How do you specify multiple AppleScript child elements of the same class? - macos

Question

I'm adding Apple Event scripting to my application. I would like to be able to use both of the statements below on it:
-- Every instance of MyObject in the app
every MyObject
-- Only the instances of MyObject the user has selected
selected MyObjects
These are relevant excerpts from of my sdef file:
<dictionary>
<suite ...>
<class name="application" code="capp" description="Top-level scripting object" plural="applications" inherits="application">
<cocoa class="MyAppClass" />
<element type="MyObject" access="r">
<cocoa key="myobjects" />
</element>
<element name="selected MyObjects" code="ABCD" type="MyObject" access="rw">
<cocoa key="selectedMyObjects" />
</element>
</class>
<class name="MyObject" code="ABcd" inherits="item" plural="MyObjects">
...
</class>
</suite>
</dictionary>
When I call every MyObject, it returns a list of objects, as expected. But when I call selected MyObjects, the Script Editor selects the "MyObjects" portion and complains:
Syntax Error
Expected end of line, etc. but found plural class name.
How can I achieve what I'm looking to do?

Answer 1

Your design is wrong. See the Scripting Interface Guidelines for a somewhat thin but better-than-nothing outline of good UX practices.
The correct idiom is to define a selection property, typically on application and/or document. This may be read-write or read-only, depending on what's appropriate to your application.
The selection property's value is either:
A single bespoke object specifier that identifies all of the currently selected objects e.g.:
selection of application "Foo"
or
selection of document X of application "Foo"
Some of the better Carbon-based apps and the occasional Cocoa app use this approach, which allows users to perform powerful queries such as:
get (name of every MyObject of selection of document X)
delete (every job whose status is completed)
but requires more work for you to implement.
A list of single-object specifiers, each identifying one selected item, e.g.:
{thing B of document X of application "Foo",
thing E of document X of application "Foo",...}
This is less powerful, since users cannot manipulate all of the selected items in a single command but must instead iterate the list processing each item one at a time, but is cheaper for you to implement. Cocoa Scripting being somewhat lame and inflexible even at the best of times, and pretty damn hopeless at moving/duplicating/deleting more than one object at a time, this is the most common approach in Cocoa-based apps.
Update your sdef file with this property:
<property name="selection" code="ABCD">
<cocoa key="selectedMyObjects" />
<type type="MyObject" list="yes" />
</property>
For an example of the first (better) design approach, take a look at Script Editor's own dictionary. For an example of the second, see Mail's dictionary.
(Tip: To view a dictionary in Script Editor, choose File>Open Dictionary and select the appropriate item from the list. To export that dictionary as an SDEF file, just make sure the dictionary viewer window is frontmost and choose File>Save As.)