All Databases Inside Mac - AppleEvents

Descriptor Lists

When extracting data from an Apple event, you use Apple Event Manager

functions to return data in a buffer specified by a pointer, or to return

descriptor records containing the data, or to return lists of

descriptor records (called descriptor lists) containing the data. As previously

noted, the descriptor record (of data type AEDesc) is the fundamental structure

in Apple events, and it contains a handle to data. A descriptor list is a data

structure of type AEDescList defined by the data type AEDesc-that is, a

descriptor list is a descriptor record that contains a list of other

descriptor records.

An example of a descriptor list that you will be using is the direct parameter

for the Open Documents event. As illustrated in the next figure, this

descriptor list is a list of descriptor records that contain alias records to

filenames. (The Alias Manager describes alias records in detail.)

A descriptor list for a list of aliases

Closely related to a descriptor list is a structure of data type AERecord; in

fact, it is defined by the data type AEDescList.

While a descriptor list is a descriptor record that contains a list of other

descriptor records, an AE record of data type AERecord contains a list of

keyword-specified descriptor records describing parameters. A descriptor list

of data type AERecord contains no attributes, only parameters.

There is one final data structure to consider: the Apple event record. An

Apple event record is a structure of data type AppleEvent defined as an

AE record. It is used for describing a full-fledged Apple event.

An Apple event record is basically a descriptor record (of descriptor type

typeAppleEvent) with a handle to a list of keyword-specified descriptor

records. These descriptor records describe the attributes and parameters for

an Apple event. When you use the AECreateAppleEvent function, the

Apple Event Manager creates an Apple event record containing the

attributes for an Apple event's event class, event ID, target address, return ID,

and transaction ID. You then use Apple Event Manager functions such as

AEPutParamDesc to add parameters to the Apple event.

The internal structure of an Apple event record is nearly identical to an

AE record. They differ in the content referred to by the data handles that they

contain: the former has a list of attributes and, possibly, parameters referred

by its handle; the latter contains only parameters. However, you can pass an

Apple event record to any Apple Event Manager function that expects an

AE record. Since both are structures of data type AEDescList, which is derived

from the data type AEDesc, you can pass Apple event records, AE records,

descriptor lists, and descriptor records to any

Apple Event Manager functions that expect records of data type AEDesc.

The data in Apple event records, AE records, and descriptor lists-all of which

are descriptor records-is private to the Apple Event Manager.

The Apple Event Manager maintains these different data structures

because it stores different kinds of information in their handles. Although all

the information you need is available by calling the appropriate

Apple Event Manager functions, the Apple Event Manager needs a way

to tell these different descriptor records apart. It does this by looking at their

data types.

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