Obj-C: Creating an object with a String name

Question

Hey all. I know this sounds simple, but I can't find a way to do it. I have a method in Obj-C that takes in a NSString and then should create a new class with the String as its title.

-(DataModel *)createDataModel:(NSString *)dataModel_name {
        DataModel *[initWithString:dataModel_name] = [[DataModel alloc] init];
        }

I know I have some problems in this. For starters, I don't know how to define a return on an object whose name could change. Second, I know this doesn't compile considering the initWithString method is wrong. I just don't know what to do or what method to use so that I can create this DataModel object with the specified name...

Answer 1

NSClassFromString() will do what you want. Also, initially declaring variables as type id allows you to set their explicit type later on. So:

id dataModel = [[NSClassFromString(dataModel_name) alloc] init];

Answer 2

If your title is setup correctly, as a property:

-(DataModel *)createDataModel:(NSString *)dataModel_name {
    DataModel *model = [[DataModel alloc] init];
    model.title = dataModel_name;
    return model;
}

That would require in your datamodel.h:

@interface DataModel {
  NSString *title;
}
@property (nonatomic, retain) NSString *title;
@end

And in your .m:

@implementation DataModel
@synthesize title;
@end

But your question isn't clear if your real purpose is trying to instantiate different classes based on the dataModel_name or if you just have a single generic class with a title that should be set to dataModel_name.

Depending on what you want to do, there are different answers. If you really want different classes based on the name, then you should do things differently. You can use the Cocoa specific type: id, to return any object from a method. Then the method, NSClassFromString() to create the object:

- (id)createDataModel:(NSString *)dataModel_name {
   id model = [[NSClassFromString(dataModel_name) alloc] init];
   [model setTitle:dataModel_name];
   return model;
}

Or you can define a Protocol (Interface in java parlance) that declares the features of your data model. Your method would return that instead.

Answer 3

To locate or create a new class:

Class arbitraryClass = NSClassFromString(dataModel_name);
if ( nil == arbitraryClass ) arbitraryClass = objc_allocateClassPair( [DataModel class] , [dataModel_name UTF8String] , 0 );

To create a new instance of an object with your newly created class:

DataModel *modelWithArbitratyClassName = [[arbitraryClass alloc] init];

Creating new classes at runtime is not usually a good idea.

Answer 4

So, it seems you want to dynamically add an Instance variable to an object at runtime. You don't get this for free. CALayer and CAAnimation can do something similar to this, you can read about it here

You could add similar functionality to your own objects using Key-value-coding, and more specifically the method valueForUndefinedKey. There will be some KVC specific caveats so you should really make sure you are familiar with and understand KVC. Take a look at this, it might be just want you want.

A dictionary is used to store the value and key, and to retrieve the value when you try to access it.