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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.

Paul D. Waite
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sayzlim
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  • Where have you read about this pattern? –  May 10 '11 at 07:42
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    This book: http://www.amazon.com/Cocoa-Design-Patterns-Erik-Buck/dp/0321535022 – sayzlim May 10 '11 at 07:44
  • which I think used to be the top Google result for `cocoa "dynamic creation"` — now, 23 minutes after you asked, [it‘s this question!](http://www.google.co.uk/search?hl=en&q=cocoa+%22dynamic+creation%22&aq=f&aqi=&aql=&oq=) – Paul D. Waite May 10 '11 at 07:46
  • Does that mean someone will clarify it sooner? – sayzlim May 10 '11 at 07:50

2 Answers2

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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.

  1. 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.

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Amy Worrall
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  • Sorry if I don't fully understand the first answer, since I'm also quite new in developing app. If you're saying "Write library", do you mean you create a number of classes that can handle parsing from several web services? Can you give a more precise example? If I want to handle parsing, can't I just create a class method that handle parsing and return the result? Thank you for explaining. – sayzlim May 10 '11 at 13:48
  • Yeah, that's what I meant. The reason I did it that way was in order to be able to customise the behaviour of my parser with configuration files, so I could have it talk to a brand new web service without having to write any more code. – Amy Worrall May 10 '11 at 13:51
  • Can you show me an example with web services like Twitter? What is your configuration file format? A more practical example like what the class method to and return. – sayzlim May 10 '11 at 14:01
  • Sorry, I'm not allowed to release my config file format (I wrote this for work). Here's another example: sometimes you have an app that works on iOS 3, but you want to use some snazzy new iOS 4 features if they're available. If you wanted to support printing, for example, you might write `if (NSClassFromString(@"UIPrintInteractionController") != nil) {}`, to check whether to show a print button in your UI. – Amy Worrall May 10 '11 at 14:40
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See this link: need advise about NSClassFromString

The only real benefit for iPhone was being able to reference classes from newer APIs and still target the old APIs. Since 4.0 you can do this anyway by setting the deployment target of your project. I can't really see any reason you would use it for iPhone programming any more.

This would only work for objective-C classes. You can't import java objects into your iphone app.

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skorulis
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  • From what I've read, this approach is suitable if we want to create a plug-in based class and use it in your design. I wonder if there is a way to make use of this approach for reusing class in other project, such as class for parsing. – sayzlim May 10 '11 at 07:41