Question

I’m doing some work with dynamic programming in Objective-C and I’ve read through the Objective-C Runtime Programming Guide front to back and been able to do most of what I need, but the one thing I haven’t figured out is how to call a method dynamically provided I have a string representation of it.

Essentially I dynamically do a property lookup to see if my object has a property that matches from a list using class_copyPropertyList and then loop through and match these via an NSMutableDictionary that is populated from a plist file. When a match is found I want to execute the property. I have no way of knowing ahead of time what matches could possibly exist as this is a lib that will be packaged into many different apps.

Answer 1

Use NSSelectorFromString to create a SEL from an NSString. Then you can execute it using one of the performSelector methods.

Setting a property dynamically:

SEL setter = NSSelectorFromString(@"setProperty:");
[myObject performSelector:setter withObject:newValue];

Getting a property dynamically:

SEL getter = NSSelectorFromString(@"property");
id myProperty = [myObject performSelector:getter];

For more complicated methods you can use NSInvocation and NSMethodSignature:

SEL action = NSSelectorFromString(@"someMethod:withArguments:");
NSMethodSignature *signature = [myObject methodSignatureForSelector:action];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
[invocation setArgument:arg1 atIndex:2]; // indices 0 and 1 are reserved.
[invocation setArgument:arg2 atIndex:3];
[invocation invokeWithTarget:myObject];
id returnedObject;
[invocation1 getReturnValue:&returnedObject];