Question

I recently read about Dynamic Creation as one of the design pattern in Cocoa. However, I don’t really understand how it works. So I need clarification from you who have implemented in your design.

1. What is it? Why and when would you use this design pattern?

2. I have read that you use NSClassFromString() to access the class. I assume that I use this when I want to use class that doesn’t exist within the project I’m working on. Usually when I want to use certain class, I imported them in header. Does using this approach skip the #import process?

   Class JavaArrayList = NSClassFromString(@"java.util.ArrayList");
   

   I quote the code above as example. If do according to the code above, that means I can create a new JavaArrayList class and use the methods in it right?

   JavaArrayList *foo = [[JavaArrayList alloc] init];
   [foo useMethodBelongJava:doWhateverTask];
   

3. What are the benefits of using this design pattern? Especially in iPhone Development.

Answer 1

Your example appears to be using that pattern to instantiate a Java class. In the old days (up to about MacOS 10.4 I think), Apple had some technology called the Cocoa-Java Bridge, which let you use Java classes within Objective-C code. You had to instantiate them in the manner specified, because they didn’t have Objective-C header files to import.

However, as of Snow Leopard, the Java Bridge no longer exists, so the code in your question won’t work any more.

The recommended solution for calling a Java class from Objective-C is now JNI. Take a look at this question if that is what you’re trying to do.

1. What is it? Why and when would you use this design pattern?

Coming back to NSClassFromString, it has other uses besides instantiating Java classes (which, as I mentioned, it doesn’t do any more!). For an example, recently I wrote a library for parsing the response from a web service. In order to make it work with different web services, I had it read in a configuration file that described the data format it was expecting. For each field in the web service, my configuration file specified which Cocoa class to instantiate. Thus, in my code, I had a Cocoa class name as a string. To instantiate the object I wanted, I used NSClassFromString to turn it into a Class object.

2. Usually when I want to use certain class, I imported them in header. Does using this approach skip the #import process?

It can do. NSClassFromString will instantiate any class that is present at run time, so you don’t need the header to be able to use it. If you don’t have the header, you’ll get a bunch of warnings of “may not respond to selector” whenever you try and use your newly instantiated class, as the compiler doesn’t have enough information to be helpful. However, in many circumstances where NSClassFromString is useful, the header files aren’t available.