AppleEvent Object Model

The AppleEvent Object Model (AEOM) was a set of protocols built on top of AppleEvents by which applications running under classic Mac OS and macOS could control each other's functions. Applications that implemented some part of the AEOM were called scriptable because they could be controlled via AppleScript. Unfortunately, scriptability support remained patchy and inconsistent throughout the history of classic Mac OS.

The AEOM provided a syntactic layer under which any application could publish its internal objects, allowing those objects to be manipulated in a standardized way. Unlike other similar-sounding concepts such as ToolTalk, there was a clear, orthogonal distinction between nouns and verbs; thus, instead of providing separate commands for "close document" and "close window", there was a single "close" verb which could take references to "document" or "window" objects, or any other object that the application published.

The objects that an application made available through its AEOM support were arranged in a hierarchy. At the top was the application itself, referenced via a null object descriptor. Other objects were referenced by (recursively) specifying their parent object, together with other information identifying it as a child of that parent, all collected in an AERecord. An iterator was provided by parents to enumerate their children, or children of a certain class, allowing applications to address a set of elements. The system was generally similar to the Document Object Model used in XML, although with some differences in access patterns.

Each object could have elements and properties; elements were other objects, which might be created or deleted, while properties could not be created or deleted but had values which might be interrogated or changed. For instance, the application might have one or more window elements representing windows showing the contents of currently-open documents. These windows might have properties such as their title, position and size.

An application could define custom verbs for operating on its objects. The AEOM also specified various standard verbs which (it was hoped) applications would implement in a consistent fashion, such as open, close, create element, delete, set data, and get data. Each verb was defined as an AppleEvent of a specific type and class, together with particular parameters of particular types that were expected to be present. For instance, the "get data" event was the standard means for obtaining the value of a property: it took essentially one parameter, which was an object descriptor identifying the property to be queried. The value of that property would be returned in the reply event. The "set data" event took two parameters, the object descriptor for the property to set and the new value for the property; the reply event was only expected to return a success status or failure error code.

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