Tutorial - Adding Python code to an existing ObjC application

In this tutorial we are going to take an existing ObjC application and add Python and PyObjC to it. One of the reasons why you may want to do this is because some things are much simpler in Python than in ObjC, mainly due to the rich library Python has.

To follow the tutorial you need:

  • PyObjC 1.3.1
  • py2app 0.2 or later (included in the binary installer for PyObjC)
  • Python 2.3 or later (note: PyObjC is NOT compatible with MacPython-OS9)
  • Mac OS X 10.3 or later
  • Xcode Tools

If you do not have a /Developer folder, then you do not have Xcode Tools installed. On MacOSX 10.7 or later you can download Xcode from the App Store, see Apple’s developer website <https://developer.apple.com/xcode/> for more information.

The application we are going to modify is Apple’s SimpleComboBox example. This example shows you how to use combo boxes, but that is not what interests us right now: the application pretends to be a database application that allows you to keep notes (such as track list) for your CD collection. With such an application it feels silly that even though you want to type notes on the CD you are currently playing in iTunes you still have to retype album title, artist and genre. This is what we are going to fix: we are going to add a button “ask iTunes”, which will use Python’s AppleScript support to ask iTunes about the currently playing track and fill in the fields for you.

Follow these steps:

Note

Before you start, download the reference source package for this tutorial.

  1. Make a copy of /Developer/Examples/AppKit/SimpleComboBox to work on. Let’s call this SimpleComboBoxPlus:
From this point on, all shell commands take place from this SimpleComboBoxPlus folder.
  1. Open it in Xcode, build it, and see what it does.

  2. Open CDInfoDocument.nib. Select the Class View, NSObject, subclass as ITunesCommunication. Give the class an askITunes: action. Instantiate the class as object ITunesCommunication. This wll be the class that we write in Python.

  3. Go to the object view again, open the Window.

  4. Move the text box down a bit to make space, add a button “ask iTunes”.

  5. Connect this button to the askITunes: action of the ITunesCommunication object.

  6. We now need to write the code implementing the ITunesCommunication class. As this tutorial is about using PyObjC in existing ObjC programs and not about PyObjC itself, we are going to skip writing the code and simply copy ITunesCommunication_1.py to ITunesCommunication.py.

  7. Now we need to create the build script for our plugin, create a file named setup.py with the following contents:

    from distutils.core import setup
    import py2app
    
    setup(
        plugin = ['ITunesCommunication.py']
    )
    

    You may also copy this file from setup.py.

  8. Run the setup script to create a temporary plugin bundle for development:

    Note that we use the -A argument to create an alias plugin bundle at dist/ITunesCommunication.py. Alias bundles contain an alias to the main script (ITunesCommunication.py) and symlinks to the data files (none in this case). This allows us to keep working on the source files without having to rebuild the application. This alias bundle is similar to a ZeroLink executable in Xcode - it is for DEVELOPMENT ONLY, and will not work on other machines.

  9. Add dist/ITunesCommunication.plugin to the Resources folder in your Xcode project. You can do this by ctrl-clicking the Resources folder and choosing “Add Existing Files...”. Make sure to choose “Create Folder References for any added folders”.

  10. Open main.m, it is in the “Other Sources” folder in your Xcode project, and change the main(...) function to the following:

    int main(int argc, const char *argv[]) {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSString *pluginPath = [[NSBundle mainBundle]
                                    pathForResource:@"ITunesCommunication"
                                             ofType:@"plugin"];
        NSBundle *pluginBundle = [NSBundle bundleWithPath:pluginPath];
        [pluginBundle load];
        [pool release];
        return NSApplicationMain(argc, argv);
    }
    

    You may also copy a full main.m from main.m. This code ensures that our ITunesCommunication plugin is loaded before the nib files.

  11. Build and run. When you press the “Ask iTunes” the “CD Title” and “Band Name” fields will be filled with one of the best albums of the last few years :-)

  12. Now we need to make the program talk to iTunes. The current MacPython interface to the Open Scripting Architecture requires an extra step when compared to AppleScript: you need to manually generate a Python package that wraps all the AppleScript terminology for an application. To make matters more complicated iTunes is one of those special cases where the standard way to generate this package (start the application, ask it for its terminology) does not work, so we have to actually look into the bowels of iTunes.app. This leads to the following hefty command line which you should run in the SimpleComboBoxPlus directory:

    $ cd SimpleComboBoxPlus
    $ pythonw -c "from gensuitemodule import main;main()" \
        --output iTunes --creator hook --resource \
        /Applications/iTunes.app/Contents/Resources/iTunes.rsrc
    
  13. Finally, add the code to ITunesCommunication.py to actually communicate with iTunes. We cop out and copy it from ITunesCommunication_2.py.

  14. Build and run. If you press the button when iTunes is playing the Title and Band names will be filled, otherwise they will be cleared. In a real application you would disable the “Ask iTunes” button unless iTunes was active. All that is left as an exercise to the reader.

  15. To make this application redistributable, perform the following commands to make the plugin redistributable:

    $ rm -rf dist
    $ python setup.py py2app
    

    Then, from Xcode, clean your project (shift-cmd-K), switch to Deployment mode, and rebuild.

A minor variation

There a several projects that improve upon the built-in AppleScript support (or to be more precise “application scripting support”). One of those is AppScript.

When you have this module installed you can replace the contents of ITunesCommuncation.py with ITunesCommunication_AppScript.py, and you can skip step 13 entirely.