Cocoa Scripting: Deletion of elements in a loop getting out of sync

Question

While adding scriptability to my Mac program, I am struggling with the common programming problem of deleting items from an indexed array where the item indexes shift due to removal of items.

Let's say my app maintains a data store in which objects of type "Person" are stored. In the sdef, I've define the Cocoa Key allPersons to access these elements. My app declares an NSArray *allPersons.

That far, it works well. E.g, this script works well:

repeat with p in every person
  get name of p
end repeat

The problem starts when I want to support deletion of items, like this:

repeat with p in (get every person)
  delete p
end repeat

(I realize that I could just write "delete every person", which works fine, but I want to show how "repeat" makes things more complicated).

This does not work because AppleScript keep using the original item numbers to reference the items even after deleting some of them, which naturally shifts the items and their numbering.

So, considering we have 3 Persons, "Adam", "Bonny" and "Clyde", this will happen:

get every person
    --> {person 1, person 2, person 3}
delete person 1
delete person 2
delete person 3
    --> error number -1719 from person 3

After deleting item 1 (Adam), the other items get renumbered to item 1 and 2. The second iteration deletes item 2 (which is now Clyde), and the third iteration attempts to delete item 3, which doesn't exist any more at that point.

How do I solve this?

Can I force the scripting engine to not address the items by their index number but instead by their unique ID so that this won't happen?

Answer 1

re: "If you want your scripable elements to be deletable, make sure you use NSUniqueIDSpecifiers to identify them."

Yes, Apple recommends using formUniqueId or formName for object specifiers, but you can't always do that. For instance, in the Text Suite, you really only have indexing to work with; e.g. character 1, word 3, paragraph 7, etc. You don't have unique IDs for text elements. In addition to deletion, ordering can be affected by other Standard Suite commands: open, close, duplicate, make, and move.

The app implementer is a programmer, but so is the scripter. So it is reasonable to expect the scripter to solve some problems themselves. For instance, if the app has 5 persons, and the scripter wants to delete persons 2 and 4, they can easily do so even with indexed deletion:

delete person 4
delete person 2

Deleting from the end of an ordered list forward solves the problem. AS also supports negative indexes, which can be used for the same purpose:

delete person -2
delete person -4

Answer 2

It's not your ObjC code, it's your misunderstanding of how repeat with VAR in EXPR loops work. (Not really your fault either: they're 1. counterintuitive, and 2. poorly explained.) When it first encounters your repeat statement, AppleScript sends your app a count event to get the number of items specified by EXPR, which in this case is an object specifier (query) that identifies all of the person elements in whatever. It then uses that information to generate its own sequence of by-index object specifiers, counting from 1 up to the result of the aforementioned count:

person 1 of whatever
person 2 of whatever
...
person N of whatever

What you need to realize is that an object specifier is a first-class query, not an object pointer (not that Apple tell you this either): it describes a request, not an object. Ignore the purloined jargon: Apple event IPC's nearest living relatives are RDBMSes, not Cocoa or SOAP or any of the OO messaging crud that modern developers so fixate on as The One True Way To Do... well, EVERYTHING.

It's only when that query is sent to your application in an Apple event that it's evaluated against the relational graph your Apple event IPC View-Controller – aka "Apple Event Object Model" – presents as an idealized, user-friendly representation of your Model's user date that it actually resolves to a specific Model object, or objects, with which the event handler should perform the requested operation.

Thus, when the delete command in your repeat loop tells your app to delete person 1 of whatever, all your remaining elements move down by one. But on the next iteration the repeat loop still generates the object specifier person 2 of whatever, which your script then sends off to your app, which resolves it to the second item in the collection – which was originally the third item, of course, until you shifted them all about.

Or, to borrow a phrase:

Nothing in AppleScript makes sense except in light of relational queries.

..

In fact, Apple events' query-based approach it actually makes a lot of sense considering it was originally designed to be efficient over very high-latency connections (i.e. System 7's abysmally inefficient process switcher), allowing a single Apple event carrying one or more complex queries to manipulate many objects at once. It's even quite elegant [when it works right], but is quite undone by idiots at Cupertino who think the best way to make programmers not hate the technology is to lie even harder about how it actually works.

So here, I suggest you go read this, which is not the best explanation either but still a damn sight better than anything you'll get from those muppets. And this, by its original designer that explains a lot of the rationale for creating a high-level coarse-grained query-based IPC system instead of the usual low-level fine-grained OO message passing crap.

Oh, and once you've done that, you might want to consider try running this instead:

delete every person whose name is "bob"

which is pretty much the whole point of creating a thick declarative-y abstraction that does all the work so the user doesn't have to.

And when nothing but an imperative client-side loop will do, you either want to get a list of by-ID object specifiers (which are the closest things to safe, persistent pointers that AEOMs can do) from the app first and then iterate over that, or at least use your own iterator loop that counts over elements in reverse:

repeat with i from (count every person) to 1 by -1
    tell person i
        ..
    end tell
end repeat 

so that, assuming it's iterating over an ordered array on the server side, will delete from last to first, and so avoid the embarrassing off-by-N errors of your original script.

HTH

Answer 3

The key to solving this lies in implementing the objectSpecifier method correctly so that it does return an NSUniqueIDSpecifier.

My code did so far only return an index specifier and that was wrong for this purpose. I guess that had I posted my code (which is, unfortunately, too complex for that), someone may have noticed my mistake.

So, I guess the rule is: If you want your scripable elements to be deletable, make sure you use NSUniqueIDSpecifiers to identify them. For read-only element arrays, using an NSIndexSpecifier is (probably) safe, though, if your element array has persistent ordering behavior.

Update

As @foo points out, it's also important that the repeat command fetches the references to the items by using … in (get every person) and not just … in every person, because only the former leads to addressing the items by their id whereas the latter keeps indexing them as item N.