Objective-C Language Low-level Runtime Environment Calling methods directly
Example
If you need to call an Objective-C method from C code, you have two ways: using objc_msgSend, or obtaining the IMP (method implementation function pointer) and calling that.
#import <objc/objc.h>
@implementation Example
- (double)negate:(double)value {
return -value;
}
- (double)invert:(double)value {
return 1 / value;
}
@end
// Calls the selector on the object. Expects the method to have one double argument and return a double.
double performSelectorWithMsgSend(id object, SEL selector, double value) {
// We declare pointer to function and cast `objc_msgSend` to expected signature.
// WARNING: This step is important! Otherwise you may get unexpected results!
double (*msgSend)(id, SEL, double) = (typeof(msgSend)) &objc_msgSend;
// The implicit arguments of self and _cmd need to be passed in addition to any explicit arguments.
return msgSend(object, selector, value);
}
// Does the same as the above function, but by obtaining the method's IMP.
double performSelectorWithIMP(id object, SEL selector, double value) {
// Get the method's implementation.
IMP imp = class_getMethodImplementation([self class], selector);
// Cast it so the types are known and ARC can work correctly.
double (*callableImp)(id, SEL, double) = (typeof(callableImp)) imp;
// Again, you need the explicit arguments.
return callableImp(object, selector, value);
}
int main() {
Example *e = [Example new];
// Invoke negation, result is -4
double x = performSelectorWithMsgSend(e, @selector(negate:), 4);
// Invoke inversion, result is 0.25
double y = performSelectorWithIMP(e, @selector(invert:), 4);
}
objc_msgSend works by obtaining the IMP for the method and calling that. The IMPs for the last several methods called are cached, so if you're sending an Objective-C message in a very tight loop you can get acceptable performance. In some cases, manually caching the IMP can give slightly better performance, although this is a last resort optimization.
