AppleEvents

Apple events are the message-based interprocess communication mechanism in Mac OS, first appearing in System 7 and supported by every version of the classic Mac OS since then and by macOS. Apple events describe "high-level" events such as "open document" or "print file", whereas earlier OSs had supported much more basic events, namely "click" and "keypress". Apple events form the basis of the Mac OS scripting system, the Open Scripting Architecture (the primary language of such being AppleScript).

The starting point is a dynamically-typed, extensible descriptor format called an AEDesc, which is just an OSType code specifying the data type, together with a block of type-dependent data. For instance, the OSType code inte indicates that the data was a four-byte signed integer in big-endian format.

Besides predefined type codes for various common simple types, there are two predefined structured descriptor types: an AERecord, which has data type reco (record), and AEList with type list (list or array). The internal structure of these contain recursively-nested AEDescs, while the AERecord also associates each element with a unique record field ID, which is an OSType. The Apple Event Manager provides API calls to construct these structures, as well as extract their contents and query the type of contents they hold.

The Apple Event Manager also supports coercions, which converts AEDescs from one data type to another. In addition to standard coercions, for instance between integer and real types, applications can install their own coercion handler callbacks, which handle conversions to and from custom data types.

An Apple event proper is an AERecord with fields that depended on the purpose of the event. In addition, it has attributes (which are distinct from record fields, which are now called the parameters of the event) from a set predefined by the Apple Event Manager. These specify what the event is supposed to do (through event class and event ID), the target address to which the event is to be sent (which could be a process on the local or a remote machine), and various other options for handling it. Remote machines initially had to be connected via AppleTalk, but Mac OS 9 added the option for connections via TCP/IP.

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