Question

I’m tasked with creating an application using XCode for OSX. This application needs to be able to load and run separate “modules” which will be determined dynamically (i.e., one user may have purchased modules 1 & 2, while user 2 would have purchased modules 3 and 6 — only purchased modules should “run”).

In C#, I would create a “library” project (that compiles to just a DLL). When the user purchases a module, I’d supply the appropriate DLL files and then my app would look for and load/run the DLL using reflection.

What would be the equivalent to this in XCode? Can I create a “library” and then load it using reflection? Keeping in mind that the app can’t have prior knowledge of the module since in some cases, the user wouldn’t even own the module files. I see various options such as “Cocoa Framework” and “Cocoa Library” as well as “C/C++ Library.” What does each do and would any work to do what I need?

Answer 1

You can create a bundle. Xcode has templates for this (it is called “Loadable Bundle” and the icon is a Lego brick). You typically load a bundle using NSBundle‘s load method.

An example of loading it would be:

- (BOOL)loadPluginAtURL:(NSURL *)URL {
  NSBundle *pluginBundle = [NSBundle bundleWithURL:URL];
  if (![pluginBundle load]) { // is false if pluginBundle == nil automatically.
    return NO;
  }
  id plugin = nil;
  @try { // Use @try-@catch in case the principle class doesn't respond to +alloc or -init. Otherwise the host application would crash and that kinda sucks.
    plugin = [[pluginBundle principalClass] alloc] init]; // Set the principle class in the bundle's info plist.
  } @catch (id e) {
    [bundle unload];
    return NO;
  }
  if (plugin) {
    [self.loadedPlugins addObject:plugin];// Define this as an NSMutableSet object.
    return YES;
  }
  [pluginBundle unload];
  return NO;
}

You can eventually provide a framework that the plugins can use, which can include protocols and classes. You may, for example check if the principle class of the bundle is a subclass of a specific class in your framework, so you don’t send any messages the plugin doesn’t respond to.