objective C using a string to call a method - objective-c

Question

Hi im new to objective C and was wondering if someone might be able to help me with this. I have a few different methods each requiring 3 input values and normally call it using
[self methodA:1 height:10 speed:3]
but the method name I want to read from a string in a plist so for example if the string was methodB i would get
[self methodB:1 height:10 speed:3]
for "methodC"
[self methodC:1 height:10 speed:3]
and so on.
Any ideas how I might do this I tried defining the string as a Selector using NSSelectorFromString
NSString *string = [plistA objectForKey:#"method"];
SEL select = NSSelectorFromString(string);
[self performSelector:select:c height:b speed:a];
However this did not work either any help would be greatly appreciated.
Have tried the solution below but could not get to work here is what i've tried.
So just to recap I have methods such as
spawnEnemyA:2 withHeight:3 withSpeed:4
spawnEnemyB:3 withHeight:2 withSpeed:5
and I want to read the values I want to pass to these methods as well as the method type from a plist file. my code is as follows, //////////////////////////////////////////////////////////////
//These are the values I read from the plist that I want my method to use
int a = [[enemySettings objectForKey:#"speed"] intValue];
int b = [[enemySettings objectForKey:#"position"] intValue];
int c = [[enemySettings objectForKey:#"delay"] intValue];
// I Also read the method name from the plist and combine it into a single string
NSString *method = [enemySettings objectForKey:#"enemytype"];
NSString *label1 = #"spawn";
NSString *label2 = #":withHeight:withSpeed:";
NSString *combined = [NSString stringWithFormat:#"%#%#%#",label1, method,label2];
//Check that the string is correct get spawnEnemyA:withHeight:withSpeed:
CCLOG(#"%#",combined);
//This is the Invocation part
NSInvocation * invocation = [ NSInvocation new ];
[ invocation setSelector: NSSelectorFromString(combined)];
[ invocation setArgument: &c atIndex: 2 ];
[ invocation setArgument: &b atIndex: 3 ];
[ invocation setArgument: &a atIndex: 4 ];
[ invocation invokeWithTarget:self ];
[invocation release ];
////////////////////////////////////////////////////////////////////
The code compiles without any errors but the methods are not called. Any ideas? Cheers

Answer 1

You can't use performSelector for a method with 3 (or more) arguments.
But for your information, here's how to use it:
SEL m1;
SEL m2;
SEL m3;
m1 = NSSelectorFromString( #"someMethodWithoutArg" );
m2 = NSSelectorFromString( #"someMethodWithAnArg:" );
m1 = NSSelectorFromString( #"someMethodWithAnArg:andAnotherOne:" );
[ someObject performSelector: m1 ];
[ someObject performSelector: m2 withObject: anArg ];
[ someObject performSelector: m2 withObject: anArg withObject: anOtherArg ];
For methods with more than 2 arguments, you will have to use the NSInvocation class.
Take a look at the documentation to learn how to use it.
Basically:
NSInvocation * invocation = [ NSInvocation new ];
[ invocation setSelector: NSStringFromSelector( #"methodWithArg1:arg2:arg3:" ) ];
// Argument 1 is at index 2, as there is self and _cmd before
[ invocation setArgument: &arg1 atIndex: 2 ];
[ invocation setArgument: &arg2 atIndex: 3 ];
[ invocation setArgument: &arg3 atIndex: 4 ];
[ invocation invokeWithTarget: targetObject ];
// If you need to get the return value
[ invocation getReturnValue: &someVar ];
[ invocation release ];

Answer 2

In general, this kind of dynamism often indicates an anti-pattern. Colluding data with implementation in this fashion is not generally a best practice.
Sometimes, though, it is necessary. If you are going down this path, then given that your various method declarations likely look like:
- (void)methodAWidth:(NSUInteger)w height:(NSUInteger)h speed:(NSUInteger)s;
- (void)methodBWidth:(NSUInteger)w height:(NSUInteger)h speed:(NSUInteger)s;
- (void)methodCWidth:(NSUInteger)w height:(NSUInteger)h speed:(NSUInteger)s;
You would probably want something like:
NSString *selName = [NSString stringWithFormat:#"method%#Width:height:speed:", ... one of #"A", #"B", or #"C" ....];
SEL selector = NSelectorFromString(selName);
Then:
if (![target respondsToSelector:selector])
return; // no can do
void (*castMsgSend)(id, SEL, NSUInteger, NSUInteger, NSUInteger) = (void*)objc_msgSend;
castMsgSend(target, selector, 1, 10, 3);
Every method call is compiled down to a call to objc_msgSend(). By doing the above, you are creating a fully type-safe/type-checked call site that goes through the normal Objective-C messaging mechanism, but the selector is dynamically defined on the fly.\
While performSelector: (and multi-arg variants) are handy, they can't deal with non-object types.
And, as MacMade pointed out in a comment, watch out for floating point and structure returns. They use different variants of objc_msgSend() that the compiler automatically handles in the normal [foo bar] case.

Answer 3

You can directly use objc_msgsend:
NSString *methodName = [plistA objectForKey:#"method"];
objc_msgSend(self, methodName, c, b, a);
Mind that the selector must include all pieces, eg #"method:height:speed:"

Answer 4

You should replace the following line with:
[self performSelector:select:c height:b speed:a];
and write following:
[self performSelector:select withObject:[NSArray arrayWithObjects:c,b,a,nil]];