Section 11.1. Messages

11.1. Messages

The fundamental activity in AppleScript is that of sending messages. Every line of code contains at least one imperative verb. There are actually two kinds of imperative verb: a handler call, which matches a handler definition in your script, and a command , which matches an event defined in a dictionary. The imperative verb is always directed to some specific target , which is supposed to obey it. The medium of communication between the imperative verb in your code and the target that you're talking to is a message .

An object is anything that can be targeted by a message. The most important targets in AppleScript are scriptable applications, but a script object can be a target too, and indeed, in some sense, every value can be a target. So, to that extent, everything in AppleScript is a kind of object. (See also "Object-likeness" in Chapter 4.) For example, count is a command. In a context where the target is the Finder, saying count causes a message to be sent to the Finder. In a context where the target is a script object, saying count causes a message to be sent to that script object. In a context where the target is a string, saying count causes a message to be sent to that string.

AppleScript tries to give the impression that all messages have the same status. Consider, for instance, what happens when an object can't obey a message. If the target is anything other than an application, we are told that the object "doesn't understand the so-and-so message." If the target is an application, we are told: "Can't continue so-and-so." The wording of the error is the same, though, no matter whether the imperative verb that generated the message is a handler call or a command.

 tell 1     count -- error: 1 doesn't understand the count message end tell   script s end script tell s     h( ) -- error: «script s» doesn't understand the h message end tell   tell application "Finder"     tell folder 1         using terms from application "iPhoto"             start slideshow             -- error: Finder got an error: Folder 1 doesn't understand the start             slideshow message         end using terms from     end tell end tell   tell application "Finder"     tell folder 1         h( ) -- error: Finder got an error: Folder 1 doesn't understand the h message     end tell end tell   tell application "Finder"     using terms from application "iPhoto"         start slideshow -- Finder got an error: Can't continue start slideshow     end using terms from end tell   tell application "Finder"     h( ) -- error: Finder got an error: Can't continue h end tell

The get and set commands (and also sometimes copy) are treated in a special way: they are not messages in the sense just mentioned, but are instead somehow short-circuited so that they always work. The reason is, I suppose, that otherwise it might be possible for code or a target application to break or interfere with them and then nothing would work at all.

The message-target metaphor is also made somewhat problematic by the existence of scripting additions (see Chapter 21). Scripting additions cannot be targeted, but the commands they implement are available everywhere.