TweetFollow Us on Twitter

An Introduction to Handlers

Volume Number: 21 (2005)
Issue Number: 2
Column Tag: Programming

AppleScript Essentials

by Benjamin S. Waldie

An Introduction to Handlers

In a previous article, you may remember that we discussed methods of writing AppleScripts to watch folders for incoming items to process. In each of the methods we discussed, we made use of handlers. This month, we are going to explore handlers in more depth. Since handlers are a fairly complex subject, the full scope of handlers will not be covered in this month's article. Rather, we will cover the basics of handlers. In future articles, we will discuss handlers in more detail.

What is a Handler?

A handler is a group of one or more AppleScript statements that are associated with a single command. As we will discuss, some handlers are user defined, and some are application, or system defined.

Once the command associated with a handler is invoked, the statements within that handler will execute. Handlers allow you to group your code into logical "chunks," which may be triggered, or called, over and over again within a script. By carefully constructing your handlers, you can make scripts very modular, giving you the ability to break the scripts apart and reuse the handlers again in future scripts. Many AppleScript developers store their handlers in code libraries. These libraries can then be loaded, and handlers within them can be triggered from other scripts.

Anatomy of a Handler

Let's take a look at a basic handler. The following handler may be used to display a basic dialog message.

on displayDialog()
   display dialog "This is code within a handler."
end displayDialog

Now, let's break handlers down into different parts, and examine how they are constructed.

Handler Definition

A handler definition refers to the handler itself. The following would be considered the handler definition from the previous example.

on displayDialog()
   display dialog "This is code within a handler."
end displayDialog

A handler definition always begins with the word on or to, which indicates to AppleScript that it is the beginning of a handler. In the example above, I chose to begin my handler with the word on. However, the method you choose to use is entirely at your discretion. You should use whichever word looks more acceptable to you.

The next part of the handler definition is the name of the handler, followed by any parameters to be passed to the handler by the script. For example:

displayDialog()

In many cases, you may need to pass information to a handler to be processed. To allow for this, handlers support input parameters. Handlers also have the ability to return data back to the script. For example, a handler may return a true/false value indicating whether or not it was successfully processed.

There are a couple different methods that you can use to specify input parameters, and we will discuss them shortly. In the previous example, the empty parentheses indicate that my handler does not require any parameters.

Following the first line of a handler definition are any AppleScript statements that should be executed by the handler. You may write as few or as many statements as you need, keeping in mind that they will not execute until the handler is called.

The final line of a handler definition always begins with the word end, followed by the name of the handler.

end displayDialog

To save time when writing a handler, you may write the word end, and when you compile your code, the handler name will automatically be inserted for you.

Handler Call

To call a handler from within a script, you must specify the name of the handler, followed by values for all of its parameters. The following is the call for the previous handler example:

displayDialog()

We'll look at calling handlers that use parameters shortly, as we explore parameters.

If you need to call a handler from within an application tell block, you must specify of me after the handler call, to indicate that the handler is to be addressed within the script, and not within the application. For example:

tell application "Finder"
   displayDialog() of me
end tell

Parameters

As previously mentioned, the first line of a handler contains the handler name, followed by any parameters to be passed into the handler. A parameter is a variable that is named in the handler definition, and is assigned a value when the handler is called. When calling a handler, all parameters are required, and must be specified. Handlers do not allow optional parameters.

There are two types of parameters that may follow a handler name - labeled parameters and positional parameters.

Labeled Parameters

Labeled parameters are parameters that are associated with labels in the handler definition. When the handler is called, these labels are used to determine which parameters are which. Therefore, when working with labeled parameters, you may pass the parameters through your handler call in any order you wish, so long as they correspond with the correct labels. The exception to this rule is that, in some cases, you may choose to assign a direct parameter, which is required to fall immediately after the handler name.

