Call a method in a class instantiated with classFromString?

Question

In another question ( how to instantiate an object of class from string in Objective-C? ), this was suggested:

id myclass = [[NSClassFromString(@"MyClass") alloc] init];
[myclass FunctioninClass];

However, [myClass FunctioninClass] yields an error because the compiler has no idea if FunctioninClass exists.

I do know the classes that can be there are all descendants of one parent. And that parent has the prototype of the method I want to call, except I want to call the overridden descendant method. Does that suggest a solution?

score 4 · Answer 1 · edited May 15 '12 at 07:49

4

You can use performSelector instead. Here is Apple doc

[myClass performSelector:@selector(FunctioninClass)];

edited May 15 '12 at 07:49

Paul Peelen

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answered May 14 '12 at 23:03

user523234

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This certainly compiles - now I'm testing it... : – Richard Grossman May 15 '12 at 01:48
Which will crash if the selector issn't available. – Paul Peelen May 15 '12 at 07:58
Thanks to user523234 - accepting Paul's edited version of your answer so future generations will see the full answer as edited. :) – Richard Grossman May 20 '12 at 17:32

Paul Peelen · Accepted Answer · 2012-05-15T07:58:13.353

2

After @user523234's comment, I decided to improve my answer and correct it. His answer, however, is in my opinion not good enough and will crash your code in case the selector is not available.

So here is the solution:

id myclass = [[NSClassFromString(@"MyClass") alloc] init];

if ([myclass respondsToSelector:@selector(FunctioninClass)])
{
    [myClass performSelector:@selector(FunctioninClass)];
}

Using the - respondsToSelector: will make sure you check wether the sector exists in the class. However, you will need to use the preformSelector everytime. I am unsure if you can dynamically type-cast an identifier.

edited May 15 '12 at 07:58

answered May 14 '12 at 22:44

Paul Peelen

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It's not clear here what you're suggesting the original poster change. – Jonathan Grynspan May 14 '12 at 22:49
2

You are assuming you know @"MyClass" is MyClass. – user523234 May 14 '12 at 23:05
I do know the classes that can be there are all descendants of one parent. Does that suggest a solution? – Richard Grossman May 15 '12 at 01:11
Yes it does, but @user523234 is right, it does assume you know MyClass. I update my answer. – Paul Peelen May 15 '12 at 07:53

David Gelhar · Answer 3 · 2012-05-15T02:06:32.773

1

Since you know that all the possible classes derive from one parent class, and that class includes the method you need, simply:

ParentClass *myclass = [[NSClassFromString(@"MyClass") alloc] init];
[myclass FunctioninClass];

The fact that you're using NSClassFromString to choose which subclass to instantiate isn't even really relevant. This is just a simple inheritance situation: myclass "is-a" ParentClass, so you can call any method of ParentClass on it.

edited May 15 '12 at 02:06

answered May 15 '12 at 02:01

David Gelhar

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Sadly the parent class has abstract methods and none of the new properties or methods introduced by the descendant, hence the descendant class ya see... – Richard Grossman May 20 '12 at 17:27

Call a method in a class instantiated with classFromString?

3 Answers3