValue:&f];
NSLog(@"operation: %@", @(f));

in which

- (CGFloat)height {
    return 42;
}

output

2017-03-28 16:22:22.378 redpacket[46656:7361082] operation: 42

Arnuad

Answer 4

To answer the second part of the question, another way to invoke a selector that returns a primitive would be to get a function pointer and invoke it as such, as in (assuming someSelector returns a float and has no arguments);

SEL selector = NSSelectorFromString(@"someSelector");
float (*func)(id,SEL) = (float (*)(id,SEL))[someInstance methodForSelector: selector];
printf("return value is: %f", (func)(someInstance, selector));

Soapbox

Float return values might not be ABI compatible on x86 (i.e 32 bit simulator). Actual devices seem fine though (arm, arm64). developer.apple.com/library/mac/documentation/Cocoa/Reference/… – Boycott

Answer 5

I tried the NSInvocation implemented as suggested by dizy, its working as expected.

I also tried the another way i.e.

int result = objc_msgSend([someArray objectAtIndex:0], @selector(currentPoint));

in the above case, we are bypassing the compiler and inserting objc_msgSend call explicitly as described in the blog: http://www.cocoawithlove.com/2011/06/big-weakness-of-objective-c-weak-typing.html

In this case, I got the following warning: Implicitly declaring library function 'objc_msgSend' with type 'id (id, SEL, ...)' which is obvious because we are calling library function directly.

So, I implemented with the NSInvocation which is working perfectly fine for me.

Signore

Did you #include <objc/message.h>? – Ransell

No, I didn't include – Signore