Re: [Pyobjc-dev] AppKit/__init__.py complexity

[Bill B. <bb...@co...>]

On Friday, Feb 28, 2003, at 14:25 US/Eastern, Chris Ryland wrote:
> Can you expand on this thought a bit for a Cocoa/PyObjC newbie?
>
> I guess I'm not sure what the informal protocols are used for, and why 
> they're interesting...

Sure -- "informal protocol" is really just a made up term that refers 
to a collection of methods that fulfill some particular purpose.  For 
example, NSTableView has three informal protocols -- one for handling 
of notifications, one for delegation, and one for the data source.

A formal protocol is like an interface in Java;   a class that declares 
compliance with a formal protocol or interface must implement all 
methods in the protocol or interface.

An informal protocol typically has methods that are purely optional.  
Some methods -- like a couple of the methods in the data source -- may 
be required.

An example of an informal protocol:

@interface NSObject(NSTableViewDelegate)
- (void)tableView:(NSTableView *)tableView willDisplayCell:(id)cell 
forTableColumn:(NSTableColumn *)tableColumn row:(int)row;
- (BOOL)tableView:(NSTableView *)tableView 
shouldEditTableColumn:(NSTableColumn *)tableColumn row:(int)row;
- (BOOL)selectionShouldChangeInTableView:(NSTableView *)aTableView;
- (BOOL)tableView:(NSTableView *)tableView shouldSelectRow:(int)row;
- (BOOL)tableView:(NSTableView *)tableView 
shouldSelectTableColumn:(NSTableColumn *)tableColumn;
- (void) tableView:(NSTableView*)tableView 
mouseDownInHeaderOfTableColumn:(NSTableColumn *)tableColumn;
- (void) tableView:(NSTableView*)tableView 
didClickTableColumn:(NSTableColumn *)tableColumn;
- (void) tableView:(NSTableView*)tableView 
didDragTableColumn:(NSTableColumn *)tableColumn;
@end

In Objective-C, the above declaration is not backed by any 
implementation.   It is just a declaration mostly for the benefit of 
the developer.   If the above declaration were missing, it wouldn't 
affect anything -- not at compile time, not at runtime.

The reason why we have to declare the methods in PyObjC is because of 
the types.

Because methods like the above are typically implemented on some class 
where none of the superclasses implement the method, then there is no 
way for the PyObjC bridge to know that the 'row' parameter should be 
handled as an (int) and not an (id).

Initially, you had to declare the types of the method when implementing 
the method.  I.e.:

     def selectionShouldChangeInTableView_(self, aTableView):
	    return YES
     selectionShouldChangeInTableView_ = 
objc.selector("selectionShouldChangeInTableView:", signature="c@:@")

Obviously, that is very ugly.  Instead, Ronald created the mechanism we 
use today that allows you to declare that a class will be a target of 
an informal protocol in this fashion:

     class MyFooTableController(NSObject, NSTableDataSource, 
NSTableViewDelegate):
         def selectionShouldChangeInTableView_(self, aTableView):
		    return YES

This allows the PyObjC runtime to automatically deduce that 
selectionShouldChangeInTableView_() really needs to return a BOOl or 
that one of the methods that takes an (int) argument really wants an 
(int) and not an object.

But there is an added benefit.   Ronald added a feature to the informal 
protocol creation mechanism such that you can mark a method as being 
required.

As such, if you declare that an object will be implementing some/all of 
the methods of an NSTableDataSource, the PyObjC bridge will 
automatically determine if your class is compliant by checking to see 
if you have implemented the one or two methods that are required if 
your class is going to truly act as an NSTableDataSource.

b.bum