Call existing object with NSString and call method - objective-c

Question

another easy one, for you guys I mean:

I need to call a method on an existing object like this:

[object method];

The name of the object however, is different (this code is for a superclass, the children all have different names) each time. So I want to call the object with an NSString like this:

NSString *objectName = @"object";
[objectName method];

I've read about NSClassFromString (I've been looking for hours now), but that doesn't work. Then I get the following error: "no known class method for selector 'method'".

Do you guys know what I'm doing wrong?

Thanks in advance, Frans

This is not a natural thing to do in Objective-C, and is in fact kind of difficult. If you'd explain what you're actually trying to do (e.g. "I want to be able to send a message to one of two objects depending on a flag"), we could probably suggest something easier and more idiomatic. — Chuck
– Chuck, Commented Dec 22, 2012 at 21:24
NSClassFromString returns a class (as its name says), not the contents of a variable. A function that did return a variable would either have to know about all variables everywhere, effectively making all variables global and thus destroying any separation of knowledge, or be able to determine the scope of any caller (e.g., what method of what class the caller is in), and know what variables are visible to that scope. A daunting problem, whose solution is of dubious utility. — Peter Hosey
– Peter Hosey, Commented Dec 23, 2012 at 3:08

kennytm · Accepted Answer · 2012-12-22 18:19:11Z

5

The name of a variable is only known at compile-time. There is no way to get a variable automatically from an NSString. You could, however, but an NSDictionary and call the method, e.g.:

NSDictionary* dict = @{@"object": object, @"obj2": obj2};
NSString* objectName;
[dict[objectName] method];

answered Dec 22, 2012 at 18:19

kennytm

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6 Comments

Daij-Djan Over a year ago

looks easiest if it were function local and the dict doesnt change.. but I think it will, ramy's way is nice

CodaFi Over a year ago

Hold up, @KennyTM, then how does VFL work? Didn't apple define NSDictionaryOfVariableBindings() specifically for this?

Peter Hosey Over a year ago

@CodaFi: What do you mean by “VFL”? If you mean valueForKey:, how it works is described in the documentation. You're right about the function—it does exactly what the first line of this answer does.

CodaFi Over a year ago

@PeterHosey You mean to tell me you've never heard of the Visual Format Language?

Peter Hosey Over a year ago

@CodaFi: Oh, that. Never heard it called “VFL” before. Anyway, you're right—you create a dictionary using that function and give it to the constraint when you create it. That's a different sense of “variable” from what KennyTM and the questioner are talking about—they're talking about variables in the C sense, not pairs in a dictionary.

|

Ramy Al Zuhouri · Accepted Answer · 2012-12-22 18:25:44Z

1

There isn't reflection in C, all you can do is to put the object in a dictionary or make it be an ivar.

For example:

@property (nonatomic,strong) id objectName1; // with ARC
@property (nonatomic,strong) id objectName2;
...
@property (nonatomic,strong) id objectNameN;

Then:

NSString* key= @"objectName1"; // objectName1 or whatever
id object= [self valueForKey: key];
[object method];

edited Dec 22, 2012 at 18:25

answered Dec 22, 2012 at 18:19

Ramy Al Zuhouri

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5 Comments

Daij-Djan Over a year ago

-1 this isnt even reflection he wants/needs ... your solution works though so +1 = 0

Ramy Al Zuhouri Over a year ago

He wants to get @"object" from object and viceversa.Isn't that reflection? With preprocessor it's unidirectional.

Daij-Djan Over a year ago

he wants to get instances from a name.. variable names as in java arent known at compile time. class members yes but then he could use KVC

Daij-Djan Over a year ago

oh and there is almost as much reflection in obC as in java

Frans Over a year ago

Thanks Guys, I've got it working now with: id object= [self valueForKey: key]; [object method]; The dictionary-idea from KennyTM was also good, but the above one suited my application better. So thanks everyone for your ideas/solutions/help and especially Ramy for the "winning" solution!

DivineDesert · Accepted Answer · 2012-12-22 18:22:08Z

-1

There is no such thing that you are searching for, but you can prevent this error by using isKindOfClass and then performing that method.

answered Dec 22, 2012 at 18:22

DivineDesert

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2 Comments

Daij-Djan Over a year ago

he wants to assign names to INSTANCES not classes

DivineDesert Over a year ago

I already mentioned that there is no such thing what he wants, but he can make use of conditions to call a particular method, that is the way I found can be done

Johannes Lund · Accepted Answer · 2012-12-22 19:06:18Z

-1

If I have understood this correctly and you are talking about class methods, the problem is that you cannot "directly" send a message to the class you´re creating. You have to use performSelector.

SEL method = NSSelectorFromString(@"method");
Class class = NSClassFromString(@"SomeClass");
[(id)class performSelector:method];

answered Dec 22, 2012 at 19:06

Johannes Lund

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2 Comments

Daij-Djan Over a year ago

he wants instances by name -- if I understood him :D I think ramy & kenny are right

estobbart Over a year ago

It'd be helpful to see this in context. There's probably a better way to solve it. Have you looked through the objc runtime ref? developer.apple.com/library/mac/#documentation/Cocoa/Reference/…

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