From Xojo Documentation
| This class is only available on the macOS platform.
For cross-platform development, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this class on an incompatible platform.
AppleEvent objects can be used to communicate with other Macintosh applications and the Macintosh System software. If you are compiling your application for use on other operating systems, be sure to check the global Boolean constants TargetWin32, TargetMacOS, and TargetLinux. These constants return True if the application is running on the respective operating system.
An AppleEvent is a self-contained block of data which consists in a sequence of key-type-value data (called an AppleEvent Descriptor, or AEDesc per Apple's terminology). Each descriptor can contain other descriptors as an ordered array or as a mixture of keyed data. The AppleEvent as a whole is itself and AppleEvent Descriptor. This flexibility makes the power of AppleEvents but it also has a price: their complexity of use.
Anatomy of an AppleEvent
An AppleEvent is composed of:
- A command, composed of an event class and an event ID (both a four-character code).
- Some so called attributes, notably the target, i.e. the application to which the AppleEvent should be sent. The target can be any already running application, either on the local or a distant computer.
- Some parameters, each being identified by a four-character code. Internally, each parameter also stores its type, its size and its data. You should use the appropriate xxxParam() properties to get or set parameters of a given type. The special parameter '----' is called the direct parameter.
- An automatically generated reply AppleEvent which holds the result of the AppleEvent.
AppleScript is a way of using AppleEvents very easily through a sort of natural speaking language. The text script is first compiled as a sequence of AppleEvents, then it can be executed any number of times with different parameters. AppleScript was part of the Apple's Open Scripting Architecture (OSA). OSA allows everyone to develop a full programming language based on AppleEvents but, as of now, only AppleScript survived as an OSA language.
See Introduction to AppleScript Language on Apple's website.
The Mandatory AppleEvents
Per Apple's programming rules, every application interacting with the user should support the following AppleEvents:
- aevt/oapp: sent to the application when it is opened ('oapp' stands for Open APPlication).
- aevt/odoc: sent to the application when one or more documents are to be opened ('odoc' stands for Open DOCument)
- aevt/quit: sent to quit the application.
- aevt/pdoc: sent to the application to print a document ('pdoc' stands for Print DOCument). Mandatory only if printing is sensible in the context of your application.
- aevt/rapp: sent to application when user reopens the application, either by double-clicking again on the icon, or clicking on the icon in the dock ('rapp' stands for Reopen APPlication).
AppleEvents are sort of object oriented as they give instructions on what to do on a given object (a button, a window...). However, they are intimately linked to AppleScript which is supposed to be a very simple and natural language, hence the use of common words (before, after, first, last, any...) which are sometimes difficult to implement in another programming language. This is why there are 9 different Get...ObjectDescriptor methods.
Use the "dot notation" to access an object and any of its property, e.g.
In AppleScript, you would use "of" instead, like in:
set s to Text of Control1 of ContainerControl1 of Window1
("of Application" is usually omitted)
Describing an Object
According to the context, there may be different ways of describing an object: the frontmost window, the next row, the first word... However, the application you sent an AppleEvent to should reply by giving you a better description of the object, such as its unique ID. In such a case, you should use the object descriptor that the application sent to you.
Also, the flexibility of AppleEvents allows a function to return any kind of data. Most notably, you can get an array (i.e. a list in AppleEvent terminology) of values instead of a single one. As an example, the following AppleScript command
tell application "Finder" to get name of windows --this is interpreted as "give me the name of every opened window"
may legally return nothing or an empty list if there is no window opened, a string or a list with only one string item if there is 1 window opened, or a list of strings when there are more than one window opened.
Getting an Object Descriptor
As said before, there are 9 different Get...ObjectDescriptor functions corresponding to the different ways to obtain a reference to an object. Choose the one which suits best your needs:
- GetIndexedObjectDescriptor: you want one object in an array. The key value is the index of the object in the array.
- GetNamedObjectDescriptor: you want to describe an object by its name.
- GetOrdinalObjectDescriptor: you want to get the first, last, middle, any (random) or all object(s).
- GetRangeObjectDescriptor: you want to get a range of objects in an array.
- GetStringComparisonObjectDescriptor: you want to get an object by comparison with another object descriptor.
- GetTestObjectDescriptor: you want to select objects using a test (like an If...Then...Else statement).
- GetUniqueIDObjectDescriptor: you want to get the object given a unique ID that was passed to you earlier by the application (you cannot guess the unique ID).
- GetPropertyObjectDescriptor: you want to access a property of an object.
Note: whenever you want to get an object at the application level, e.g. a window, you should pass Nil as the Object parameter.
AppleEvent objects are used to send and receive information between your application and other Macintosh applications or the Mac OS. To send an AppleEvent from your application to another application, create an AppleEvent with the AppleEvent constructor, fill in the AppleEvent's properties with any necessary data, then call the AppleEvent's Send function to send it.
AppleEvents can also be received by your application. When an AppleEvent is received, the Application object's HandleAppleEvent event is executed and the AppleEvent is passed to the event as a parameter. All intrinsic AppleEvents are first passed to the Application object's HandleAppleEvent event handler. If you return True from this event, the default behavior of the AppleEvent will not be executed. For more information on receiving AppleEvents, see the Application class.
Replying To An AppleEvent
When an AppleEvent is received (via an AppleEvent handler) the ReplyBoolean, ReplyInteger, and ReplyString properties can be used to automatically send a reply back to the application that sent the AppleEvent. When sending an AppleEvent (via the Send method) the Reply* properties, such as ReplyInteger, can be used to get any reply the target application has sent back once it receives the AppleEvent.
For example, the line:
indicates that the application successfully handled the AppleEvent message.
To determine whether the other application successfully handled a message, use code such as:
Dim s As String
If ae.Send Then // AE successfully sent
If ae.ReplyBoolean Then
s = "Yes (handled)"
s = "Yes (unhandled)"
s = "No"
In this code, the TextEdit application (which must be running for this code to work) is instructed to open two documents ("My Document" and "My Other Document") that are located in the default folder:
Dim list As AppleEventDescList
a = New AppleEvent("aevt", "odoc", "com.apple.textedit")
list = New AppleEventDescList
list.AppendFolderItem(GetFolderItem("My Other Document"))
a.DescListParam("----") = list
If Not a.Send Then
MsgBox "The AppleEvent could not be sent."
- AppleEventDescList, AppleEventObjectSpecifier classes; Application object; If…#Else…#Endif statement; TargetLinux, TargetMacOS, TargetWin32 constants.
- UserGuide:Advanced Apple Events
- Apple Events Programming Guide, a legacy guide on Apple's website.
- Introduction to AppleScript Language on Apple's website.
- For a list of system-defined errors, see file /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/CarbonCore.framework/Headers/MacErrors.h