There are two ways that you can assign labeled parameters in a handler. The first is to assign the parameters using one of the following predefined labels: about, above, against, apart from, around, aside from, at, below, beneath, beside, between, by, for, from, instead of, into, on, onto, out of, over, since, through, thru, under. The label of may also be used, though only to define a direct parameter. If you add a direct parameter, using the label of, then you are required to have at least one or more additional parameters. The following is an example of a handler with labeled parameters:

to displayDialog of theText above theButtons aside from theIconNumber
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

In the example above, the parameter theText is a direct parameter, as indicated by the label of. The parameter theButtons is associated with the label above, and the parameter theIconNumber is associated with the label aside from. The handler above would be called using the following line of code:

displayDialog of "Hello" above {"OK"} aside from 1

Again, since labeled parameters are associated with their labels, then you can rearrange the non-direct parameters, if desired. For example, the following handler call would function identically as the previous one:

displayDialog of "Hello" aside from 1 above {"OK"}

Another way to assign labeled parameters is to use custom labels. This is done by creating label definitions in the format labelName:parameterName, separating them by commas, and preceding them with the label given. Custom labeled parameters must follow any predefined label parameters. An example of a handler with custom labeled parameters is the following:

to displayDialog of theText given someButtons:theButtons, someIconNumber:theIconNumber
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

In the example above, the parameter theText is a direct parameter, preceded by a predefined label. The remaining parameters are custom labeled parameters. The parameter theButtons is associated with the custom label someButtons, and the parameter theIconNumber is associated with the custom label someIconNumber. This handler can be called using the following line of code:

displayDialog of "Hello" given someButtons:{"OK"}, someIconNumber:1

Again, since this handler makes use of labeled parameters, the non-direct parameters may be rearranged, if desired:

displayDialog of "Hello" given someIconNumber:1, someButtons:{"OK"}

Positional Parameters

Another type of parameter that you can use when defining a handler is a positional parameter. Positional parameters are separated by commas, and do not contain any labels. Because they do not contain labels, they are identified by their position in the handler definition. Therefore, they must be listed in the same position when the handler is called. The following is an example of a handler that makes use of positional parameters:

on displayDialog(theText, theButtons, theIconNumber)
   display dialog theText buttons theButtons with icon theIconNumber
end displayDialog

The handler above may be called using the following line of code:

displayDialog("Hello", {"OK"}, 1)

Return Value

In some cases, a handler may return a value to the script that called it. This value may be placed into a variable for later usage. By default, a handler will return the result of the last AppleScript statement that executes within the handler, assuming that this statement produces a result. If desired, you may configure your handler to return a different value. For example, the following code returns the name of the button clicked in the dialog.

set theChoice to displayDialog()

on displayDialog()
   display dialog "Would you like to continue processing?" buttons {"Yes", "No"}
   return button returned of result
end displayDialog

Next, I could add additional code at the root level of my script to check the button that was clicked, now stored in a variable called theChoice, and take the appropriate course of action.

Within a handler, you may return a value at any time to cease further execution of that handler, or you may return no value to cease execution without returning a specific value. For example, the following handler will stop processing, returning no value, if the user clicks the "No" button.

displayDialog()

on displayDialog()
   display dialog "Would you like to continue processing?" buttons {"Yes", "No"}
   set theChoice to button returned of result
   if theChoice = "No" then return
   display dialog "Continuing..."
end displayDialog

Types of Handlers

There are two types of handlers in AppleScript - subroutine handlers and command handlers.

Subroutine Handlers

Subroutine handlers are groups of statements, which are defined by the developer, and called throughout a script, or from another script. Subroutines can be extremely useful if you need to perform the same exact task over and over throughout your script. For example, let's say that you need to display an informational dialog to the user 10 times throughout your script to let the user know what is occurring.

display dialog "Beginning next task..." buttons {"*"} 
   default button "*" with icon 1 giving up after 3

To display the above dialog 10 times, I could write out all of this code 10 times - or, I could write a handler 1 time.

on displayNextTaskMessage()
   display dialog "Beginning next task..." buttons {"*"} default 
      button "*" with icon 1 giving up after 3
end displayNextTaskMessage

Once written, this handler can be triggered from anywhere within my script. Rather than writing the entire lengthy line of display dialog code each time I need to display the dialog to the user, I can call my handler instead. The following code would call the displayNextTaskMessage handler.

displayNextTaskMessage()

In the example above, you may be wondering where the handler name "displayNewTaskMessage" came from. This is something that I defined myself when writing the handler. Remember, since subroutine handlers are defined by a developer, their names are user definable. So, I could have named this handler anything I wanted, as long as it was not the name of another existing handler in my script.

