pseudofish

Using a sheet to display a save panel adds for a significant user interface improvement. Based on the simplicity of a range of other Cocoa features, I was surprised at how tricky it was to get working.

The issues had nothing to do with Cocoa per-se, but with the PyObjC bridge.

Creating the panel and creating the sheet is as follows:

sp = NSSavePanel.savePanel()
sp.setRequiredFileType_('ics')
sp.setNameFieldLabel_('Export As:')
sp.beginSheetForDirectory_file_modalForWindow_modalDelegate_didEndSelector_contextInfo_(\
    userDocumentFolder(),'default.ics',\
    NSApp().mainWindow(),self,\
    'didEndSheet:returnCode:contextInfo:',0)

The tricky bit is the asynchronous callback, which is passed in as an Objective C selector: 'didEndSheet:returnCode:contextInfo:'

In Python terms, the following method does the trick:

def didEndSheet_returnCode_contextInfo_(self, sheet, returnCode, info):
    if returnCode == NSCancelButton:
        return
    # Save the file returned by sheet.filename()
    print sheet.filename()

I only had one slight problem. The application crashed when the user clicked a button on the sheet.

The issue is that the Python bridge has no idea as to the type of the arguments in the method listed as the callback from the sheet. When the callback occurs from Cocoa, it goes searching for a method which matches a specific type signature, as documented in NSSavePanel.

An explicit type signature is required for the method to be discoverable. PyObjC provides a few ways of doing this.

Note: the name of the callback function is up to programmer, which is why the type signature is important.

The Python 2.4 version (using decorators) looks like:

from objc import *

@objc.signature('v@:@ii')
def didEndSheet_returnCode_contextInfo_(self, sheet, returnCode, info):
    if returnCode == NSCancelButton:
        return

    # Save the file returned by sheet.filename()

With the standard version like:

from objc import *

def didEndSheet_returnCode_contextInfo_(self, sheet, returnCode, info):
    if returnCode == NSCancelButton:
        return

   # Save the file returned by sheet.filename()
   print sheet.filename()
   didEndSheet_returnCode_contextInfo_ = objc.selector(\
       didEndSheet_returnCode_contextInfo_ , signature='v@:@ii')

With the type information added in, the callback can locate the method on the object and all is well in the world. Files are saved, memory protected, everybody is happy.

Almost.

I had a simple question, where did this type string come from?

Searching on Google had resulted in two different strings. Both worked. However they were quite different.

@objc.signature('v16@4:8@12i16i20')@objc.signature('v@:@ii')

Somewhere, there had to be an explanation for all of this.

The PyObjC documentation provided some good hints. The intro mentions the different syntax for specifying the type signature, and provides an example.

Using a convenience method on AppHelper, the type information does not need to be explicitly included in the code for the endSheet case:

from PyObjCTools import AppHelper

# Python 2.4
@AppHelper.endSheetMethod
def didEndSheet_returnCode_contextInfo_(self, sheet, returnCode, info):
    pass

# Python 2.3
def didEndSheet_returnCode_contextInfo_(self, sheet, returnCode, info):
    passdidEndSheet_returnCode_contextInfo_ = AppHelper.endSheetMethod(\
        didEndSheet_returnCode_contextInfo_)

This third option provides cleaner code, but still didn’t explain where type signatures come from.

(I must have been a difficult child, a solution isn’t enough, the need to know why/how is the motivator)

The key came in reading through the documentation on wrapping an Objective C class. When the documentation refers to a ‘raw Objective-C method signature’, it means what it says. The string appears to be Objective-C’s internal representation of the type signature.

Apple provides documentation on the syntax. (Note: Try here for updated docs)

Probably a bit further into Objective-C than I wanted to delve, but much was learnt along the way. A good abstraction allows you to peek through it. I’m happy to learn that PyObjC provides that flexibility, and that most of the time, it isn’t needed.