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.