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A Quick Look at Working With Apple Events

Apple events are designed to provide a flexible mechanism for interprocess communication. An Apple event specifies a target application (or other process) and provides a detailed description of an operation to perform. The operating system locates the target, delivers the event and, if necessary, delivers a reply Apple event back to the sender. While simple in concept, this mechanism provides a basis for powerful interaction between processes and for automating tasks that use multiple applications.

In this section:

When Applications Use Apple Events
Apple Event Terminology
Sending and Receiving Apple Events
Responding to Apple Events

When Applications Use Apple Events

An application is most likely to work with Apple events for the following reasons:

• To respond to Apple events received from the Mac OS.

  For applications that present a graphical user interface, Mac OS X sends Apple events to initiate certain operations, such as launching or quitting the application.

  For Cocoa applications, most of the work of responding to these events happens automatically. Carbon applications need to provide more of their own implementation—for details, see “Handling Apple Events Sent by the Mac OS.”

• To make its services or data available to other processes.

  Most applications that provide services through Apple events are scriptable applications—they provide a scripting terminology that lets users write AppleScript scripts to access the application’s operations and data. When a script is executed, some of its statements result in Apple events being sent to the application. Other applications can also send Apple events directly to scriptable applications.

  Scriptable applications make it possible for users to automate their work. And starting in Mac OS X version 10.4, your scriptable application can also provide users with Automator actions. (Automator is an application that lets users work in a graphical interface to put together complex, automated workflows, made up of actions that perform discrete operations.)

  Scriptable Carbon applications can use the techniques for working with Apple events that are described throughout this document. For information on scriptable Cocoa applications, see “Framework and Language Support.”

  For information on designing and creating scriptable applications, and on creating Automator actions, see the learning paths in Getting Started with AppleScript.

• To communicate directly with other applications.

  An application can send an Apple event to ask another application to perform an operation or return data. For example, you might create a scriptable server application, running locally or remotely. Your other applications create Apple events and send them to the scriptable server to access its services. This is, in effect, another way to factor your code, with the shared functionality made available through Apple events.

• To support recording in a scriptable application.

  Recording refers to the assembling of Apple events that represent a user’s actions into a script. Users can turn on recording in the Script Editor application, then perform actions with scriptable applications that support recording. Any Apple events generated by those actions are recorded into an AppleScript script. To support recording as fully as possible, you can take these steps:

  • Factor code that implements the user interface in your application from code that actually performs operations—this is a standard approach for applications that follow the model-view-controller design paradigm.

  • Send Apple events within the application to connect these two parts of your application. The Apple Event Manager provides a mechanism for doing this with a minimum of overhead, described in “Addressing an Apple Event for Direct Dispatching.”

  • Make sure that any significant action within your application generates an Apple event that can be recorded in a script.

  Recording is beyond the scope of this document, but you can read more about it in the sections “Recordable Applications” and “Making Your Application Recordable” in Inside Macintosh: Interapplication Communication.

Apple Event Terminology

A scriptable application specifies the terminology that can be used in scripts that target the application. There are currently three formats for this information:

• aete: This is the original dictionary format and is still used in Carbon applications. The name comes from the Resource Manager resource type in which the information is stored ('aete').

• script suite: This is the original format used by Cocoa applications. A script suite contains a pair of information property list (plist) files.

• sdef: “sdef” is short for “scripting definition.” This XML-based format is a superset of the other two formats and supports additional features.

For more information on these formats, including pointers to additional documentation, see "Scriptable Applications" in Open Scripting Architecture in AppleScript Overview.

Sending and Receiving Apple Events

Applications typically use Apple events to request services and information from other applications or to provide services and information in response to such requests. In client-server terms, the client application sends an Apple event to request a service or information from the server application. The recipient of an Apple event is also known as the target application because it is the target of the event. A client application must know which kinds of Apple events the server supports.

Figure 1-1 shows two applications communicating with Apple events. The client uses Apple Event Manager functions to create and send an Apple event to the server, the FileMaker Pro database application. The event might, for example, request employee information from a payroll database. FileMaker Pro uses other Apple Event Manager functions to extract information from the event and identify the requested operation. Depending on the event, FileMaker Pro may need to add a record, delete a record, or return specified information in a reply Apple event.

The most common Apple event client is a script editor application executing an AppleScript script. Statements in a script that target an application may result in Apple events being sent to the application. Another common client is the Mac OS, which sends Apple events to applications to open documents and perform other operations.

The most common servers are scriptable applications and scriptable parts of the Mac OS, such as the Finder and the System Events application (located in /System/Library/CoreServices). You can read more about script editors and scriptable applications in AppleScript Overview.

Responding to Apple Events

Your application should be prepared to respond to Apple events sent by the Mac OS, as well as to other Apple events the application supports. An Apple event typically contains information that specifies the target application, the action to perform, and optionally the objects on which to operate. For example, Figure 1-2 shows an open documents Apple event sent by Mac OS X to the AppleWorks application. This type of Apple event provides a list of files for the target application to open.

For an application to handle a specific Apple event such as the open documents event, it must register with the Apple Event Manager a function that handles events of that type. The Apple Event Manager dispatches a received Apple event to the handler registered for it. An Apple event handler is an application-defined function that extracts pertinent data from an Apple event, performs the requested action, and if necessary, returns a result.

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