Command Handlers

A command handler is a group of statements that is triggered by a specific application or system related event.

Every AppleScript application contains an implied run handler. Any AppleScript statements at the top level of the script, excluding global variables, properties, other handlers, and script objects, fall within that run handler. If you prefer to make the run handler visible in your code, you may wrap these statements within an on run handler call. When the script is run, both methods will behave identically. For example, each of the examples below will perform the exact same function:

Example 1:

display dialog "Hello!"

Example 2:

on run
   display dialog "Hello!"
end run

As you can see, any code within the run handler of a script will be executed whenever the script is run. However, in some cases, you may want to execute code when other types of actions occur within your script.

The open handler may be used to initiate specific code when items are dragged and dropped onto a script in the Finder. For example:

on open theDroppedItems
   -- Process the items
end open

By adding an open handler into a script, and then saving that script as an application, the script will automatically accept dropped files and folders. It will also receive a new icon, indicating that it is now a drop script.

In the previous example code, the parameter theDroppedItems will contain a list of paths to any items dropped onto the script. If you want your script to only process folders, then you would need to add custom code within the open handler to determine which dropped items were folders, and process only these items.

Two other types of command handlers that may be added to an AppleScript application are the idle handler and the quit handler.

An idle handler is particularly useful when creating stay open AppleScript applications. In a stay open AppleScript application, by default, AppleScript will send the script an idle command every 30 seconds. At that time, any code within the idle handler will execute. For example, if I save the following code as a stay opened AppleScript application, and trigger it, the script will beep every 30 seconds:

on idle
   beep
end idle

Though the default time period between idle messages is 30 seconds, I can change this behavior if desired, by returning an integer value indicating how many seconds of a delay should occur before the next idle. In the following example, the script would beep every 10 seconds.

on idle
   beep
   return 10
end idle

The quit handler may be used in order to execute code when the script quits, whether manually quit by the user or not.

on quit
   display dialog "Task complete."
end quit

Example Handlers

Now that we have covered the basics of handlers, let's take a look at some example handlers, which may be useful to you as you write scripts in the future. Each of these handlers has been written generically, so that you can use it in virtually any script in the future.

The following handler will make a new folder in a specified output folder, using a specified name:

on makeNewFolder(theNewFolderName, theOutputFolder)
   tell application "Finder" to make new folder at folder theOutputFolder with properties 
      {name:theNewFolderName}
end makeNewFolder

The following handler will delete a folder of a file:

on moveItemToTrash(theItemPath)
   tell application "Finder" to delete item theItemPath
end moveItemToTrash

The following handler will mount an afp volume:

on mountVolume(theVolumeName, theServerIPAddress, theUserName, thePassword)
   mount volume "afp://" & theUserName & ":" & thePassword & "@" & theServerIPAddress 
      & "/" & theVolumeName as string
end mountVolume

In Closing

Again, handlers are a fairly complex aspect of AppleScript development for many users. Today, I use them regularly, and I try to make them as modular as possible. My theory is that if I write code to perform a specific task, such as opening a document in QuarkXPress, then I don't want to ever write that code again. Rather, I store the handler for later usage in future scripts, and the next time I need to perform the task, which could be a year down the line, I simply add the handler into my new script.

I encourage you to begin using handlers more in your scripting, as it will benefit you greatly. We'll discuss other aspects of handlers in more detail in future articles. In the meantime, for additional information about handlers, you may want to check out an AppleScript book, or browse the AppleScript Language Guide at <http://developer.apple.com/documentation/AppleScript/>.

Until next time, keep scripting!


Benjamin Waldie is president of Automated Workflows, LLC, a firm specializing in AppleScript and workflow automation consulting. In addition to his role as a consultant, Benjamin is an evangelist of AppleScript, and can frequently be seen presenting at Macintosh User Groups, Seybold Seminars, and MacWorld. For additional information about Benjamin, please visit http://www.automatedworkflows.com, or email Benjamin at applescriptguru@mac.com.

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Are We Now In The Post-Post-PC Era?
A longtime and thoroughgoing laptop aficionado, I was more than a little dismayed by Steve Jobs’s declaration back in 2010 when he sprang the iPad on an unsuspecting world. that we’d entered a “post-... Read more
Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.