This tutorial is aimed primarily at people with little or no background in Objective-C and Cocoa, and it will help you to understand PyObjC programs written by other people, such as the examples included with the distribution. This document is actually not a true tutorial: you do not get to build anything, only read and examine things.
It is strongly suggested that you first do the Creating your first PyObjC application tutorial to get some hands-on experience with PyObjC, Cocoa and especially Interface Builder.
If you have used another GUI toolkit in the past it is essential that you understand that Cocoa is different. For this once this isn't marketing-speak: Cocoa is inherently different from common toolkits such as Tk, wxWindows, Carbon, MFC, etc. Apple's documentation explains this, but such introductory text is often skipped. It is a good idea to refer back to Application Architecture after reading this section. If you want, you can write code that does not follow the Model-View-Controller paradigm, but you would be on your own. Cocoa and Interface Builder are designed to suit this model.
Cocoa is built on the Model-View-Controller paradigm (MVC). What this means is that the application code should be split into three parts:
To summarize: the Model knows nothing about the user, the View knows nothing about the data and operations, and the Controller only knows how to relate the Model and the View. For really tiny applications, such as the currency converter, it may be tempting to do away with the Model and simply put that code in the Controller. You probably shouldn't do this, as it can make your code harder to read since it will be a mix of algorithms and glue code, however there is no technical limitation that prevents you from doing this. If you do combine the functionality of the model and controller, it is customary to name it as if it represented the document (without "Controller"). Note that the MVC paradigm is not specific to Cocoa and can be used with almost any GUI toolkit, but Cocoa is explicitly designed for this paradigm.
You should have an MVC trio for every distinct unit of information in your program. In case of a simple dialog-style application such as Currency Converter you will have one such trio. Most applications, however, will have at least two: one for the application itself and one for the "documents" the application handles. These may be real documents (i.e. files), but a document can be more abstract. For example, if your application does scientific simulations that run in separate windows, each simulation could be a document.
Cocoa and Interface Builder strongly encourage you to use a NIB file per MVC trio. You should follow this encouragement unless you are sure that you know what you are doing.
This brings us to the second big difference between Cocoa and other GUI toolkits: almost all of the boilerplate code is replaced by the NIB. The source of Cocoa programs that do little work, especially example programs, will typically be much shorter than the equivalent with other toolkits.
The NIB file is not a description of dialogs and menus and buttons, as you would get out of interface-builders for other toolkits. A NIB file is more: it contains a archived object graph that represents the GUI, conceptually similar to a pickle in Python. You tell Interface Builder about all the relevant classes in your application, the instances you want to create from those classes, and how the classes should connect to each other. Interface Builder the actually instantiates the classes, makes all the connections and at that point freezes and stores the whole lot.
Unarchival of a NIB happens in two phases. The objects are restored using the NSCoding protocol (initWithCoder: is similar to __setstate__ of Python's pickle protocol), and then each object is sent an awakeFromNib: message so that they may do any initialization that depends on a fully restored object graph (pickle does not have this functionality built-in).
The section above explains a lot of the strangeness in AppKit-based PyObjC applications:
This also explains why you want separate NIB files for each MVC trio: the objects and classes in a NIB file are all unarchived together. In other words, if you had created your document window in your application NIB (even if you set it to "hidden" initially so it does not show up) it would become very difficult to create a second window for a new document.
If you think about the consequences of this section for a while it will become clear why all the boilerplate code is missing from Cocoa applications: you don't need it. Like the output of other gui-builders, a NIB usually contains enough information to recreate the view objects, but a NIB can also contain a large proportion of the setup for your Model and Controller functionality. This is especially true when using Cocoa Bindings.
If you are familiar with other object-oriented GUI toolkits such as MFC another thing to notice is that Cocoa applications often use a delegate object where other toolkits would use a subclass. For example: it is not common to use your own subclass of NSApplication for the main application object. NSApplication objects have a helper called its delegate. The application object will attempt to inform its delegate many interesting events, and the delegate implements only the methods for the events it is interested in.
For example, the method applicationShouldTerminate: of the delegate is called just before the application quits, and it has a chance to return NO if it is not appropriate to quit just yet.
Let us examine the final NIB of the Currency Converter tutorial with this in mind. If you open it and look at the main window (titled "MainMenu.nib") and select the "Instances" pane you should see six objects. Two of these have greyed-out names ("File's Owner" and "First Responder"), these are present in every nib can not be changed. The "File's Owner" is either the Controller or the combined Model-Controller object, and is specified by the application when it loads the NIB. For the main nib, which is loaded automatically by NSApplicationMain or PyObjCTools.AppHelper.runEventLoop, this will be the instance of NSApplication. Currency Converter is not a document-based application, so the MVC trio for the conversion window are in here too. These are named Converter, Window and ConverterController respectively.
Let us have a look at the ConverterController object by double clicking it. The "MainMenu.nib" window goes to the "Classes" tab, and an info window shows up. In the "MainMenu.nib" window the ConverterController class is selected, and you can see it is a subclass of NSObject. Having the same name for the class and the instance is common in Cocoa programs, the main exception being the File Owner object.
The info window shows more information on the ConverterController class. It should pop open to the "attributes" page. In the "Outlets" tab you see that instances of this class have four attributes, converter, rateField, dollarField and totalField. In any instance of ConverterController you can connect these to other objects, as we shall see below. The "Actions" tab shows that there are two methods convert: and invertRate:, and again you can arrange for these to be called on instances of your ConverterController on certain events by making connections.
So let us now look at the connections for our ConverterController instance. Select the "Instances" tab in the main window, select ConverterController and set the info window to show "Connections". You now see all the outlets defined in the class. Select one, and in the lower half of the info window you will see which object it connects to. Moreover, a blue line will also link the object representations in the main window and in the dialog preview window.
Finding out who calls your convert: method is more difficult, though, with this view. But, if you select the "Convert" button in the dialog you will see that its target action will go to the ConverterController.convert_ method.
Luckily there is a way to find such incoming connections without reverting to guessing. For instance, you will be hard put to find who, if anyone, calls ConverterController.invertRate_. The solution: go to the "MainMenu.nib" window and look at the top of the vertical scrollbar. There are two little icons there, one with lines and one with squares, with the squares being highlighted. Press it. The view will change to a scrollable list with objects in the left column and an indication of connections in the right column. You can now see our ConverterController object has four outgoing connections (the ones we found earlier) and two incoming connections. Click on the incoming connections icon. The view will change again and ConverterController will probably scroll out of sight. Locate it, and see that there are two lines going out of the ConverterController object. One goes to NSButton(Convert) and is labeled convert:, we knew about that already. The other one goes to an object NSMenuItem(Invert Exchange Rate) and is labeled invertRate:, so that is where calls to invertRate: come from. And if you look at where this NSMenuItem sits in the object hierarchy you find that it is an entry in the "Edit" menu in the menubar.
This section remains to be written. Contributions will be gratefully accepted :-)